linux-kernel - Re: [PATCH rc] iommu/arm-smmu-v3: Fix smmu_domain->nr_ats

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Message-ID: <aIx3Y3qIstTceTC0@google.com>
Date: Fri, 1 Aug 2025 08:14:27 +0000
From: Pranjal Shrivastava <praan@...gle.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: jgg@...dia.com, will@...nel.org, joro@...tes.org, robin.murphy@....com,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH rc] iommu/arm-smmu-v3: Fix smmu_domain->nr_ats_masters
 decrement

On Thu, Jul 31, 2025 at 08:01:27PM -0700, Nicolin Chen wrote:
> The arm_smmu_attach_commit() updates master->ats_enabled before calling
> arm_smmu_remove_master_domain() that is supposed to clean up everything
> in the old domain, including the old domain's nr_ats_masters. So, it is
> supposed to use the old ats_enabled state of the device, not an updated
> state.
> 
> This isn't a problem if switching between two domains where:
>  - old ats_enabled = false; new ats_enabled = false
>  - old ats_enabled = true;  new ats_enabled = true
> but can fail cases where:
>  - old ats_enabled = false; new ats_enabled = true
>    (old domain should keep the counter but incorrectly decreased it)
>  - old ats_enabled = true;  new ats_enabled = false
>    (old domain needed to decrease the counter but incorrectly missed it)
> 
> Update master->ats_enabled after arm_smmu_remove_master_domain() to fix
> this.
> 
> Fixes: 7497f4211f4f ("iommu/arm-smmu-v3: Make changing domains be hitless for ATS")
> Cc: stable@...r.kernel.org
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>

arm_smmu_remove_master_domain() needs to see the old ats_enabled state
for the cleanup.

Reviewed-by: Pranjal Shrivastava <praan@...gle.com>

Thanks,
Praan
> ---
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> index 312bc31d7e8eb..3c81139faecae 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -2997,9 +2997,9 @@ void arm_smmu_attach_commit(struct arm_smmu_attach_state *state)
>  		/* ATS is being switched off, invalidate the entire ATC */
>  		arm_smmu_atc_inv_master(master, IOMMU_NO_PASID);
>  	}
> -	master->ats_enabled = state->ats_enabled;
>  
>  	arm_smmu_remove_master_domain(master, state->old_domain, state->ssid);
> +	master->ats_enabled = state->ats_enabled;
>  }
>  
>  static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
> -- 
> 2.43.0
> 
>