Re: [PATCH v3 1/2] iommu/arm-smmu-v3: fix unexpected CMD_SYNC timeout

From: Robin Murphy
Date: Wed Aug 15 2018 - 08:26:52 EST

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
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@xxxxxxxxxx>
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...


+ 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_MSIDATA, ent->sync.msidata);
cmd[1] |= ent->sync.msiaddr & CMDQ_SYNC_1_MSIADDR_MASK;
@@ -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;