Re: [PATCH v4 4/5] drivers/perf: Add support for ARMv8.2 Statistical Profiling Extension

[Will Deacon]

Hi Mark,

Thanks for the extensive review. Replies inline.

On Thu, Jun 15, 2017 at 03:57:29PM +0100, Mark Rutland wrote:
> On Mon, Jun 05, 2017 at 04:22:56PM +0100, Will Deacon wrote:
> > +/* ID registers */
> > +#define PMSIDR_EL1			sys_reg(3, 0, 9, 9, 7)
> 
> Nit: could we please give the sysreg definitions a SYS_ prefix, for
> consistency with other (architected) sysregs?
> 
> Ideally, as these are architected they'd live in <asm/sysreg.h>, but I'm
> happy to factor that out later.

I don't see the point in adding the prefixes without moving the #defines
into sysreg.h, so I'll look at just doing it in one go.

> > +#define PMSIRR_EL1			sys_reg(3, 0, 9, 9, 3)
> > +#define PMSIRR_EL1_RND_SHIFT		0
> > +#define PMSIRR_EL1_IVAL_MASK		0xffUL
> 
> This is a little odd. This covers RND and (some) RES0 bits, not
> INTERVAL, so it's the opposite polarity to the rest of the masks, and
> incomplete (missing the upper 32 RES0 bits).
> 
> Could we please have this in the same polarity as the other masks,
> precisely covering the INTERVAL field? i.e.
> 
> #define PMSIRR_EL1_INTERVAL_SHIFT	8
> #define PMSIRR_EL1_INTERVAL_MASK	0xffffffUL

Yup, I'll fix that.

> > +#define PMBSR_EL1_EC_BUF		(0x0UL << PMBSR_EL1_EC_SHIFT)
> > +#define PMBSR_EL1_EC_FAULT_S1		(0x24UL << PMBSR_EL1_EC_SHIFT)
> > +#define PMBSR_EL1_EC_FAULT_S2		(0x25UL << PMBSR_EL1_EC_SHIFT)
> > +
> > +#define PMBSR_EL1_FAULT_FSC_SHIFT	0
> > +#define PMBSR_EL1_FAULT_FSC_MASK	0x3fUL
> 
> Nit: it might be worth having MSS in the name to show the field
> hierarchy, e.g. PMBSR_EL1_MSS_FAULT_FSC_MASK

I'd rather avoid growing the name, and I don't find this confusing as-is.

> > +struct arm_spe_pmu {
> > +	struct pmu				pmu;
> > +	struct platform_device			*pdev;
> > +	cpumask_t				supported_cpus;
> > +	struct hlist_node			hotplug_node;
> > +
> > +	int					irq; /* PPI */
> > +
> > +	u16					min_period;
> > +	u16					cnt_width;
> 
> Elsewhere, we refer to this as the counter size (e.g.
> SPE_PMU_CAP_CNT_SZ, and count_size in the sysfs interface).
> 
> Could we please pick one of "width" and "size" and use it consistently?
> 
> I guess "size" is preferable, given the architectural name is
> "CountSize".

Will fix.

> > +/* Convert a free-running index from perf into an SPE buffer offset */
> > +#define PERF_IDX2OFF(idx, buf)	((idx) & (((buf)->nr_pages << PAGE_SHIFT) - 1))
> 
> The masking logic here assumes nr_pages is a power of two.
> 
> That's not always true, as arm_spe_pmu_setup_aux() only ensures that
> nr_pages is a multiple of 2 * PAGE_SIZE, and only when using snapshot
> mode.
> 
> For example, with ten 4K pages:
> 
> nr_pages:			0b1010
> (nr_pages << PAGE_SHIFT):	0b1010000000000000
> (nr_pages << PAGE_SHIFT) - 1:	0b1001111111111111

Nice. Fixed (using a bloody mod operator).

> > +
> > +/* Keep track of our dynamic hotplug state */
> > +static enum cpuhp_state arm_spe_pmu_online;
> > +
> > +/* This sysfs gunk was really good fun to write. */
> > +enum arm_spe_pmu_capabilities {
> > +	SPE_PMU_CAP_ARCH_INST = 0,
> 
> No need for the initializer; enums start at zero.

I know, but I do like to make it explicit that I'm relying on that here.
This is done all over the place in the kernel.

