Re: [PATCH] cxl/hdm: Fix hdm decoder init by adding COMMIT field check

[Jonathan Cameron]

On Fri, 3 Mar 2023 17:21:13 +0000
Fan Ni <fan.ni@xxxxxxxxxxx> wrote:

> On Fri, Mar 03, 2023 at 02:36:05PM +0000, Jonathan Cameron wrote:
> 
> > On Thu, 2 Mar 2023 08:36:59 -0700
> > Dave Jiang <dave.jiang@xxxxxxxxx> wrote:
> >   
> > > On 3/1/23 11:23 PM, Fan Ni wrote:  
> > > > On Wed, Mar 01, 2023 at 11:54:08AM -0700, Dave Jiang wrote:    
> > > >>    
> > > > Hi Dave,
> > > > Thanks for looking into this.    
> > > >>
> > > >> On 2/28/23 3:40 PM, Fan Ni wrote:    
> > > >>> Add COMMIT field check aside with existing COMMITTED field check during
> > > >>> hdm decoder initialization to avoid a system crash during module removal
> > > >>> after destroying a region which leaves the COMMIT field being reset while
> > > >>> the COMMITTED field still being set.    
> > > >>
> > > >> Hi Fan. Are you seeing this issue on qemu emulation or hardware? The    
> > > > I run into the issue with qemu emulation.    
> > > >> situation does not make sense to me. If we clear the COMMIT bit, then the
> > > >> COMMITTED bit should be cleared by the hardware shortly after right?    
> > > > 
> > > >  From the spec, I cannot find any statement saying clearing the COMMIT bit
> > > > will automatically clear the COMMITTED. If I have not missed the statement in
> > > > the spec, I assume we should not make the assumption that it will be
> > > > cleared automatically for real hardware. But you may be right, leaving the
> > > > COMMITTED bit set can potentially cause some issue? Need to check more.    
> > > 
> > > I have not been able to find direct verbiage that indicates this either. 
> > > However, logically it would make sense. Otherwise, the COMMITTED field 
> > > never clears and prevents reprogramming of the HDM decoders. The current 
> > > QEMU implementation is creating a situation where the HDM decoder is 
> > > always active after COMMIT bit is set the first time, regardless whether 
> > > COMMIT field has been cleared later on during a teardown. It does sound 
> > > like a bug with QEMU emulation currently.  
> > 
> > I agree that one sane interpretation is that unsetting commit should result in
> > the decoder being deactivated and hence the commit bit dropping.  However
> > I'm not sure that's the only sane interpretation.
> > 
> > There is no verbage that I'm aware of that says the committed bit being
> > set means that the current register values are in use.  It simply says that
> > when the commit bit was set, the HDM decoder was successfully committed
> > (using registers as set at that time).  There is a specific statement about
> > not changing the registers whilst checks are in progress, but those checks
> > are only required if lock on commit is set, so it doesn't cover this case.
> > 
> > Wonderfully there isn't actually anything says what a commit transition to 0
> > means.  Does that result in the decoder become uncommitted, or does that only
> > happen when the next 0 to 1 transition happens?
> > 
> > The only stuff we have is what happens when lock on commit = 1, which isn't
> > the case here.
> > 
> > So is there another valid implementation? I think yes.
> > In some implementations, there will be a complex state machine that is
> > triggered when commit is set.  That will then write some entirely invisible
> > internal state for decode logic based on the contents of the registers.
> > As such, once it's set committed, it typically won't look at the registers
> > again until another commit 0->1 transition happens.  At that point the
> > committed bit drops and raised again once the commit state machine finishes
> > (given QEMU doesn't emulate that delay the upshot is if you set commit then
> > check committed it will be set ;)
> > 
> > In that implementation the commit 1->0 transition is an irrelevance and
> > it won't change the committed bit state.
> > 
> > So whilst the QEMU code is doing the less obvious implementation, I think
> > the spec still allows it.  I don't mind QEMU changing to the more obvious
> > one though if someone wants to send a patch.
> > 
> > Jonathan
> >   
> 
> In current qemu emulation, when COMMITTED bit is set when the decoder is
> committed and at the same time the COMMIT field will be cleared. Does
> the following fix make sense?
> 1. At qemu side, when the commit completes, just set the COMMITTED bit,
> but leave the COMMIT bit as set, also check LOCK ON COMMIT bit,
> if it is set, clear it, which will allow further reset of COMMIT bit.

QEMU definitely can't do anything to the Commit bit, other than prevent it being
cleared if lock on commit is set.
Right now the QEMU emulation doesn't handle LOCK ON COMMIT at all.
It would be sensible to add this support, but we don't have an
open software stack that ever sets that yet so any testing is likely to be
one time only via some hacks.

> 2. for the kernel side, if it needs to reprogram the decoder, it needs to
> check the COMMITTED bit, if it is set, then OS need to reset COMMIT bit
> first, which will also clear COMMITTED bit automatically at qemu side.

Could do it that way, or simplify it by always clearing commit before setting
it to make sure the transition happens.

Looks like commit is cleared in cxl_decoder_reset() already so this may
already happen - I haven't checked the flow.

> 3. when the OS needs to reset the decoder, it does similar thing as 2 to
> reset COMMIT bit and qemu will clear COMMITTED bit.

No the point of the above argument is that the spec doesn't say anything
about when committed is cleared. 2 options.
1) Hardware clears it when commit 1->0.
2) Hardware clears it when commit 0->1

Given that spec only talks about after a commit 0->1 transition whilst commit
remains 1, the state after a commit 0->1 transition is implementation defined.

