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:
> > On Mon, Aug 10, 2026 at 01:37:11PM +0200, Marek Szyprowski wrote:
> > > On 10.08.2026 12:20, Will Deacon wrote:
> > > > On Mon, Aug 10, 2026 at 02:59:42PM +0530, Aneesh Kumar K.V wrote:
> > > >> Marek Szyprowski <[email protected]> writes:
> > > >>
> > > >>> On 07.08.2026 20:20, Jason Gunthorpe wrote:
> > > >>>> On Fri, Aug 07, 2026 at 06:13:49PM +0000, Mostafa Saleh wrote:
> > > >>>>> 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/[email protected]/.
> > > > 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/[email protected]/

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/[email protected]/

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

https://lore.kernel.org/all/[email protected]/

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