> 
> > +	SPE_PMU_CAP_ERND,
> > +	SPE_PMU_CAP_FEAT_MAX,
> > +	SPE_PMU_CAP_CNT_SZ = SPE_PMU_CAP_FEAT_MAX,
> > +	SPE_PMU_CAP_MIN_IVAL,
> > +};
> 
> IMO, it would be worth having s/CAP/CAP_FEAT/ for the HW features.
> 
> We could get rid of the confusing SPE_PMU_CAP_FEAT_MAX definition here,
> if we were to:
> 
> > +
> > +static int arm_spe_pmu_feat_caps[SPE_PMU_CAP_FEAT_MAX] = {
> > +	[SPE_PMU_CAP_ARCH_INST]	= SPE_PMU_FEAT_ARCH_INST,
> > +	[SPE_PMU_CAP_ERND]	= SPE_PMU_FEAT_ERND,
> > +};
> 
> .. change this to:
> 
> static int arm_spe_pmu_feat_caps[] = {
> 	...
> };
> 
> ... and:
> 
> > +
> > +static u32 arm_spe_pmu_cap_get(struct arm_spe_pmu *spe_pmu, int cap)
> > +{
> > +	if (cap < SPE_PMU_CAP_FEAT_MAX)
> 
> ... change this to:
> 
> 	if (cap < ARRAY_SIZE(arm_spe_pmu_feat_caps))

I quite liked this suggestion at first and I even implemented it, but the
result was IMO less maintainable than the above. There's ABI involved here,
so I want to make it as difficult as possible to break the ABI when adding a
new hardware capability to the driver. The current code does a good job of
that:

  - If you add a boolean feature in the wrong place in arm_spe_capabilities,
    then you'll get a WARN

  - If you add a non-boolean feature in the wrong place then it will be
    reported as non-present, unless you add an entry in
    arm_spe_pmu_feat_caps (at which point you'd realise the mistake)

  - If you only update arm_spe_pmu_feat_caps, then you'll get a build error.

With your change, it's a lot easier to break things subtly, so I'd rather
keep this as-is unless you have non-cosmetic reasons to change it.

> > +#define SPE_EXT_ATTR_ENTRY(_name, _func, _var)				\
> > +	&((struct dev_ext_attribute[]) {				\
> > +		{ __ATTR(_name, S_IRUGO, _func, NULL), (void *)_var }	\
> > +	})[0].attr.attr
> > +
> > +#define SPE_CAP_EXT_ATTR_ENTRY(_name, _var)				\
> > +	SPE_EXT_ATTR_ENTRY(_name, arm_spe_pmu_cap_show, _var)
> > +
> > +static struct attribute *arm_spe_pmu_cap_attr[] = {
> > +	SPE_CAP_EXT_ATTR_ENTRY(arch_inst, SPE_PMU_CAP_ARCH_INST),
> > +	SPE_CAP_EXT_ATTR_ENTRY(ernd, SPE_PMU_CAP_ERND),
> > +	SPE_CAP_EXT_ATTR_ENTRY(count_size, SPE_PMU_CAP_CNT_SZ),
> > +	SPE_CAP_EXT_ATTR_ENTRY(min_interval, SPE_PMU_CAP_MIN_IVAL),
> > +	NULL,
> > +};
> 
> I'd have expected GCC to warn about (integer/enum) _var values being
> cast straight to void *, given the size mismatch.
> 
> Is that not the case, or do we need an unsigned long cast in
> SPE_CAP_EXT_ATTR_ENTRY()?
> 
> Maybe GCC only complains the other way around.

GCC 7 seems to be perfectly happy with this code.

> 
> [...]
> 
> > +/* Convert between user ABI and register values */
> > +static u64 arm_spe_event_to_pmscr(struct perf_event *event)
> > +{
> > +	struct perf_event_attr *attr = &event->attr;
> > +	u64 reg = 0;
> > +
> > +	reg |= ATTR_CFG_GET_FLD(attr, ts_enable) << PMSCR_EL1_TS_SHIFT;
> > +	reg |= ATTR_CFG_GET_FLD(attr, pa_enable) << PMSCR_EL1_PA_SHIFT;
> 
> We should limit PA access to privileged users.
> 
> > +
> > +	if (!attr->exclude_user)
> > +		reg |= BIT(PMSCR_EL1_E0SPE_SHIFT);
> > +
> > +	if (!attr->exclude_kernel)
> > +		reg |= BIT(PMSCR_EL1_E1SPE_SHIFT);
> > +
> > +	if (IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR))
> > +		reg |= BIT(PMSCR_EL1_CX_SHIFT);
> 
> ... maybe likewise for CONTEXTIDR, too.

Yes, agreed. Fixed.

> > +static void arm_spe_event_sanitise_period(struct perf_event *event)
> > +{
> > +	struct arm_spe_pmu *spe_pmu = to_spe_pmu(event->pmu);
> > +	u64 period = event->hw.sample_period & ~PMSIRR_EL1_IVAL_MASK;
> 
> ... as noted above, the upper 32 bits are RES0 in addition to the low 8
> bits, so we need to explicitly check bits 31:8, e.g.
> 
> 	u64 period = event->hw.sample_period;
> 	period &= (PMSIRR_EL1_INTERVAL_MASK << PMSIRR_EL1_INTERVAL_SHIFT);

Fixed.

