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iommu/amd: move wait_on_sem() out of spinlock

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With iommu.strict=1, the existing completion wait path can cause soft
lockups under stressed environment, as wait_on_sem() busy-waits under the
spinlock with interrupts disabled.

Move the completion wait in iommu_completion_wait() out of the spinlock.
wait_on_sem() only polls the hardware-updated cmd_sem and does not require
iommu->lock, so holding the lock during the busy wait unnecessarily
increases contention and extends the time with interrupts disabled.

Signed-off-by: Ankit Soni <Ankit.Soni@amd.com>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
This commit is contained in:
Ankit Soni 2025-12-01 14:39:40 +00:00 committed by Joerg Roedel
parent 9ace4753a5
commit d2a0cac105
1 changed files with 18 additions and 9 deletions
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27
drivers/iommu/amd/iommu.c
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@ -1185,7 +1185,12 @@ static int wait_on_sem(struct amd_iommu *iommu, u64 data)
{
int i = 0;
while (*iommu->cmd_sem != data && i < LOOP_TIMEOUT) {
/*
* cmd_sem holds a monotonically non-decreasing completion sequence
* number.
*/
while ((__s64)(READ_ONCE(*iommu->cmd_sem) - data) < 0 &&
i < LOOP_TIMEOUT) {
udelay(1);
i += 1;
}
@ -1437,14 +1442,13 @@ static int iommu_completion_wait(struct amd_iommu *iommu)
raw_spin_lock_irqsave(&iommu->lock, flags);
ret = __iommu_queue_command_sync(iommu, &cmd, false);
raw_spin_unlock_irqrestore(&iommu->lock, flags);
if (ret)
goto out_unlock;
return ret;
ret = wait_on_sem(iommu, data);
out_unlock:
raw_spin_unlock_irqrestore(&iommu->lock, flags);
return ret;
}
@ -3121,13 +3125,18 @@ static void iommu_flush_irt_and_complete(struct amd_iommu *iommu, u16 devid)
raw_spin_lock_irqsave(&iommu->lock, flags);
ret = __iommu_queue_command_sync(iommu, &cmd, true);
if (ret)
goto out;
goto out_err;
ret = __iommu_queue_command_sync(iommu, &cmd2, false);
if (ret)
goto out;
wait_on_sem(iommu, data);
out:
goto out_err;
raw_spin_unlock_irqrestore(&iommu->lock, flags);
wait_on_sem(iommu, data);
return;
out_err:
raw_spin_unlock_irqrestore(&iommu->lock, flags);
return;
}
static inline u8 iommu_get_int_tablen(struct iommu_dev_data *dev_data)