linux-kernel - Re: [PATCH rfcv2 6/8] iommu/arm-smmu-v3: Populate smmu

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Message-ID: <20251001162516.GE3024065@nvidia.com>
Date: Wed, 1 Oct 2025 13:25:16 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: will@...nel.org, robin.murphy@....com, joro@...tes.org,
	jean-philippe@...aro.org, miko.lenczewski@....com,
	balbirs@...dia.com, peterz@...radead.org, smostafa@...gle.com,
	kevin.tian@...el.com, praan@...gle.com,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH rfcv2 6/8] iommu/arm-smmu-v3: Populate smmu_domain->invs
 when attaching masters

On Tue, Sep 30, 2025 at 01:19:29PM -0700, Nicolin Chen wrote:
> I see a cleaner way of handling this is to update invs->num_invs
> inside arm_smmu_invs_unref():
> ----------------------------------------------------------------
> @@ -1209,6 +1216,13 @@ size_t arm_smmu_invs_unref(struct arm_smmu_invs *invs,
>                         j++;
>                 }
>         }
> +
> +       /* The lock is required to fence concurrent ATS operations. */
> +       write_lock_irqsave(&invs->rwlock, flags);
> +       /* Trim the size by removing tailing trash entries */
> +       WRITE_ONCE(invs->num_invs, num_invs);
> +       write_unlock_irqrestore(&invs->rwlock, flags);

That seems Ok

It means the arm_smmu_invs_unref() becomes the fence that guarentees
the ATS is stopped for anything marked as trash.

Then the next steps can just be normal RCU and don't need rwlocking.

Jason