> > +
> > +	if (period < spe_pmu->min_period)
> > +		period = spe_pmu->min_period;
> 
> We already verify this in arm_spe_pmu_event_init(), so we don't need to
> check this here.
> 
> We can drop arm_spe_event_sanitise_period() entirely. Given we validate
> the period at event_init() time, there's no need to sanitize the value.

What about PERF_IOC_PERIOD? I don't think that re-inits the event.

> > +static bool arm_spe_pmu_buffer_mgmt_pending(u64 pmbsr)
> > +{
> > +	const char *err_str;
> > +
> > +	/* Service required? */
> > +	if (!(pmbsr & BIT(PMBSR_EL1_S_SHIFT)))
> > +		return false;
> > +
> > +	/* We only expect buffer management events */
> > +	switch (pmbsr & (PMBSR_EL1_EC_MASK << PMBSR_EL1_EC_SHIFT)) {
> > +	case PMBSR_EL1_EC_BUF:
> > +		/* Handled below */
> > +		break;
> > +	case PMBSR_EL1_EC_FAULT_S1:
> > +	case PMBSR_EL1_EC_FAULT_S2:
> > +		err_str = "Unexpected buffer fault";
> > +		goto out_err;
> > +	default:
> > +		err_str = "Unknown error code";
> > +		goto out_err;
> > +	}
> 
> For the error cases, I take it the assumption is that we leave
> PMBSR_EL1.S set, so that the HW doesn't start again?

No, I don't think I actually handle these cases at all. Whilst they're
probably catastrophic (vmapped mappings are faulting!), I should at least
try to park the profiler. Will fix for the next version.

> > +out_err:
> > +	pr_err_ratelimited("%s on CPU %d [PMBSR=0x%08llx]\n", err_str,
> > +			   smp_processor_id(), pmbsr);
> 
> It might be worth dumping pmbsr with %016lx. The upper 64 bits are
> currently RES0, but they do exist.

Ok.

> > +	return false;
> > +}
> > +
> 
> Could we have a comment block here to describe (roughly) what 
> we're trying to do for the snapshot case?

Ok, I'll try to think of something to say.

> > +
> > +	/*
> > +	 * If we're within max_record_sz of the limit, we must
> > +	 * pad, move the head index and recompute the limit.
> > +	 */
> > +	if (limit - head < spe_pmu->max_record_sz) {
> > +		memset(buf->base + head, 0, limit - head);
> 
> Could we have a mnemonic for the padding byte, and/or a helper that
> wraps memset? e.g.
> 
> static void pad_buffer(void *start, u64 size)
> {
> 	/* The padding packet is a single zero byte */
> 	memset(start, 0, size);
> }

Sure.

> > +		handle->head = PERF_IDX2OFF(limit, buf);
> > +		limit = ((buf->nr_pages * PAGE_SIZE) >> 1) + handle->head;
> > +	}
> > +
> > +	return limit;
> > +}
> > +
> > +static u64 __arm_spe_pmu_next_off(struct perf_output_handle *handle)
> > +{
> > +	struct arm_spe_pmu_buf *buf = perf_get_aux(handle);
> > +	u64 head = PERF_IDX2OFF(handle->head, buf);
> > +	u64 tail = PERF_IDX2OFF(handle->head + handle->size, buf);
> > +	u64 wakeup = PERF_IDX2OFF(handle->wakeup, buf);
> > +	u64 limit = buf->nr_pages * PAGE_SIZE;
> > +
> > +	/*
> > +	 * Set the limit pointer to either the watermark or the
> > +	 * current tail pointer; whichever comes first.
> > +	 */
> > +	if (handle->head + handle->size <= handle->wakeup) {
> > +		/* The tail is next, so check for wrapping */
> > +		if (tail >= head) {
> > +			/*
> > +			 * No wrapping, but need to align downwards to
> > +			 * avoid corrupting unconsumed data.
> > +			 */
> > +			limit = round_down(tail, PAGE_SIZE);
> > +
> > +		}
> > +	} else if (wakeup >= head) {
> 
> When wakeup == head, do we need to signal a wakeup event somehow?
> Currently we'll pad the buffer, signal truncation, and end output, which
> seems a little odd, but maybe that's what perf expects.

Wakeup can never be equal to head here. We know that the wakeup is next (by
the first if above) and therefore it is within handle->size of head. Since
we're starting a session, then we either have:

  1. Wakeup calculated by perf_aux_output_{skip,end}, or
  2. Wakeup calculated by rb_alloc_aux (initial mmap)

In case (1), the wakeup will have been signalled when it was calculated to
be equal to head, and then the wakeup will have been moved to the next
watermark point. In case (2), the only way it can be equal to head is if
the watermark was 0, but an initial watermark is converted to half the
buffer size by the core.

