vfio: Add VFIO I/O backend capability flags for feature support

[PATCH] vfio: Add VFIO I/O backend capability flags for feature support

Posted by Cédric Le Goater 2 days, 3 hours ago

Different VFIO I/O backends support different features. For example,
the kernel VFIO backend supports DMA-BUF creation, while vfio-user
does not. Currently, this is handled by attempting the operation and
checking for -ENOTTY, which can lead to misleading warnings when a
feature is simply not supported by a particular backend.

Introduce a capability flags mechanism in VFIODeviceIOOps that allows
backends to explicitly advertise which features they support. Callers
can check these capabilities before attempting operations, avoiding
spurious errors and warnings.

Cc: John Levon <john.levon@nutanix.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
---
 include/hw/vfio/vfio-device.h | 13 +++++++++++++
 hw/vfio/device.c              |  2 ++
 hw/vfio/region.c              |  5 +++++
 3 files changed, 20 insertions(+)

diff --git a/include/hw/vfio/vfio-device.h b/include/hw/vfio/vfio-device.h
index a95c5bf50302a7394582dcc493d96aef00b47dc8..380a55d6e5ea280c8f44016c7a8a1dcb8f4c00fe 100644
--- a/include/hw/vfio/vfio-device.h
+++ b/include/hw/vfio/vfio-device.h
@@ -172,12 +172,25 @@ typedef QLIST_HEAD(VFIODeviceList, VFIODevice) VFIODeviceList;
 extern VFIODeviceList vfio_device_list;
 
 #ifdef CONFIG_LINUX
+/*
+ * VFIO backend I/O operation capabilities
+ */
+#define VFIO_IO_CAP_DMA_BUF        (1ULL << 0)
+
 /*
  * How devices communicate with the server.  The default option is through
  * ioctl() to the kernel VFIO driver, but vfio-user can use a socket to a remote
  * process.
  */
 struct VFIODeviceIOOps {
+    /**
+     * @capabilities
+     *
+     * Bitmask of VFIO_IO_CAP_* flags indicating which features this
+     * backend supports.
+     */
+    uint64_t capabilities;
+
     /**
      * @device_feature
      *
diff --git a/hw/vfio/device.c b/hw/vfio/device.c
index 8f7ae919a55dd9e08bdd072eee33f5b46a91aa2a..3ffd69a579b8ac135ef729d3d836c8545e36786b 100644
--- a/hw/vfio/device.c
+++ b/hw/vfio/device.c
@@ -648,6 +648,8 @@ static int vfio_device_io_region_write(VFIODevice *vbasedev, uint8_t index,
 }
 
 static VFIODeviceIOOps vfio_device_io_ops_ioctl = {
+    .capabilities = VFIO_IO_CAP_DMA_BUF,
+
     .device_feature = vfio_device_io_device_feature,
     .get_region_info = vfio_device_io_get_region_info,
     .get_irq_info = vfio_device_io_get_irq_info,
diff --git a/hw/vfio/region.c b/hw/vfio/region.c
index 47fdc2df349b65c6be6c9605b7a38a4e367f0475..0342ca712acd0ad846685115c29432c15aa966a0 100644
--- a/hw/vfio/region.c
+++ b/hw/vfio/region.c
@@ -293,6 +293,11 @@ static bool vfio_region_create_dma_buf(VFIORegion *region, Error **errp)
     size_t total_size;
     int i, ret;
 
+    /* Check if backend supports DMA-BUF creation */
+    if (!(vbasedev->io_ops->capabilities & VFIO_IO_CAP_DMA_BUF)) {
+        return true;
+    }
+
     total_size = sizeof(*feature) + sizeof(*dma_buf) +
                  sizeof(struct vfio_region_dma_range) * region->nr_mmaps;
     feature = g_malloc0(total_size);
-- 
2.53.0

