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

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Date:   Thu, 10 Aug 2023 16:34:39 +0800
From:   Michael Shavit <mshavit@...gle.com>
To:     Will Deacon <will@...nel.org>
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 9, 2023 at 9:50 PM Will Deacon <will@...nel.org> wrote:
>
> 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?

I have no idea what the practical impact of this optimization is, but
the motivation here was to make the overall series as close to a nop
as possible. This optimization existed before but is "broken" by the
previous patch. This patch restores it.

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

The arm_smmu_write_ctx_desc call added in arm_smmu_detach_dev() was
inserted after zapping the STE precisely so that we could skip the
sync. Is there a concern that a stale CD could be used when the
CDtable is re-inserted into the STE?

> >               /*
> > -              * 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?

This is a drive-by to make this comment more accurate. Note how
(before this patch series) arm_smmu_domain_finalise_s1 explicitly
mentions that it calls arm_smmu_write_ctx_desc while the STE isn't
installed yet. Yet this comment asserts the STE *is* live.