Re: [RFC] arm64: Enforce gettimeofday vdso structure read ordering

[Mark Rutland]

Hi Brent,

Thanks for the thorough reply. Comments inline below.

On Mon, Aug 22, 2016 at 01:32:47PM -0400, bdegraaf@xxxxxxxxxxxxxx wrote:
> On 2016-08-22 07:37, Mark Rutland wrote:
> >* What problem does this patch address?
> 
> Initially, I set out to fix a control-flow problem, as I originally
> wrote this against the code prior to the refactoring of commit
> b33f491f5a9aaf171b7de0f905362eb0314af478, back when there was still a
> do_get_tspec subroutine.  That one used a dmb to order the
> data accesses prior to the isb/mrs sequence that read the virtual
> counter.  Our Senior Director, who has done extensive work with the ARM
> cpu and is intimately familiar with the instruction set, indicated that
> the dmb which was used in that code was not enough to ensure ordering
> between the loads from the structure and the read of the virtual
> counter.
> Since that code had no control-flow (e.g., some conditional governing
> the code's progress) prior to the isb, he suggested that some form of
> dsb would be required to ensure proper order of access between the
> loads from the vdso structure and the mrs read of the the virtual
> counter.

Ok. So if I've parsed the above correctly, the fear was that an ISB was
insufficient to guarantee the ordering of prior loads w.r.t. the
subsequent MRS, and a control dependency between the two was necessary,
in addition to the ISB.

> I went to the latest armv8 ARM at that time and found a concrete example
> of how the code should be structured to ensure an ordered read of the
> virtual counter.  In the most recent copy to which I have access
> (ARM DDI 0487A.j), that code is given on page D6-1871, under section
> D6.2.2.  I moved the sequence count check immediately prior to the
> isb to satisfy the particular ordering requirements in that code.

My reading of that example is that the control dependency alone was
insufficient (given speculation), and the ISB provided the necessary
ordering between the signal variable being updated and the MRS. To me,
the example does not imply that both are required in all cases, only
that a control dependency alone is insufficient.

Per the description of ISB on page B2-87 of ARM DDI 0487A.j, my
understanding (which may be flawed), is that the instructions prior to
the ISB must be completed before the subsequent instructions are
fetched+issued, and hence the MRS should be (locally) ordered w.r.t. the
loads.

> When the refactored code went in recently, however, the seqcnt_read
> prior to the isb changed to a seqcnt_check, which addressed that
> ordering requirement.

Have you seen an issue in practice prior to this? If so, we may need to
go digging further into this, and consider stable kernels.

> >* Is this a bug fix? If so, what problem can be seen currently?
> 
> The most obvious problem with the existing code is where the timezone
> data gets loaded: after sequence counts have been checked and
> rechecked, completely outside the area of the code protected by the
> sequence counter.  While I realize that timezone code does not change
> frequently, this is still a problem as the routine explicitly reads
> data that could be in the process of being updated.

I take that you specifically mean the following line in
__kernel_gettimeofday, which occurs after the usual seqcnt_acquire +
seqcnt_check sequence:

	ldp     w4, w5, [vdso_data, #VDSO_TZ_MINWEST]

I see that the writer side for the timezone data is also not protected,
since commit bdba0051ebcb3c63 ("arm64: vdso: remove broken, redundant
sequence counting for timezones"). Following comments in commits I found
x86 commit 6c260d586343f7f7 ("x86: vdso: Remove bogus locking in
update_vsyscall_tz()").

Per the x86 commit, this is not atomic in the usual syscall path, and
thus it is a cross-architecture property that timezone updates are not
atomic, and that reordering of accesses may occur around a change of
timezone. If we want to tighten up the VDSO, we'll also need to tighten
up the syscall.

[...]

> The second problem is that the timing between the reading of the vdso
> data and the virtual counter is treated as secondary in the code, as a
> few register manipulations using the structure data are performed
> prior to reading the virtual counter.  While the cpu itself is free to
> reorder these shifts and loads somewhat, depending on the
> implementation, the read of the virtual counter should be performed as
> close as possible to the time the vdso data itself is read to minimize
> variability.  As these manipulations and loads are not dependent on
> the result of the mrs read, putting them after the virtual counter
> isb/mrs sequence allows these independent register manipulations to
> issue while the mrs is still in process.

This is rather dependent on the microarchitecture. Do we have any
numbers as to the variability?

> >* Is this an optimisation? If so, how much of an improvement can be
> >  seen?
> 
> Optimization was not the main goal of this patch, yet performance did
> improve on my target, with the average time improving marginally
> (350 nsec after vs 360 nsec before the change), compared to the
> refactored code.  In fact, performance is now slightly better (around
> a miniscule 2 nsec) than the original code, before the refactor, which
> hurt performance.

Ok.

> >>+	.macro	seqdata_acquire fallback, tzonly=NO_TZ, skipvcnt=0, getdata
> >>+9999:	ldar	seqcnt, [vdso_data, #VDSO_TB_SEQ_COUNT]
> >>+8888:	tbnz	seqcnt, #0, 9999b
> >> 	ldr	w_tmp, [vdso_data, #VDSO_USE_SYSCALL]
> >>-	cbnz	w_tmp, \fail
> >>+	cbnz	w_tmp, \fallback
> >>+	\getdata
> >>+	dmb	ishld	/* No loads from vdso_data after this point */
> >
> >What ordering guarantee is the DMB attempting to provide? Given we have
> >the acquire, I assume some prior load, but I couldn't figure out what
> >specifically.
> 
> That barrier specifically ensures that loads performed by the
> "getdata" sequence do not get accessed after the subsequent ldar check
> of the sequence counter, since, as you know, ldar may allow loads that
> come before it in program order to be accessed after it in much the
> same way as stlr may allow stores that come after it to accessed
> before it.

