Re: [PATCH v6 6/8] dma: swiotlb: Centralize memory-encryption pool sizing

[Will Deacon]

On Wed, Oct 07, 2026 at 11:04:05AM +0100, Catalin Marinas wrote:
> On Tue, Oct 06, 2026 at 10:49:17PM +0100, Will Deacon wrote:
> > On Thu, Sep 24, 2026 at 11:37:54AM +0530, Aneesh Kumar K.V (Arm) wrote:
> > > @@ -496,7 +516,8 @@ swiotlb_select_pool_policy(unsigned int flags)
> > >  	if (swiotlb_force_disable)
> > >  		return SWIOTLB_POOL_NONE;
> > >  
> > > -	if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT))
> > > +	if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) &&
> > > +	    !restricted_dma_pool_present)
> > >  		return SWIOTLB_POOL_CC_GUEST;
> > 
> > I think this check on the restricted DMA pool is too general -- the pool
> > could be tied to a specific DMA-capable peripheral and so treating its
> > presence as a global property isn't right.
> 
> I agree it's a hack but that was the simplest way to avoid the pVMs
> getting a bounce buffer after this patch. More than happy to leave it
> out and reduce the buffer on cmdline or we come up with some better
> heuristics.

Hrm, that does mean that reverting just this part will regress pVMs
because they'll suddenly be allocating a tonne more memory for an
entirely unused swiotlb buffer. So I think I'd prefer to drop the entire
series until this has been worked out properly.

> Another option could be the arch code passing another flag that it
> doesn't want an encrypted pool (e.g. when running in a pKVM guest) but I
> don't particularly this either. The arch code doesn't know whether
> there's an alternative pool.

At that point, the default size may as well be driven by the
drivers/virt/coco driver.

> That said, such heuristics should have been a separate patch to make it
> easier to review/drop.

I think the only right way to get a semi-accurate heuristic is to take
into account the set of dma-capable devices that will use the swiotlb
pool, but that's fiddly and should probably be tackled as a separate
series. Maybe a simpler hack in that direction would be to take the
SWIOTLB_POOL_CC_GUEST if _any_ device is going to use swiotlb? You'll
run into the usual problem of not being able to tell if a device is
DMA-capable or not, but you could probably look for a global restricted
DMA pool and, if that doesn't exist, check for per-device restricted pools
on dma-coherent devices (since restricted DMA isn't supported by ACPI) as
a reasonable approximation.

Will