Will Deacon <[email protected]> writes: > 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. >
The patch restores the behavior to what it was before that commit. That is, for both CCA and pKVM, we don't do the max_pfn check, and hence the swiotlb is never resized. > >> 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? That is definitely possible. However, on arm64, the current code further reduces the default swiotlb size when max_pfn < arm64_dma_phys_limit. That reduction is only correct when swiotlb is being used as a bounce buffer because of arm64_dma_phys_limit. With CCA, that is not the case. We use swiotlb to bounce DMA for untrusted devices. Therefore, reducing the default swiotlb size based on max_pfn is not correct for CCA configurations. This change in behavior is also a regression for CCA. CCA configurations that booted successfully before now gives the error below. [ 39.848004] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 256 (slots), used 159 (slots) [ 40.002654] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 256 (slots), used 159 (slots) [ 40.165933] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 256 (slots), used 159 (slots) [ 40.325465] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 512 (slots), used 415 (slots) [ 40.485932] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 204800 bytes), total 768 (slots), used 671 (slots) [ 42.485337] EXT4-fs (vda): re-mounted 6870157e-2795-475e-aad6-3a05725fb7cf. [ 43.799800] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 1280 (slots), used 1153 (slots) [ 44.016682] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 1280 (slots), used 1153 (slots) [ 44.226566] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 1536 (slots), used 1409 (slots) [ 44.437194] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 1792 (slots), used 1665 (slots) -aneesh