Admittedly, I'd not realised this at the time (hence the >= check), and it
looks like we're going to rewrite this anyway :)

> > +		/*
> > +		 * The wakeup is next and doesn't wrap. Align upwards to
> > +		 * ensure that we do indeed reach the watermark.
> > +		 */
> > +		limit = round_up(wakeup, PAGE_SIZE);
> > +
> > +		/*
> > +		 * If rounding up crosses the tail, then we have to
> > +		 * round down to avoid corrupting unconsumed data.
> > +		 * Hopefully the tail will have moved by the time we
> > +		 * hit the new limit.
> > +		 */
> > +		if (wakeup < tail && limit > tail)
> > +			limit = round_down(wakeup, PAGE_SIZE);
> > +	}
> 
> It took me a while to grok that we must consider the wakeup in
> free-running counter space to avoid early wakeups, while we must
> consider the tail in ring-buffer offset space to avoid clobbering data.
> 
> With that understanding, I think we have an issue here. If wakeup is
> more than buffer size in the future, and the buffer is empty, I think we
> set the limit too low.
> 
> In that case, we'd evaluate:
> 
> 	handle->head + handle->size <= handle->wakeup
> 
> ... as true, since size is at most buffer size. Thus we'd go into the
> first if block. There we'd evaluate:

If the buffer is empty, then size is exactly buffer size - 1, but I take
your point.

> 
> 	tail >= head
> 
> ... as true, since when the buffer is empty, head == tail. Thus, we'd
> set the limit to:
> 
> 	round_down(tail, PAGE_SIZE)
> 
> ... which'll leave us with limit <= head, since head == tail. Thus,
> we'll hit the case below:
> 
> > +
> > +	/*
> > +	 * If rounding down crosses the head, then the buffer is full,
> > +	 * so pad to tail and end the session.
> > +	 */
> > +	if (limit <= head) {
> > +		memset(buf->base + head, 0, handle->size);
> > +		perf_aux_output_skip(handle, handle->size);
> > +		perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
> > +		perf_aux_output_end(handle, 0);
> > +		limit = 0;
> > +	}
> > +
> > +	return limit;
> > +}
> 
> ... and end all output, even though the entire buffer was empty, and we
> could have returned the end of the buffer as the limit.
> 
> It might be that something prevents wakeup from being that far in the
> future, but in previous discussions we'd assumed that it could be any
> arbitrary value.

Yes, I think this case does indeed go wrong. Well spotted!

> I believe we can solve that, and simplify the logic as below. I've left
> the wakeup < head and wakeup == head cases as above, ignored and
> terminating respectively.

I think this mostly works, some suggestions/questions below.

> static u64 __arm_spe_pmu_next_off(struct perf_output_handle *handle)
> {
> 	struct arm_spe_pmu_buf *buf = perf_get_aux(handle);
> 	const u64 bufsize = buf->nr_pages * PAGE_SIZE;
> 	u64 limit = bufsize;
> 	u64 head = PERF_IDX2OFF(handle->head, buf);
> 	u64 tail = PERF_IDX2OFF(handle->head + handle->size, buf);
> 	u64 wakeup = PERF_IDX2OFF(handle->wakeup, buf);
> 
> 	if (!handle->size)
> 		goto no_space;

We can avoid the memset/output_skip in this case.

> 	/*
> 	 * Avoid clobbering unconsumed data. We know we have space, so
> 	 * if we see head == tail we know that the buffer is empty. If
> 	 * head > tail, then there's nothing to clobber prior to
> 	 * wrapping.
> 	 */
> 	if (head < tail)
> 		limit = round_down(tail, PAGE_SIZE);
> 	
> 	/*
> 	 * Wakeup may be arbitrarily far into future. If it's not in the
> 	 * current generation, either we'll wrap before hitting it, or
> 	 * it's in the past and has been handled already.
> 	 *
> 	 * If there's a wakeup before we wrap, arrange to be woken up by
> 	 * the page boundary following it. Keep the tail boundary if
> 	 * that's lower.
> 	 */
> 	if ((handle->wakeup / bufsize) == (handle->head / bufsize)) &&

I'd really like to get rid of these divisions, since we're not working with
nice powers of 2 here. Can't you just do:

  handle->wakeup < (handle->head + handle->size)

to establish that they're in the same "generation"?

> 	    head <= wakeup)
> 		limit = min(limit, round_up(wakeup, PAGE_SIZE));
> 
> 	if (limit <= head)
> 		goto no_space;

Does this correctly handle the case where the buffer is full and head ==
tail, but limit == bufsize? AFAICT, we can return a limit of bufsize and
corrupt the whole buffer.

