Subject: [PATCH 1/1] iommu/arm-smmu-v3: fix unexpected CMD_SYNC timeout

From: [email-masked] (Will Deacon)
Date: Thu, 9 Aug 2018 09:49:04 +0100

On Thu, Aug 09, 2018 at 09:30:51AM +0800, Leizhen (ThunderTown) wrote:
> On 2018/8/8 18:12, Will Deacon wrote:
> > On Mon, Aug 06, 2018 at 08:31:29PM +0800, Zhen Lei wrote:
> >> The condition "(int)(VAL - sync_idx) >= 0" to break loop in function
> >> __arm_smmu_sync_poll_msi requires that sync_idx must be increased
> >> monotonously according to the sequence of the CMDs in the cmdq.
> >>
> >> But ".msidata = atomic_inc_return_relaxed(&smmu->sync_nr)" is not protected
> >> by spinlock, so the following scenarios may appear:
> >> cpu0			cpu1
> >> msidata=0
> >> 			msidata=1
> >> 			insert cmd1
> >> insert cmd0
> >> 			smmu execute cmd1
> >> smmu execute cmd0
> >> 			poll timeout, because msidata=1 is overridden by
> >> 			cmd0, that means VAL=0, sync_idx=1.
> > 
> > Oh yuck, you're right! We probably want a CC stable on this. Did you see
> > this go wrong in practice?
> Just misreported and make the caller wait for a long time until TIMEOUT. It's
> rare to happen, because any other CMD_SYNC during the waiting period will break
> it.

Thanks. Please mention that in the commit message, because I think it's
useful to know.

> >> Signed-off-by: Zhen Lei <[email-masked]>
> >> ---
> >>  drivers/iommu/arm-smmu-v3.c | 7 +++----
> >>  1 file changed, 3 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> >> index 1d64710..4810f61 100644
> >> --- a/drivers/iommu/arm-smmu-v3.c
> >> +++ b/drivers/iommu/arm-smmu-v3.c
> >> @@ -566,7 +566,7 @@ struct arm_smmu_device {
> >>
> >>  	int				gerr_irq;
> >>  	int				combined_irq;
> >> -	atomic_t			sync_nr;
> >> +	u32				sync_nr;
> >>
> >>  	unsigned long			ias; /* IPA */
> >>  	unsigned long			oas; /* PA */
> >> @@ -836,7 +836,6 @@ static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
> >>  			cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_CS, CMDQ_SYNC_0_CS_SEV);
> >>  		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSH, ARM_SMMU_SH_ISH);
> >>  		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIATTR, ARM_SMMU_MEMATTR_OIWB);
> >> -		cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, ent->sync.msidata);
> >>  		cmd[1] |= ent->sync.msiaddr & CMDQ_SYNC_1_MSIADDR_MASK;
> >>  		break;
> >>  	default:
> >> @@ -947,7 +946,6 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
> >>  	struct arm_smmu_cmdq_ent ent = {
> >>  		.opcode = CMDQ_OP_CMD_SYNC,
> >>  		.sync	= {
> >> -			.msidata = atomic_inc_return_relaxed(&smmu->sync_nr),
> >>  			.msiaddr = virt_to_phys(&smmu->sync_count),
> >>  		},
> >>  	};
> >> @@ -955,6 +953,8 @@ static int __arm_smmu_cmdq_issue_sync_msi(struct arm_smmu_device *smmu)
> >>  	arm_smmu_cmdq_build_cmd(cmd, &ent);
> >>
> >>  	spin_lock_irqsave(&smmu->cmdq.lock, flags);
> >> +	ent.sync.msidata = ++smmu->sync_nr;
> >> +	cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, ent.sync.msidata);
> > 
> > I really don't like splitting this out from building the rest of the
> > command. Can you just move the call to arm_smmu_cmdq_build_cmd into the
> > critical section, please?
> OK. I have considered that before, just worry it will increase the
> compition of spinlock.

If you can provide numbers showing that it's a problem, then we could add
a helper function e.g. arm_smmu_cmdq_sync_set_msidata(arm_smmu_cmdq_ent *cmd)

> In addition, I will append a optimization patch: the adjacent CMD_SYNCs,
> we only need one.

Ok, but please keep them separate, since I don't want to fix up fixes and
optimisations.

Thanks,

Will