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

On Fri, Aug 07, 2026 at 10:58:10AM -0300, Jason Gunthorpe wrote:

On Fri, Aug 07, 2026 at 02:18:31PM +0100, Will Deacon wrote:

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I was under the impression that there is a possibility of using swiotlb
instead of restricted-dma-pool with pKVM.

Yes, that patch enables swiotlb as a possibility for protected guests
but with your patch we avoid shrinking the swiotlb buffer even when
restricted dma pools are being used and that's a waste of memory.

I also thought we switched pkvm to use CC-like swiotlb as part of the
alignment in this rework? Mostafa ?

Yes, now protected VMs can use the SWIOTLB for bouncing. However for
Android (and kvmtool), use restricted DMA. so SWIOTLB is not used.

However, I do not think we should over engineer this in the code,
swiotlb can be set from the cmdline and now through Kconfig also.

Thanks,
Mostafa

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If that is not the case, then we could change:

!cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) &&

to

!is_realm_world() &&

Perhaps, or you could just pass the swiotlb= option if the defaults don't
work for you. Can you give more details about the slots exhaustion you're
seeing under CCA?

We see badness with swiotlb too, it basically doesn't work out of the
box if you have to use it 100% for real devices. It easily runs out
of memory.

Auto tuning to higher levels makes sense to me, but I'd rather the
core code handled adjusting its size to the estimated need, not arch
code.

Jason