I think that closing that corner case requires a clarification to the spec.

Which leaves us with a sticky question of what to do...

> 
> Fan
> 
> > >
> > > DJ
> > >   
> > > > 
> > > > Fan
> > > >     
> > > >> Otherwise, how would one reprogram the decoder if the decoder is indicating
> > > >> to be active?
> > > >>
> > > >> DJ
> > > >>    
> > > >>>
> > > >>> In current kernel implementation, when destroying a region (cxl
> > > >>> destroy-region),the decoders associated to the region will be reset
> > > >>> as that in cxl_decoder_reset, where the COMMIT field will be reset.
> > > >>> However, resetting COMMIT field will not automatically reset the
> > > >>> COMMITTED field, causing a situation where COMMIT is reset (0) while
> > > >>> COMMITTED is set (1) after the region is destroyed. Later, when
> > > >>> init_hdm_decoder is called (during modprobe), current code only check
> > > >>> the COMMITTED to decide whether the decoder is enabled or not. Since
> > > >>> the COMMITTED will be 1 and the code treats the decoder as enabled,
> > > >>> which will cause unexpected behaviour.
> > > >>>
> > > >>> Before the fix, a system crash was observed when performing following
> > > >>> steps:
> > > >>> 1. modprobe -a cxl_acpi cxl_core cxl_pci cxl_port cxl_mem
> > > >>> 2. cxl create-region -m -d decoder0.0 -w 1 mem0 -s 256M
> > > >>> 3. cxl destroy-region region0 -f
> > > >>> 4. rmmod cxl_acpi cxl_pci cxl_port cxl_mem cxl_pmem cxl_core
> > > >>> 5. modprobe -a cxl_acpi cxl_core cxl_pci cxl_port cxl_mem (showing
> > > >>> "no CXL window for range 0x0:0xffffffffffffffff" error message)
> > > >>> 6. rmmod cxl_acpi cxl_pci cxl_port cxl_mem cxl_pmem cxl_core (kernel
> > > >>> crash at cxl_dpa_release due to dpa_res has been freed when destroying
> > > >>> the region).
> > > >>>
> > > >>> The patch fixed the above issue, and is tested based on follow patch series:
> > > >>>
> > > >>> [PATCH 00/18] CXL RAM and the 'Soft Reserved' => 'System RAM' default
> > > >>> Message-ID: 167601992097.1924368.18291887895351917895.stgit@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > >>>
> > > >>> Signed-off-by: Fan Ni <fan.ni@xxxxxxxxxxx>
> > > >>> ---
> > > >>>    drivers/cxl/core/hdm.c | 8 +++++---
> > > >>>    1 file changed, 5 insertions(+), 3 deletions(-)
> > > >>>
> > > >>> diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
> > > >>> index 80eccae6ba9e..6cf854c949f0 100644
> > > >>> --- a/drivers/cxl/core/hdm.c
> > > >>> +++ b/drivers/cxl/core/hdm.c
> > > >>> @@ -695,6 +695,7 @@ static int init_hdm_decoder(struct cxl_port *port, struct cxl_decoder *cxld,
> > > >>>    	struct cxl_endpoint_decoder *cxled = NULL;
> > > >>>    	u64 size, base, skip, dpa_size;
> > > >>>    	bool committed;
> > > >>> +	bool should_commit;
> > > >>>    	u32 remainder;
> > > >>>    	int i, rc;
> > > >>>    	u32 ctrl;
> > > >>> @@ -710,10 +711,11 @@ static int init_hdm_decoder(struct cxl_port *port, struct cxl_decoder *cxld,
> > > >>>    	base = ioread64_hi_lo(hdm + CXL_HDM_DECODER0_BASE_LOW_OFFSET(which));
> > > >>>    	size = ioread64_hi_lo(hdm + CXL_HDM_DECODER0_SIZE_LOW_OFFSET(which));
> > > >>>    	committed = !!(ctrl & CXL_HDM_DECODER0_CTRL_COMMITTED);
> > > >>> +	should_commit = !!(ctrl & CXL_HDM_DECODER0_CTRL_COMMIT);
> > > >>>    	cxld->commit = cxl_decoder_commit;
> > > >>>    	cxld->reset = cxl_decoder_reset;
> > > >>> -	if (!committed)
> > > >>> +	if (!should_commit || !committed)
> > > >>>    		size = 0;
> > > >>>    	if (base == U64_MAX || size == U64_MAX) {
> > > >>>    		dev_warn(&port->dev, "decoder%d.%d: Invalid resource range\n",
> > > >>> @@ -727,7 +729,7 @@ static int init_hdm_decoder(struct cxl_port *port, struct cxl_decoder *cxld,
> > > >>>    	};
> > > >>>    	/* decoders are enabled if committed */
> > > >>> -	if (committed) {
> > > >>> +	if (should_commit && committed) {
> > > >>>    		cxld->flags |= CXL_DECODER_F_ENABLE;
> > > >>>    		if (ctrl & CXL_HDM_DECODER0_CTRL_LOCK)
> > > >>>    			cxld->flags |= CXL_DECODER_F_LOCK;
> > > >>> @@ -772,7 +774,7 @@ static int init_hdm_decoder(struct cxl_port *port, struct cxl_decoder *cxld,
> > > >>>    		return 0;
> > > >>>    	}
> > > >>> -	if (!committed)
> > > >>> +	if (!should_commit || !committed)
> > > >>>    		return 0;
> > > >>>    	dpa_size = div_u64_rem(size, cxld->interleave_ways, &remainder);    
> >