> > +static void arm_spe_perf_aux_output_begin(struct perf_output_handle *handle,
> > +					  struct perf_event *event)
> > +{
> > +	u64 base, limit;
> > +	struct arm_spe_pmu_buf *buf;
> > +
> > +	/* Start a new aux session */
> > +	buf = perf_aux_output_begin(handle, event);
> > +	if (!buf) {
> > +		event->hw.state |= PERF_HES_STOPPED;
> > +		/*
> > +		 * We still need to clear the limit pointer, since the
> > +		 * profiler might only be disabled by virtue of a fault.
> > +		 */
> > +		limit = 0;
> > +		goto out_write_limit;
> > +	}
> > +
> > +	limit = buf->snapshot ? arm_spe_pmu_next_snapshot_off(handle)
> > +			      : arm_spe_pmu_next_off(handle);
> > +	if (limit)
> > +		limit |= BIT(PMBLIMITR_EL1_E_SHIFT);
> > +
> > +	base = (u64)buf->base + PERF_IDX2OFF(handle->head, buf);
> > +	write_sysreg_s(base, PMBPTR_EL1);
> > +	limit += (u64)buf->base;
> > +
> 
> I believe an isb() is necessary here to ensure the write to PMBPTR_EL1
> occurs before the write to PMBLIMITR_EL1 enables the PMU. Otherwise, the
> CPU could execute those out-of-order.

This function is always called in a context where the profiler is disabled
due to some other control (e.g. in PMSCR or because we're in fault context)
so the isb isn't necessary.

> > +out_write_limit:
> > +	write_sysreg_s(limit, PMBLIMITR_EL1);
> > +}
> > +
> > +static bool arm_spe_perf_aux_output_end(struct perf_output_handle *handle,
> > +					struct perf_event *event,
> > +					bool resume)
> > +{
> > +	u64 pmbptr, pmbsr, offset, size;
> > +	struct arm_spe_pmu *spe_pmu = to_spe_pmu(event->pmu);
> > +	struct arm_spe_pmu_buf *buf = perf_get_aux(handle);
> > +	bool truncated;
> > +
> > +	/*
> > +	 * We can be called via IRQ work trying to disable the PMU after
> > +	 * a buffer full event. In this case, the aux session has already
> > +	 * been stopped, so there's nothing to do here.
> > +	 */
> > +	if (!buf)
> > +		return false;
> > +
> > +	/*
> > +	 * If there isn't a pending management event and we're not stopping
> > +	 * the current session, then just leave everything alone.
> > +	 */
> > +	pmbsr = read_sysreg_s(PMBSR_EL1);
> 
> When we call from arm_spe_pmu_irq_handler(), I think we need
> synchronisation before reading PMBSR_EL1.
> 
> AFAICT from the spec, a context synchronisation event doesn't ensure
> that the PMU's indirect write to PMBSR_EL1 is visible to the PE's direct
> read above. I beleive we need a PSB CSYNC (and subsequent ISB) to ensure
> that.

I don't think that's right, but the spec isn't completely clear. PSB CSYNC
is about the profiling data itself, but in this case we've taken an IRQ
already so the PMBSR will be up-to-date. I'll seek clarification anyway.

> The only other caller is from arm_spe_pmu_stop(), which first calls
> arm_spe_pmu_disable_and_drain_local(), so I guess the new barriers
> should live in arm_spe_pmu_irq_handler(). I'll comment there.
> 
> > +	if (!arm_spe_pmu_buffer_mgmt_pending(pmbsr) && resume)
> > +		return false; /* Spurious IRQ */
> > +
> > +	/* Ensure hardware updates to PMBPTR_EL1 are visible */
> > +	isb();
> 
> Can we please move this into arm_spe_pmu_buffer_mgmt_pending(), after
> the associated PSB CSYNC?

Hmmm, I deliberately *didn't* do that because I wanted
arm_spe_pmu_buffer_mgmt_pending to ensure the buffer writes are visible, and
then the caller can decide if it cares about indirect SPE register writes
being visible. In reality, I ended up with a single caller, but let's see
how it looks when I rework it to deal with fatal aborts.

> > +	/*
> > +	 * Work out how much data has been written since the last update
> > +	 * to the head index.
> > +	 */
> > +	pmbptr = round_down(read_sysreg_s(PMBPTR_EL1), spe_pmu->align);
> 
> I don't believe we need to align this.
> 
> Per the spec, PMBPTR_EL1[M:0] are RES0 in HW unless sync external abort
> reporting is present, in which case they're valid. We write these bits
> as zero, unless we have a bug elsewhere.
> 
> ... so either the bits are zero, and we're fine, or an external abourt
> has been hit. In the external abort case, we have no idea how far we
> need to reverse the base pointer anyhow.

As part of the fatal abort handling, I should probably round down to
max_record_sz (keyed off DL==1, which I don't think can ever happen
at the moment).

> > +	offset = pmbptr - (u64)buf->base;
> > +	size = offset - PERF_IDX2OFF(handle->head, buf);
> > +
> > +	if (buf->snapshot)
> > +		handle->head = offset;
> 
> It's be worth a /* see arm_spe_pmu_next_snapshot_off() */ comment
> or similar to explain what we're going for the snapshot case here.

