On Thu, Aug 10, 2023 at 04:34:39PM +0800, Michael Shavit wrote:

On Wed, Aug 9, 2023 at 9:50 PM Will Deacon [off-list ref] wrote:

quoted

On Wed, Aug 09, 2023 at 01:12:04AM +0800, Michael Shavit wrote:

quoted

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.

quoted

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.

quoted

quoted

              /*
-              * 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

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