On Fri, Aug 07, 2026 at 06:33:14PM +0530, Aneesh Kumar K.V wrote: > Will Deacon <[email protected]> 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) <[email protected]> > >> --- > >> 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
