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SubjectRe: [PATCH v3 1/2] iommu/arm-smmu-v3: fix unexpected CMD_SYNC timeout
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On 15/08/18 11:23, 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.
>
> This is not a functional problem, just make the caller wait for a long
> time until TIMEOUT. It's rare to happen, because any other CMD_SYNCs
> during the waiting period will break it.
>
> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> ---
> drivers/iommu/arm-smmu-v3.c | 12 ++++++++----
> 1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> index 1d64710..3f5c236 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 */
> @@ -775,6 +775,11 @@ static int queue_remove_raw(struct arm_smmu_queue *q, u64 *ent)
> return 0;
> }
>
> +static inline void arm_smmu_cmdq_sync_set_msidata(u64 *cmd, u32 msidata)

If we *are* going to go down this route then I think it would make sense
to move the msiaddr and CMDQ_SYNC_0_CS_MSI logic here as well; i.e.
arm_smmu_cmdq_build_cmd() always generates a "normal" SEV-based sync
command, then calling this guy would convert it to an MSI-based one.
As-is, having bits of mutually-dependent data handled across two
separate places just seems too messy and error-prone.

That said, I still don't think that just building the whole command
under the lock is really all that bad - even when it doesn't get
optimised into one of the assignments that memset you call out is only a
single "stp xzr, xzr, ...", and a couple of extra branches doesn't seem
a huge deal compared to the DSB and MMIO accesses (and potentially
polling) that we're about to do anyway. I've tried hacking things up
enough to convince GCC to inline a specialisation of the relevant switch
case when ent->opcode is known, and that reduces the "overhead" down to
just a handful of ALU instructions. I still need to try cleaning said
hack up and double-check that it doesn't have any adverse impact on all
the other SMMUv3 stuff in development, but watch this space...

Robin.

> +{
> + cmd[0] |= FIELD_PREP(CMDQ_SYNC_0_MSIDATA, msidata);
> +}
> +
> /* High-level queue accessors */
> static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
> {
> @@ -836,7 +841,6 @@ 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 +951,6 @@ 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 +958,8 @@ 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;
> + arm_smmu_cmdq_sync_set_msidata(cmd, ent.sync.msidata);
> arm_smmu_cmdq_insert_cmd(smmu, cmd);
> spin_unlock_irqrestore(&smmu->cmdq.lock, flags);
>
> @@ -2179,7 +2184,6 @@ static int arm_smmu_init_structures(struct arm_smmu_device *smmu)
> {
> int ret;
>
> - atomic_set(&smmu->sync_nr, 0);
> ret = arm_smmu_init_queues(smmu);
> if (ret)
> return ret;
> --
> 1.8.3
>
>

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