linux-kernel - Re: [PATCH v5 8/9] iommu/arm-smmu-v3: Skip cd sync if CD table isn't active

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Message-ID: <20230809135011.GC4226@willie-the-truck>
Date:   Wed, 9 Aug 2023 14:50:13 +0100
From:   Will Deacon <will@...nel.org>
To:     Michael Shavit <mshavit@...gle.com>
Cc:     iommu@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, robin.murphy@....com,
        nicolinc@...dia.com, jgg@...dia.com, jean-philippe@...aro.org
Subject: Re: [PATCH v5 8/9] iommu/arm-smmu-v3: Skip cd sync if CD table isn't
 active

On Wed, Aug 09, 2023 at 01:12:04AM +0800, Michael Shavit wrote:
> This commit explicitly keeps track of whether a CD table is installed in
> an STE so that arm_smmu_sync_cd can skip the sync when unnecessary. This
> was previously achieved through the domain->devices list, but we are
> moving to a model where arm_smmu_sync_cd directly operates on a master
> and the master's CD table instead of a domain.

Why is this path worth optimising?

> 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 f5ad386cc8760..488d12dd2d4aa 100644
> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> @@ -985,6 +985,9 @@ static void arm_smmu_sync_cd(struct arm_smmu_master *master,
>  		},
>  	};
>  
> +	if (!master->cd_table.installed)
> +		return;

Doesn't this interact badly with the sync in arm_smmu_detach_dev(), which I
think happens after zapping the STE?

>  	cmds.num = 0;
>  	for (i = 0; i < master->num_streams; i++) {
>  		cmd.cfgi.sid = master->streams[i].id;
> @@ -1091,7 +1094,7 @@ int arm_smmu_write_ctx_desc(struct arm_smmu_master *master, int ssid,
>  		cdptr[3] = cpu_to_le64(cd->mair);
>  
>  		/*
> -		 * STE is live, and the SMMU might read dwords of this CD in any
> +		 * STE may be live, and the SMMU might read dwords of this CD in any
>  		 * order. Ensure that it observes valid values before reading
>  		 * V=1.
>  		 */

Why does this patch need to update this comment?

Will