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

[Will Deacon <will@kernel.org>]

On Fri, Aug 07, 2026 at 06:33:14PM +0530, Aneesh Kumar K.V wrote:
> Will Deacon <will@kernel.org> writes:
> 
> > On Fri, Aug 07, 2026 at 02:56:12PM +0530, Aneesh Kumar K.V (Arm) wrote:
> >> Realm guests and protected KVM guests may require swiotlb for device DMA
> >> even when all system RAM is addressable by the DMA zones.
> >> 
> >> Do not reduce the SWIOTLB buffer to the kmalloc-bouncing size for these
> >> guests, as the reduced number of slots can be exhausted during CCA guest
> >> operation.
> >> 
> >> Fixes: 30c5e45ee2c5 ("dma-direct: make dma_direct_map_phys() honor DMA_ATTR_CC_SHARED")
> >> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
> >> ---
> >>  arch/arm64/mm/init.c | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >> 
> >> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> >> index e308a7cabd12..24caaf10e755 100644
> >> --- a/arch/arm64/mm/init.c
> >> +++ b/arch/arm64/mm/init.c
> >> @@ -339,7 +339,8 @@ void __init arch_mm_preinit(void)
> >>  {
> >>  	unsigned int flags = SWIOTLB_VERBOSE;
> >>  
> >> -	if (max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) {
> >> +	if (!cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) &&
> >> +	    max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) {
> >
> > Protected guests under pKVM rely on restricted DMA, so won't this just
> > waste memory for them?
> >
> 
> commit 30c5e45ee2c5 ("dma-direct: make dma_direct_map_phys() honor DMA_ATTR_CC_SHARED"),
> which this patch fixes, has
> 
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -339,9 +339,7 @@ void __init arch_mm_preinit(void)
>  {
>         unsigned int flags = SWIOTLB_VERBOSE;
>  
> -       if (is_realm_world() || is_protected_kvm_guest()) {
> -               flags |= SWIOTLB_FORCE;
> -       } else if (max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) {
> +       if (max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) {
>                 /*
>                  * If no bouncing needed for ZONE_DMA, reduce the swiotlb
>                  * buffer for kmalloc() bouncing to 1MB per 1GB of RAM.
> 
> The is_protected_kvm_guest() part was added by the 
> commit e62decaf98e7 ("arm64/coco: Add pKVM as a CC platform")
> 
> @@ -337,7 +339,7 @@ void __init arch_mm_preinit(void)
>  {
>         unsigned int flags = SWIOTLB_VERBOSE;
>  
> -       if (is_realm_world()) {
> +       if (is_realm_world() || is_protected_kvm_guest()) {
>                 flags |= SWIOTLB_FORCE;
>         } else if (max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) {
>                 /*
> @@ -412,6 +414,17 @@ void dump_mem_limit(void)
>         }
>  }
> 
> 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.

> 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?

Will