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

Will Deacon [off-list ref] writes:

On Fri, Aug 07, 2026 at 02:56:12PM +0530, Aneesh Kumar K.V (Arm) wrote:

quoted

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. If that is not the case, then
we could change:

!cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) &&

to

!is_realm_world() &&

-aneesh