linux-kernel - Re: [PATCH] iommu/amd: serialize sequence allocation under concurrent TLB invalidations

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Message-ID: <20260120092427.00001794@linux.microsoft.com>
Date: Tue, 20 Jan 2026 09:24:27 -0800
From: Jacob Pan <jacob.pan@...ux.microsoft.com>
To: Ankit Soni <Ankit.Soni@....com>
Cc: <iommu@...ts.linux.dev>, <vasant.hegde@....com>,
 <suravee.suthikulpanit@....com>, <joro@...tes.org>, <will@...nel.org>,
 <robin.murphy@....com>, <linux-kernel@...r.kernel.org>,
 <Srikanth.Aithal@....com>
Subject: Re: [PATCH] iommu/amd: serialize sequence allocation under
 concurrent TLB invalidations

Hi Ankit,

On Tue, 20 Jan 2026 09:05:07 +0000
Ankit Soni <Ankit.Soni@....com> wrote:

> With concurrent TLB invalidations, completion wait randomly gets
> timed out because cmd_sem_val was incremented outside the IOMMU
> spinlock, allowing CMD_COMPL_WAIT commands to be queued out of
> sequence and breaking the ordering assumption in wait_on_sem().
> Move the cmd_sem_val increment under iommu->lock so completion
> sequence allocation is serialized with command queuing.
> And remove the unnecessary return.
> 
> Fixes: d2a0cac10597 ("iommu/amd: move wait_on_sem() out of spinlock")
> 
> Tested-by: Srikanth Aithal <sraithal@....com>
> Reported-by: Srikanth Aithal <sraithal@....com>
> Signed-off-by: Ankit Soni <Ankit.Soni@....com>
> ---
>  drivers/iommu/amd/iommu.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
> index d7f457338de7..593fb879b7b0 100644
> --- a/drivers/iommu/amd/iommu.c
> +++ b/drivers/iommu/amd/iommu.c
> @@ -1422,6 +1422,12 @@ static int iommu_queue_command(struct
> amd_iommu *iommu, struct iommu_cmd *cmd) return
> iommu_queue_command_sync(iommu, cmd, true); }
>  
> +static u64 get_cmdsem_val(struct amd_iommu *iommu)
> +{
> +	lockdep_assert_held(&iommu->lock);
> +	return atomic64_inc_return(&iommu->cmd_sem_val);
Do we still need this to be atomic now that it’s protected by a
spinlock?

> +}
> +
>  /*
>   * This function queues a completion wait command into the command
>   * buffer of an IOMMU
> @@ -1436,11 +1442,11 @@ static int iommu_completion_wait(struct
> amd_iommu *iommu) if (!iommu->need_sync)
>  		return 0;
>  
> -	data = atomic64_inc_return(&iommu->cmd_sem_val);
> -	build_completion_wait(&cmd, iommu, data);
> -
>  	raw_spin_lock_irqsave(&iommu->lock, flags);
>  
> +	data = get_cmdsem_val(iommu);
> +	build_completion_wait(&cmd, iommu, data);
> +
>  	ret = __iommu_queue_command_sync(iommu, &cmd, false);
>  	raw_spin_unlock_irqrestore(&iommu->lock, flags);
>  
> @@ -3119,10 +3125,11 @@ static void
> iommu_flush_irt_and_complete(struct amd_iommu *iommu, u16 devid)
> return; 
>  	build_inv_irt(&cmd, devid);
> -	data = atomic64_inc_return(&iommu->cmd_sem_val);
> -	build_completion_wait(&cmd2, iommu, data);
>  
>  	raw_spin_lock_irqsave(&iommu->lock, flags);
> +	data = get_cmdsem_val(iommu);
> +	build_completion_wait(&cmd2, iommu, data);
> +
>  	ret = __iommu_queue_command_sync(iommu, &cmd, true);
>  	if (ret)
>  		goto out_err;
> @@ -3136,7 +3143,6 @@ static void iommu_flush_irt_and_complete(struct
> amd_iommu *iommu, u16 devid) 
>  out_err:
>  	raw_spin_unlock_irqrestore(&iommu->lock, flags);
> -	return;
>  }
>  
>  static inline u8 iommu_get_int_tablen(struct iommu_dev_data
> *dev_data)