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

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Will Deacon <will@kernel.org> writes:

> 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.
>

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