Ok.

> > +
> > +	/*
> > +	 * Either the buffer is full or we're stopping the session. Check
> > +	 * that we didn't write a partial record, since this can result
> > +	 * in unparseable trace and we must disable the event.
> > +	 */
> > +	if (pmbsr & BIT(PMBSR_EL1_COLL_SHIFT))
> > +		perf_aux_output_flag(handle, PERF_AUX_FLAG_COLLISION);
> > +
> > +	truncated = pmbsr & BIT(PMBSR_EL1_DL_SHIFT);
> > +	if (truncated)
> > +		perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
> > +
> > +	perf_aux_output_end(handle, size);
> 
> The comment block above perf_aux_output_end() says:
> 
>   It is the pmu driver's responsibility to observe ordering rules of the
>   hardware, so that all the data is externally visible before this is
>   called.
> 
> ... but in arm_spe_pmu_buffer_mgmt_pending() we only ensured that the
> data was visible in the current NSH domain (i.e. only to this CPU).
> 
> I followed the callchain for updating head:
> 
> perf_aux_output_end()
> -> perf_event_aux_event()
> -> perf_output_end()
> -> perf_output_put_handle()
> 
> ... I see that there's an smp_wmb() (i.e. a DMB ISHST) on that path, but
> it's not clear to me if that's sufficient to ensure that the PMU's
> writes are made visible to other CPUs.

With the new memory model, it should be sufficient; the DSB NSH ensures
data is visible to us locally, and then we order that before the update
of the ring buffer.

> Given the comment, I'd feel happier if we had something here or in
> arm_spe_pmu_buffer_mgmt_pending() to ensure that the PMU's prior writes
> are visible to other CPUs.

I could add a comment?

> > +	/*
> > +	 * If we're not resuming the session, then we can clear the fault
> > +	 * and we're done, otherwise we need to start a new session.
> > +	 */
> > +	if (!resume)
> > +		write_sysreg_s(0, PMBSR_EL1);
> > +	else if (!truncated)
> > +		arm_spe_perf_aux_output_begin(handle, event);
> > +
> > +	return true;
> > +}
> > +
> > +/* IRQ handling */
> > +static irqreturn_t arm_spe_pmu_irq_handler(int irq, void *dev)
> > +{
> > +	struct perf_output_handle *handle = dev;
> > +
> > +	if (!perf_get_aux(handle))
> > +		return IRQ_NONE;
> > +
> > +	if (!arm_spe_perf_aux_output_end(handle, handle->event, true))
> > +		return IRQ_NONE;
> 
> As commented in arm_spe_perf_aux_output_end(), I think we need a
> psb_csync(); isb() sequence prior to the read of PMBSR_EL1 in
> arm_spe_perf_aux_output_end() to ensure that it is up-to-date w.r.t. the
> interrupt.

As above, I don't agree but am checking this with the architects.

> > +	irq_work_run();
> > +	isb(); /* Ensure the buffer is disabled if data loss has occurred */
> 
> What exactly are we synchronising here?
> 
> AFAICT, when truncation occurs we don't clear PMBLIMITR_EL1.E, so the
> buffer is only implicitly disabled by the PMU's indirect write
> PMBSR_EL1.S, which we must have already synchronised prior to reading
> PMBSR_EL1.

When you report truncation to perf_aux_output_end, it eventually (the
irq_work_run() above) calls back into arm_spe_pmu_stop, and I want to make
sure we've nobbled the limit pointer.

It would probably be clearer just to add an ISB to the end of
arm_spe_pmu_disable_and_drain_local, which goes back to your previous comments
about the mgmt_pending code.

> ... so I can't see why this is necessary.
> 
> > +	write_sysreg_s(0, PMBSR_EL1);
> 
> ... and regardless, we clear PMBSR_EL1.S here, which'll start the PMU
> again, even if truncation occured, which I don't think we want.
> 
> Can we have arm_spe_perf_aux_output_end() clear PMBLIMITR_EL1.E when
> truncation occurs?

Right, that's exactly what happens. It's just highly convoluted.

> 
> > +	return IRQ_HANDLED;
> > +}
> > +
> > +/* Perf callbacks */
> > +static int arm_spe_pmu_event_init(struct perf_event *event)
> > +{
> > +	u64 reg;
> > +	struct perf_event_attr *attr = &event->attr;
> > +	struct arm_spe_pmu *spe_pmu = to_spe_pmu(event->pmu);
> > +
> > +	/* This is, of course, deeply driver-specific */
> > +	if (attr->type != event->pmu->type)
> > +		return -ENOENT;
> > +
> > +	if (event->cpu >= 0 &&
> > +	    !cpumask_test_cpu(event->cpu, &spe_pmu->supported_cpus))
> > +		return -ENOENT;
> 
> We're not rejecting cpu < 0, so I take it we're trying to handle
> per-task events?

