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 17:27:45 +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 Thu, Aug 10, 2023 at 04:34:39PM +0800, Michael Shavit wrote:
> 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.

I'm not sure it's necessary, tbh. It's not like we're calling
arm_smmu_sync_cd() all over the place -- it's used when we're actually
working with the CD.

> > 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?

Ah, sorry, I went and looked at the architecture and it says for
CMD_CFGI_STE:

  | This command invalidates all Context descriptors (including L1CD)
  | that were cached using the given StreamID.

so as long as we make the CD unreachable in the STE before the STE
invalidation (which I think we do by setting the Config field to bypass or
abort), then I agree that we don't need the subsequent CD invalidation.

> > >               /*
> > > -              * 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.

Can you do it as its own patch then, please?

Will