Ok. I wonder what the relative performance of a DMB ISHLD; LDAR is
relative to a DMB ISH LD, and whether that's actually a win across
microarchitectures.

> >>+	mov	w9, seqcnt
> >>+	ldar	seqcnt, [vdso_data, #VDSO_TB_SEQ_COUNT]
> >
> >Usually, acquire operations pair with a release operation elsewhere.
> >What does this pair with?
> 
> It was for that reason that I introduced stlr's into the writer code,
> but the barrier provided by stlr was insufficient for my purposes, as
> Will pointed out.  There is no requirement or even suggestion in the
> ARM that every use of ldar needs to be paired with stlr's.

Sure. I guess I was asking which updater does this pair with, and I
having dug, I see it's just update_vsyscall().

> >>+	cmp	w9, seqcnt
> >>+	bne	8888b	/* Do not needlessly repeat ldar and its implicit
> >>barrier */
> >>+	.if (\tzonly) != NO_TZ
> >>+		cbz	x0, \tzonly
> >>+	.endif
> >>+	.if (\skipvcnt) == 0
> >>+		isb
> >>+		mrs	x_tmp, cntvct_el0
> >>+	.endif
> >> 	.endm
> >
> >All this conitional code makes the callers somehwat painful to read.
> >
> >It might be nicer to have this explicit in the calelrs that require it
> >rather than conditional in the macro.
> 
> The general use-case of the acquire sequence made this the cleanest safe
> implementation I could come up.  If this isb/mrs sequence is split out
> into each clock handler, it would serve to obscure the relationship
> between the control-flow dependency (in this case, the "bne 8888b") and
> the isb.  Keeping this acquire sequence intact helps to ensure that
> future modifications adhere to the correct sequence.  Note that if the
> caller specifies neither option, the default is to leave these items
> in place.

As above, I'm not sure that the control dependency is key. Even if so,
the logical sequence is:

	seqdata_acquire
	isb
	mrs

I can't fathom why someone would move the ISB (and/or MRS)  before the
seqcnt_acquire.

> >> 	.macro get_nsec_per_sec res
> >>@@ -64,9 +70,6 @@ x_tmp		.req	x8
> >> 	 * shift.
> >> 	 */
> >> 	.macro	get_clock_shifted_nsec res, cycle_last, mult
> >>-	/* Read the virtual counter. */
> >>-	isb
> >>-	mrs	x_tmp, cntvct_el0
> >> 	/* Calculate cycle delta and convert to ns. */
> >> 	sub	\res, x_tmp, \cycle_last
> >> 	/* We can only guarantee 56 bits of precision. */
> >>@@ -137,17 +140,12 @@ x_tmp		.req	x8
> >> ENTRY(__kernel_gettimeofday)
> >> 	.cfi_startproc
> >> 	adr	vdso_data, _vdso_data
> >>-	/* If tv is NULL, skip to the timezone code. */
> >>-	cbz	x0, 2f
> >>-
> >>-	/* Compute the time of day. */
> >>-1:	seqcnt_acquire
> >>-	syscall_check fail=4f
> >>-	ldr	x10, [vdso_data, #VDSO_CS_CYCLE_LAST]
> >>-	/* w11 = cs_mono_mult, w12 = cs_shift */
> >>-	ldp	w11, w12, [vdso_data, #VDSO_CS_MONO_MULT]
> >>-	ldp	x13, x14, [vdso_data, #VDSO_XTIME_CLK_SEC]
> >>-	seqcnt_check fail=1b
> >>+	seqdata_acquire fallback=4f tzonly=2f getdata=__stringify(\
> >>+		ldr	x10, [vdso_data, #VDSO_CS_CYCLE_LAST];\
> >>+		/* w11 = cs_mono_mult, w12 = cs_shift */;\
> >>+		ldp	w11, w12, [vdso_data, #VDSO_CS_MONO_MULT];\
> >>+		ldp	x13, x14, [vdso_data, #VDSO_XTIME_CLK_SEC];\
> >>+		ldp	w4, w5, [vdso_data, #VDSO_TZ_MINWEST])
> >
> >Why do we need the stringify? Is that just so we can pass the code as a
> >macro parameter? If so, it really doesn't look like the way to go...
> >
> >This is unfortunately painful to read.
> >
> 
> I implemented it this way to remain as similar as possible with the
> refactored code that was recently merged, while at the same time
> ensuring that, as I explained above, the reads of the vdso_data
> performed by each clock type are completely contained within a set of
> proper sequence count checks.  That they were not contained led to
> problems such as the improper handling of the timezone data before,
> and it ensures that the isb follows the sequence check closely.  This
> use is not entirely dissimilar to other code which uses stringify
> currently present in the arm64 kernel code which passes code as a
> parameter. See, for example, arch/arm64/lib/copy_*_user.S.
> All this said, however, I was never thrilled about going the stringify
> route, but it was the most readable of any other variants I could
> come up with (and far better than adding the extra ".if's" in the
> macro).
> Do you happen to have a better suggestion?

I think that:

	ACQUIRE, blah, blah, blah
	< long >	
	< code >
	< sequence >
	CONDITION_FAIL, blah, blah, blah

Is clearer than dropping the code sequence into a macro parameter, even
if there's come implicit dependency between the ACQUIRE and
CONDITION_FAIL macro.

Thanks,
Mark.