Re: [PATCH v1 1/1] iommu/amd: Don't split flush for amd_iommu_domain_flush_all()

Samiullah Khawaja
2026-04-14T23:55:25Z

On Tue, Apr 14, 2026 at 09:06:26PM +0000, Weinan Liu wrote:
>We have observed multiple full invalidations occurring during device
>detach when we are done using the vfio-device.
>
>blocked_domain_attach_device()
> -> detach_device()
> -> amd_iommu_domain_flush_all()
> -> amd_iommu_domain_flush_pages(..., CMD_INV_IOMMU_ALL_PAGES_ADDRESS)
>
> while (size != 0) {
>
> -> __domain_flush_pages( flush_size /* power of 2 flush_size */)
> -> domain_flush_pages_v1()
> -> build_inv_iommu_pages()
> -> build_inv_address()
>
> }
>
>build_inv_address() will trigger a full invalidation if the chunk
>size > (1 << 51). Consequently, the guest will issue multiple full
>invalidations for a single call to amd_iommu_domain_flush_all()
>
>Without this patch, we will see 10 time instead of 1 time full
>invalidations for every amd_iommu_domain_flush_all().
>
>Fixes: a270be1b3fdf ("iommu/amd: Use only natural aligned flushes in a VM")
>
>Suggested-by: Josef Bacik <josef@toxicpanda.com>
>Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
>Signed-off-by: Weinan Liu <wnliu@google.com>
>---
> drivers/iommu/amd/iommu.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
>diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
>index 760d5f4623b5..bcec8721d228 100644
>--- a/drivers/iommu/amd/iommu.c
>+++ b/drivers/iommu/amd/iommu.c
>@@ -1769,7 +1769,8 @@ void amd_iommu_domain_flush_pages(struct protection_domain *domain,
> {
> lockdep_assert_held(&domain->lock);
>
>- if (likely(!amd_iommu_np_cache)) {
>+ if (likely(!amd_iommu_np_cache) ||
>+ size == CMD_INV_IOMMU_ALL_PAGES_ADDRESS) {
> __domain_flush_pages(domain, address, size);
>
> /* Wait until IOMMU TLB and all device IOTLB flushes are complete */
>--
>2.54.0.rc0.605.g598a273b03-goog
>
>

Reviewed-by: Samiullah Khawaja <skhawaja@google.com>