[Linaro-mm-sig] Re: [PATCH v2 9/9] vfio/pci: Add mmap() attributes to DMABUF feature

On Wed, 27 May 2026 03:23:12 -0700 Matt Evans mattev@meta.com wrote:

A new VFIO feature, VFIO_DEVICE_FEATURE_DMA_BUF_MEMATTR, is added to set (and get) CPU-facing memory type attributes for a DMABUF exported from vfio-pci. These are used for subsequent mmap()s of the buffer.

There are two attributes supported:

• The default, VFIO_DEVICE_FEATURE_DMA_BUF_MEMATTR_UC
• VFIO_DEVICE_FEATURE_DMA_BUF_MEMATTR_WC, which results in WC PTEs for the DMABUF's BAR region.

Signed-off-by: Matt Evans mattev@meta.com

drivers/vfio/pci/vfio_pci_core.c | 2 + drivers/vfio/pci/vfio_pci_dmabuf.c | 70 +++++++++++++++++++++++++++++- drivers/vfio/pci/vfio_pci_priv.h | 12 +++++ include/uapi/linux/vfio.h | 27 ++++++++++++ 4 files changed, 110 insertions(+), 1 deletion(-)

diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c index 5184b3cac160..e256a925e7ce 100644 --- a/drivers/vfio/pci/vfio_pci_core.c +++ b/drivers/vfio/pci/vfio_pci_core.c @@ -1590,6 +1590,8 @@ int vfio_pci_core_ioctl_feature(struct vfio_device *device, u32 flags, return vfio_pci_core_feature_token(vdev, flags, arg, argsz); case VFIO_DEVICE_FEATURE_DMA_BUF: return vfio_pci_core_feature_dma_buf(vdev, flags, arg, argsz);

• case VFIO_DEVICE_FEATURE_DMA_BUF_MEMATTR:

• return vfio_pci_core_feature_dma_buf_memattr(vdev, flags, arg, argsz);
  

  default: return -ENOTTY; }

diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c index 3fa14760898f..db8b95ddbe18 100644 --- a/drivers/vfio/pci/vfio_pci_dmabuf.c +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c @@ -42,7 +42,10 @@ static int vfio_pci_dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct * * contained within the DMABUF size before calling this. */

• vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);

• if (READ_ONCE(priv->memattr) == VFIO_DEVICE_FEATURE_DMA_BUF_MEMATTR_WC)

• vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
  

• else

• vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
  

  vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);

/* See comments in vfio_pci_core_mmap() re VM_ALLOW_ANY_UNCACHED. */ @@ -464,6 +467,7 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags, priv->vdev = vdev; priv->nr_ranges = get_dma_buf.nr_ranges; priv->size = length;

• priv->memattr = VFIO_DEVICE_FEATURE_DMA_BUF_MEMATTR_NC; ret = vdev->pci_ops->get_dmabuf_phys(vdev, &priv->provider, get_dma_buf.region_index, priv->phys_vec, dma_ranges,

@@ -731,4 +735,68 @@ int vfio_pci_dma_buf_revoke(struct vfio_pci_core_device *vdev, int dmabuf_fd) return ret; }

+int vfio_pci_core_feature_dma_buf_memattr(

• struct vfio_pci_core_device *vdev, u32 flags,
• struct vfio_device_feature_dma_buf_memattr __user *arg,
• size_t argsz)

+{

• struct vfio_device_feature_dma_buf_memattr db_attr;
• struct vfio_pci_dma_buf *priv;
• struct dma_buf *dmabuf;
• int ret;
• if (!vdev->pci_ops || !vdev->pci_ops->get_dmabuf_phys)

• return -EOPNOTSUPP;
  

• ret = vfio_check_feature(flags, argsz,

• 		 VFIO_DEVICE_FEATURE_GET |
  

• 		 VFIO_DEVICE_FEATURE_SET,
  

• 		 sizeof(db_attr));
  

I don't see why this needs to support GET. Are we solving a userspace problem that doesn't exist?