Yes, like intel-pt does.

> As I've mentioned before, that case worries me. One thing I've just
> realised we need to figure out is what happens if attr.inherit is set.
> The core doesn't reject that, and I suspect we may need to here.

That's already rejected by perf_mmap.

> > +
> > +	if (arm_spe_event_to_pmsevfr(event) & PMSEVFR_EL1_RES0)
> > +		return -EOPNOTSUPP;
> > +
> > +	if (event->hw.sample_period < spe_pmu->min_period ||
> > +	    event->hw.sample_period & PMSIRR_EL1_IVAL_MASK)
> > +		return -EOPNOTSUPP;
> 
> As mentioned in the sysreg comments, we need to check the upper 32 bits
> of the PMSIRR value are zero, so we'll need something like:
> 
> 	if (event->hw.sample_period < spe_pmu->min_period)
> 		return -EOPNOTSUPP;
> 	
> 	if (event->hw.sample_period &
> 	    ~(PMSIRR_EL1_INTERVAL_MASK << PMSIRR_EL1_INTERVAL_SHIFT))
> 		return -EOPNOTSUPP;

I wonder if we're actually better off just truncating the interval. That
way, if the interval is extended in the future, then new software won't
get an error on older cores. It feels a bit weird putting the max interval
in sysfs.

> > +	if (attr->exclude_idle)
> > +		return -EOPNOTSUPP;
> > +
> > +	/*
> > +	 * Feedback-directed frequency throttling doesn't work when we
> > +	 * have a buffer of samples. We'd need to manually count the
> > +	 * samples in the buffer when it fills up and adjust the event
> > +	 * count to reflect that. Instead, force the user to specify a
> > +	 * sample period instead.
> > +	 */
> > +	if (attr->freq)
> > +		return -EINVAL;
> > +
> > +	reg = arm_spe_event_to_pmsfcr(event);
> > +	if ((reg & BIT(PMSFCR_EL1_FE_SHIFT)) &&
> > +	    !(spe_pmu->features & SPE_PMU_FEAT_FILT_EVT))
> > +		return -EOPNOTSUPP;
> > +
> > +	if ((reg & BIT(PMSFCR_EL1_FT_SHIFT)) &&
> > +	    !(spe_pmu->features & SPE_PMU_FEAT_FILT_TYP))
> > +		return -EOPNOTSUPP;
> > +
> > +	if ((reg & BIT(PMSFCR_EL1_FL_SHIFT)) &&
> > +	    !(spe_pmu->features & SPE_PMU_FEAT_FILT_LAT))
> > +		return -EOPNOTSUPP;
> 
> Does anything prevent this event from being added to a group?

PERF_PMU_CAP_EXCLUSIVE should take care of that in the core.

> > +static void arm_spe_pmu_start(struct perf_event *event, int flags)
> > +{
> > +	u64 reg;
> > +	struct arm_spe_pmu *spe_pmu = to_spe_pmu(event->pmu);
> > +	struct hw_perf_event *hwc = &event->hw;
> > +	struct perf_output_handle *handle = this_cpu_ptr(spe_pmu->handle);
> > +
> > +	hwc->state = 0;
> > +	arm_spe_perf_aux_output_begin(handle, event);
> > +	if (hwc->state)
> > +		return;
> 
> I was expecting we'd do this last, since PMBLIIMITR.E enables profiling.
> 
> I understand that we're relying on the PMSCR_EL1 filtering value to
> prevent anything being written to tbe buffer until we've vonfigured the
> options, but I'd feel a lot happier if we consistently relied upon
> PMBLIMITR.E for that.

I actually prefer failing fast if we can, so I'd rather keep this as-is
given that it works and your objections are down to personal taste.

> > +
> > +	reg = arm_spe_event_to_pmsfcr(event);
> > +	write_sysreg_s(reg, PMSFCR_EL1);
> > +
> > +	reg = arm_spe_event_to_pmsevfr(event);
> > +	write_sysreg_s(reg, PMSEVFR_EL1);
> > +
> > +	reg = arm_spe_event_to_pmslatfr(event);
> > +	write_sysreg_s(reg, PMSLATFR_EL1);
> > +
> > +	if (flags & PERF_EF_RELOAD) {
> > +		reg = arm_spe_event_to_pmsirr(event);
> > +		write_sysreg_s(reg, PMSIRR_EL1);
> > +		isb();
> > +		reg = local64_read(&hwc->period_left);
> > +		write_sysreg_s(reg, PMSICR_EL1);
> > +	}
> > +
> > +	reg = arm_spe_event_to_pmscr(event);
> > +	isb();
> > +	write_sysreg_s(reg, PMSCR_EL1);
> > +}
> > +
> > +static void arm_spe_pmu_disable_and_drain_local(void)
> > +{
> > +	/* Disable profiling at EL0 and EL1 */
> > +	write_sysreg_s(0, PMSCR_EL1);
> > +	isb();
> > +
> > +	/* Drain any buffered data */
> > +	psb_csync();
> > +	dsb(nsh);
> > +
> > +	/* Disable the profiling buffer */
> > +	write_sysreg_s(0, PMBLIMITR_EL1);
> 
> Can't this be done when we clear PMSCR_EL1? Surely buffered data would
> be written out regardless?

