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:
> 
> > > 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

> 
> > > 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