[RFC 9/9] vfio/listener: Skip DMA mapping for VFIO-owned RAM-device regions

[<mhonap@nvidia.com>]

From: Manish Honap <mhonap@nvidia.com>

vfio_container_region_add() attempts an IOMMU DMA mapping for every
RAM section that enters the guest address space. For VFIO mmap-backed
regions (PCI BAR windows, CXL.mem regions), this mapping always fails:
the backing VMAs carry VM_IO | VM_PFNMAP flags and pin_user_pages()
refuses to pin VM_IO pages, so IOMMU_IOAS_MAP returns -EFAULT.

CPU access to these regions goes through KVM Stage-2 page faults
independently of the SMMU/IOMMU, so no IOMMU entry is required for
correct operation.

Add an early return for RAM-device sections owned by a VFIO device.
vfio_get_vfio_device(memory_region_owner(section->mr)) returns non-NULL
for any mmap subregion created by vfio_region_mmap(), since
memory_region_init_ram_device_ptr() propagates the VFIOPCIDevice owner
from the containing region. Matching on ownership covers both normal
PCI BAR windows and CXL.mem regions uniformly; non-VFIO RAM-device
regions such as NVDIMMs are unaffected and continue through the normal
mapping path.

Signed-off-by: Manish Honap <mhonap@nvidia.com>
---
 hw/vfio/listener.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/hw/vfio/listener.c b/hw/vfio/listener.c
index 31c3113f8f..46cad18357 100644
--- a/hw/vfio/listener.c
+++ b/hw/vfio/listener.c
@@ -608,6 +608,20 @@ void vfio_container_region_add(VFIOContainer *bcontainer,
                 pgmask + 1);
             return;
         }
+
+        /*
+         * VFIO MMAP backed regions (CXL.mem) uses VM_IO | VM_PFNMAP VMAs
+         * backed by physical device addresses. Skip vfio_container_dma_map
+         * as mapping is not needed for this region.
+         */
+        if (vfio_get_vfio_device(memory_region_owner(section->mr))) {
+            trace_vfio_listener_region_add_no_dma_map(
+                memory_region_name(section->mr),
+                section->offset_within_address_space,
+                int128_getlo(section->size),
+                pgmask + 1);
+            return;
+        }
     }
 
     ret = vfio_container_dma_map(bcontainer, iova, int128_get64(llsize),
-- 
2.25.1