Re: [PATCH] arm64: swiotlb: Keep the default size for protected guests

On Mon, Aug 10, 2026 at 10:08:18AM -0300, Jason Gunthorpe wrote:

On Mon, Aug 10, 2026 at 12:46:51PM +0100, Will Deacon wrote:

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On Mon, Aug 10, 2026 at 01:37:11PM +0200, Marek Szyprowski wrote:

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On 10.08.2026 12:20, Will Deacon wrote:

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On Mon, Aug 10, 2026 at 02:59:42PM +0530, Aneesh Kumar K.V wrote:

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Marek Szyprowski [off-list ref] writes:

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On 07.08.2026 20:20, Jason Gunthorpe wrote:

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On Fri, Aug 07, 2026 at 06:13:49PM +0000, Mostafa Saleh wrote:

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But the whole thing is best effort anyway, the kernel picks
IO_TLB_DEFAULT_SIZE which does not depend on the system topology or
how many devices or how much DMA they do.
SWIOTLB memory is wasted if unused so we should be careful around
that as it would be the other way around and users would have to
decrease it manually.

Yeah, it is why the arch code shouldn't really be sizing it directly,
it should be done in common code and, yes, we are probably going to
have to do something alot smarter to have the common code better
auto-tune this for the CC case..

What about the $subject patch? I assume that it is still needed to

restore the behavior that was altered by the "[PATCH v8 00/23]

dma-mapping: Track shared DMA state through direct, pool and swiotlb

paths?" patchset?

I would request that we pick this patch to fix the regression described
in https://lore.kernel.org/all/yq5azeyxyfol.fsf@kernel.org/ (local).

I really don't think we need it. CCA hardware isn't exactly widespread
and the KVM host side patches don't appear close to being merged.

Does this mean that the branch for-next/coco [1] won't go to v7.3-rc1?

Let's not make progress on guest support contingent on KVM CCA host
side patches please. I expect the CSPs will have VM instance types
available based on CCA within quarters, and Linux as a Guest should
work in those environments regardless of what KVM is doing. I don't
really expect full KVM support for years, frankly, the patchset is
massive. Even Intel and AMD don't have full KVM support yet.

To be clear: the only part I'm pushing back on is the realm-specific hack
to size the SWIOTLB area in arch/arm64/. Even with that hack applied, I
don't believe a one-size-fits-all value is going to work for everybody,
so it was really great to see that special-case removed in:

https://lore.kernel.org/all/20260717180442.110954-18-aneesh.kumar@kernel.org/ (local)

but now, because it regresses some test configuration, the proposal was
to penalise protected VMs too (bearing in mind that the patch above
hasn't yet landed upstream):

https://lore.kernel.org/all/20260807092612.2202005-1-aneesh.kumar@kernel.org/ (local)

with the alternative being to reintroduce the original hack that we
just removed!

https://lore.kernel.org/all/yq5acxvu2ict.fsf@kernel.org/ (local)

I'm saying: merge the patches as they are, without reintroducing this
horrible bodge in the architecture code to drive a heuristic that most
people seem to agree should be handled more robustly elsewhere.

People already have CCA capable HW, are already testing this stuff and
the closer upstream can get to being workable as a guest without a
mountain of OOT patches the better.

This specific part is about a single line of code, to control something
which is already configurable in Kconfig and on the cmdline. It's not
a mountain of out-of-tree patches.

I agree the thing is not ideal, but it was merged to ARM like this a
long time ago, this patch is just fixing a small oopsie (was it a
merge conflict?) to put it back. I don't the objection.

My main objection is that I have very little confidence in people trying
to fix this properly if we take the realm-specific bodge in the arch
code. With all the CCA patches floating about already, why would they?

Will