Re: [PATCH] vfio: Add VFIO I/O backend capability flags for feature support

Posted by Cédric Le Goater 1 day, 8 hours ago

On 4/9/26 13:43, Cédric Le Goater wrote:
> Different VFIO I/O backends support different features. For example,
> the kernel VFIO backend supports DMA-BUF creation, while vfio-user
> does not. Currently, this is handled by attempting the operation and
> checking for -ENOTTY, which can lead to misleading warnings when a
> feature is simply not supported by a particular backend.
> 
> Introduce a capability flags mechanism in VFIODeviceIOOps that allows
> backends to explicitly advertise which features they support. Callers
> can check these capabilities before attempting operations, avoiding
> spurious errors and warnings.
> 
> Cc: John Levon <john.levon@nutanix.com>
> Signed-off-by: Cédric Le Goater <clg@redhat.com>
> ---
>   include/hw/vfio/vfio-device.h | 13 +++++++++++++
>   hw/vfio/device.c              |  2 ++
>   hw/vfio/region.c              |  5 +++++
>   3 files changed, 20 insertions(+)
> 
> diff --git a/include/hw/vfio/vfio-device.h b/include/hw/vfio/vfio-device.h
> index a95c5bf50302a7394582dcc493d96aef00b47dc8..380a55d6e5ea280c8f44016c7a8a1dcb8f4c00fe 100644
> --- a/include/hw/vfio/vfio-device.h
> +++ b/include/hw/vfio/vfio-device.h
> @@ -172,12 +172,25 @@ typedef QLIST_HEAD(VFIODeviceList, VFIODevice) VFIODeviceList;
>   extern VFIODeviceList vfio_device_list;
>   
>   #ifdef CONFIG_LINUX
> +/*
> + * VFIO backend I/O operation capabilities
> + */
> +#define VFIO_IO_CAP_DMA_BUF        (1ULL << 0)
> +
>   /*
>    * How devices communicate with the server.  The default option is through
>    * ioctl() to the kernel VFIO driver, but vfio-user can use a socket to a remote
>    * process.
>    */
>   struct VFIODeviceIOOps {
> +    /**
> +     * @capabilities
> +     *
> +     * Bitmask of VFIO_IO_CAP_* flags indicating which features this
> +     * backend supports.
> +     */
> +    uint64_t capabilities;
> +
>       /**
>        * @device_feature
>        *
> diff --git a/hw/vfio/device.c b/hw/vfio/device.c
> index 8f7ae919a55dd9e08bdd072eee33f5b46a91aa2a..3ffd69a579b8ac135ef729d3d836c8545e36786b 100644
> --- a/hw/vfio/device.c
> +++ b/hw/vfio/device.c
> @@ -648,6 +648,8 @@ static int vfio_device_io_region_write(VFIODevice *vbasedev, uint8_t index,
>   }
>   
>   static VFIODeviceIOOps vfio_device_io_ops_ioctl = {
> +    .capabilities = VFIO_IO_CAP_DMA_BUF,
> +
>       .device_feature = vfio_device_io_device_feature,
>       .get_region_info = vfio_device_io_get_region_info,
>       .get_irq_info = vfio_device_io_get_irq_info,
> diff --git a/hw/vfio/region.c b/hw/vfio/region.c
> index 47fdc2df349b65c6be6c9605b7a38a4e367f0475..0342ca712acd0ad846685115c29432c15aa966a0 100644
> --- a/hw/vfio/region.c
> +++ b/hw/vfio/region.c
> @@ -293,6 +293,11 @@ static bool vfio_region_create_dma_buf(VFIORegion *region, Error **errp)
>       size_t total_size;
>       int i, ret;
>   
> +    /* Check if backend supports DMA-BUF creation */
> +    if (!(vbasedev->io_ops->capabilities & VFIO_IO_CAP_DMA_BUF)) {
> +        return true;
> +    }
> +
>       total_size = sizeof(*feature) + sizeof(*dma_buf) +
>                    sizeof(struct vfio_region_dma_range) * region->nr_mmaps;
>       feature = g_malloc0(total_size);

Applied to vfio-next.

Thanks,

C.

Re: [PATCH] vfio: Add VFIO I/O backend capability flags for feature support

Posted by John Levon 2 days, 2 hours ago

On Thu, Apr 09, 2026 at 01:43:12PM +0200, Cédric Le Goater wrote:

> !-------------------------------------------------------------------|
>   CAUTION: External Email
> 
> |-------------------------------------------------------------------!
> 
> Different VFIO I/O backends support different features. For example,
> the kernel VFIO backend supports DMA-BUF creation, while vfio-user
> does not. Currently, this is handled by attempting the operation and
> checking for -ENOTTY, which can lead to misleading warnings when a
> feature is simply not supported by a particular backend.
> 
> Introduce a capability flags mechanism in VFIODeviceIOOps that allows
> backends to explicitly advertise which features they support. Callers
> can check these capabilities before attempting operations, avoiding
> spurious errors and warnings.
> 
> Cc: John Levon <john.levon@nutanix.com>
> Signed-off-by: Cédric Le Goater <clg@redhat.com>

Thanks!

Reviewed-by: John Levon <john.levon@nutanix.com>

regards
john