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

[Will Deacon <will@kernel.org>]

On Mon, Aug 10, 2026 at 02:59:42PM +0530, Aneesh Kumar K.V wrote:
> Marek Szyprowski <m.szyprowski@samsung.com> 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/yq5azeyxyfol.fsf@kernel.org/.

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.

We have time to fix this properly, rather than papering over it in the
arch code.

Please consider this a NAK from the arm64 side on this patch.

Will