I think the buffered data could be silently dropped if you did that.

> > +static void *arm_spe_pmu_setup_aux(int cpu, void **pages, int nr_pages,
> > +				   bool snapshot)
> > +{
> > +	int i;
> > +	struct page **pglist;
> > +	struct arm_spe_pmu_buf *buf;
> > +
> > +	/*
> > +	 * We require an even number of pages for snapshot mode, so that
> > +	 * we can effectively treat the buffer as consisting of two equal
> > +	 * parts and give userspace a fighting chance of getting some
> > +	 * useful data out of it.
> > +	 */
> > +	if (!nr_pages || (snapshot && (nr_pages & 1)))
> > +		return NULL;
> 
> As noted above, we may need to ensure that this is a pwoer of two.

Sorry, I don't follow. The power-of-two bug was in my IDX2OFF macro, which
I've fixed. For snapshot mode, we just need an even number of pages.

> > +	buf = kzalloc_node(sizeof(*buf), GFP_KERNEL, cpu_to_node(cpu));
> > +	if (!buf)
> > +		return NULL;
> > +
> > +	pglist = kcalloc(nr_pages, sizeof(*pglist), GFP_KERNEL);
> > +	if (!pglist)
> > +		goto out_free_buf;
> > +
> > +	for (i = 0; i < nr_pages; ++i) {
> > +		struct page *page = virt_to_page(pages[i]);
> > +
> > +		if (PagePrivate(page)) {
> > +			pr_warn("unexpected high-order page for auxbuf!");
> 
> It looks like the intel-pt driver expects high-order pages.
> 
> What prevents us from seeing those?

The fact that we don't set the PERF_PMU_CAP_AUX_NO_SG capability. The intel
driver uses physical address and scatter/gather lists, whereas ours just
takes virtual addresses for the buffer.

> Why can't we handle those?

We never get them, so we don't need to.

> How are these pages pinned? Does the core ensure that?

Yes, they're GFP_KERNEL pages underneath.

> > +	spe_pmu->pmu = (struct pmu) {
> > +		.capabilities	= PERF_PMU_CAP_EXCLUSIVE | PERF_PMU_CAP_ITRACE,
> > +		.attr_groups	= arm_spe_pmu_attr_groups,
> > +		/*
> > +		 * We hitch a ride on the software context here, so that
> > +		 * we can support per-task profiling (which is not possible
> > +		 * with the invalid context as it doesn't get sched callbacks).
> > +		 * This requires that userspace either uses a dummy event for
> > +		 * perf_event_open, since the aux buffer is not setup until
> > +		 * a subsequent mmap, or creates the profiling event in a
> > +		 * disabled state and explicitly PERF_EVENT_IOC_ENABLEs it
> > +		 * once the buffer has been created.
> > +		 */
> > +		.task_ctx_nr	= perf_sw_context,
> 
> While other tracing PMUs do this, I think this is a horrible bodge, and
> a bad idea, given it violates assumptions made in the core code.
> 
> For example, unlike true SW events, add() and start() can fail, so a
> tracing event can unexpectedly stop SW events from being scheduled.
> 
> AFAICT, we could also try to move a tracing event into a later-created
> HW PMU group, which is very worrying.
> 
> I really think we should have a separate tracing context for this class
> of PMU, or we make it so that the invalid context can receive sched
> callbacks.

Fair point, and I already have a comment to call this out. Whilst I'm not
against seeing this fixed, I think it should be a separate patch series
given that this is a common idiom amongst system/uncore PMU drivers.

> > +static void __arm_spe_pmu_dev_probe(void *info)
> > +{
> > +	int fld;
> > +	u64 reg;
> > +	struct arm_spe_pmu *spe_pmu = info;
> > +	struct device *dev = &spe_pmu->pdev->dev;
> > +
> > +	fld = cpuid_feature_extract_unsigned_field(read_cpuid(ID_AA64DFR0_EL1),
> > +						   ID_AA64DFR0_PMSVER_SHIFT);
> > +	if (!fld) {
> > +		dev_err(dev,
> > +			"unsupported ID_AA64DFR0_EL1.PMSVer [%d] on CPU %d\n",
> > +			fld, smp_processor_id());
> > +		return;
> > +	}
> 
> Given we only bail out when PMSver is zero, surely we can just say:
> 
> 	dev_err(dev, "SPE not supported on cpu %d", smp_processor_id())

I'd rather leave it like this, so we have the information when we start
supporting additional values of fld.

Will