• if (ret != 1)

• return ret;
  

• if (copy_from_user(&db_attr, arg, sizeof(db_attr)))

• return -EFAULT;
  

• dmabuf = dma_buf_get(db_attr.dmabuf_fd);
• if (IS_ERR(dmabuf))

• return PTR_ERR(dmabuf);
  

• /* Verify DMABUF: see comments in vfio_pci_dma_buf_revoke() */
• priv = dmabuf->priv;
• if (dmabuf->ops != &vfio_pci_dmabuf_ops || priv->vdev != vdev) {

• ret = -ENODEV;
  

• goto out_put_buf;
  

• }
• ret = 0;
• scoped_guard(rwsem_write, &vdev->memory_lock) {

Why? This doesn't serialize against mmap. Just use a WRITE_ONCE() to match the READ_ONCE() on mmap?

• uint32_t old_attr = priv->memattr;
  

• if (flags & VFIO_DEVICE_FEATURE_SET) {
  

• 	switch(db_attr.memattr) {
  

• 	case VFIO_DEVICE_FEATURE_DMA_BUF_MEMATTR_NC:
  

• 	case VFIO_DEVICE_FEATURE_DMA_BUF_MEMATTR_WC:
  

• 		priv->memattr = db_attr.memattr;
  

• 		break;
  

• 	default:
  

• 		ret = -ENOTSUPP;
  

-EINVAL

• 	}
  

• }
  

• db_attr.memattr = old_attr;
  

• }
• if (!ret && (flags & VFIO_DEVICE_FEATURE_GET)) {

• if (copy_to_user(arg, &db_attr, sizeof(db_attr)))
  

• 	ret = -EFAULT;
  

• }
• out_put_buf:
• dma_buf_put(dmabuf);
• return ret;

+} #endif /* CONFIG_VFIO_PCI_DMABUF */ diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h index a1e0f4fcb1dc..8067be45beb0 100644 --- a/drivers/vfio/pci/vfio_pci_priv.h +++ b/drivers/vfio/pci/vfio_pci_priv.h @@ -41,6 +41,7 @@ struct vfio_pci_dma_buf { struct kref kref; struct completion comp; unsigned long vma_pgoff_adjust;

• u32 memattr; enum vfio_pci_dma_buf_status status;

}; @@ -154,6 +155,10 @@ void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked); int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags, struct vfio_device_feature_dma_buf __user *arg, size_t argsz); +int vfio_pci_core_feature_dma_buf_memattr(

• struct vfio_pci_core_device *vdev, u32 flags,
• struct vfio_device_feature_dma_buf_memattr __user *arg,
• size_t argsz);

int vfio_pci_dma_buf_revoke(struct vfio_pci_core_device *vdev, int dmabuf_fd); #else static inline int @@ -163,6 +168,13 @@ vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags, { return -ENOTTY; } +static inline int vfio_pci_core_feature_dma_buf_memattr(

• struct vfio_pci_core_device *vdev, u32 flags,
• struct vfio_device_feature_dma_buf_memattr __user *arg,
• size_t argsz)

+{

• return -ENODEV;

-ENOTTY

Thanks, Alex

+} static inline int vfio_pci_dma_buf_revoke(struct vfio_pci_core_device *vdev, int dmabuf_fd) { diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h index 02366e9f8e16..9b0b68f8a1ef 100644 --- a/include/uapi/linux/vfio.h +++ b/include/uapi/linux/vfio.h @@ -1564,6 +1564,33 @@ struct vfio_device_feature_dma_buf { */ #define VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2 12 +/**

• • Given a dma_buf fd previously created by
• • VFIO_DEVICE_FEATURE_DMA_BUF, GET or SET the memory attribute that
• • will be used by future mmap()s of that fd. SETting a new attribute
• • does not affect existing VMAs.
• • The default, if no previous SET has been performed, is NC.
• • Return: 0 on success, -1 and errno is set on failure:
• • ENOTSUPP: The given memattr is not supported.
• • EBADF, EINVAL: dmabuf_fd is not a DMABUF fd.
• • ENODEV: The dmabuf_fd does not match this VFIO device.
• */

+#define VFIO_DEVICE_FEATURE_DMA_BUF_MEMATTR 13

+/* Valid memory attributes for the memattr field */ +enum vfio_device_dma_buf_memattr {

• VFIO_DEVICE_FEATURE_DMA_BUF_MEMATTR_NC = 0, /* pgprot_noncached */
• VFIO_DEVICE_FEATURE_DMA_BUF_MEMATTR_WC = 1, /* pgprot_writecombine */

+};

+struct vfio_device_feature_dma_buf_memattr {

• __s32 dmabuf_fd;
• __u32 memattr;

+};

/* -------- API for Type1 VFIO IOMMU -------- */ /**