Re: [PATCH v8 2/4] drm/virtgpu api: add blob userptr resource

[Huang, Honglei]

On 9/18/2026 8:12 PM, Akihiko Odaki wrote:
> On 2026/09/18 18:59, Honglei Huang wrote:
> > Add a userptr address field so userspace can request a blob backed
> > by an existing process mapping. A non-zero userptr selects that
> > path; probe VIRTGPU_PARAM_USERPTR before using the field.
> >
> > GPU-readonly backing is requested with the virtio CREATE_BLOB wire
> > flag VIRTIO_GPU_BLOB_FLAG_USE_READONLY (also
> > VIRTGPU_BLOB_FLAG_USE_READONLY). The device MUST NOT write such a
> > resource. The guest pins without FOLL_WRITE only when the device
> > advertised VIRTIO_GPU_F_BLOB_READONLY (VIRTGPU_PARAM_BLOB_READONLY).
> >
> > Signed-off-by: Honglei Huang <honghuan@xxxxxxx>
> > ---
> >   include/uapi/drm/virtgpu_drm.h  | 13 +++++++++++++
> >   include/uapi/linux/virtio_gpu.h |  6 ++++++
> >   2 files changed, 19 insertions(+)
> >
> > diff --git a/include/uapi/drm/virtgpu_drm.h b/include/uapi/drm/ 
> > virtgpu_drm.h
> > index 95587e12ae..80f73b3276 100644
> > --- a/include/uapi/drm/virtgpu_drm.h
> > +++ b/include/uapi/drm/virtgpu_drm.h
> > @@ -99,6 +99,8 @@ struct drm_virtgpu_execbuffer {
> >   #define VIRTGPU_PARAM_SUPPORTED_CAPSET_IDs 7 /* Bitmask of supported 
> > capability set ids */
> >   #define VIRTGPU_PARAM_EXPLICIT_DEBUG_NAME 8 /* Ability to set debug 
> > name from userspace */
> >   #define VIRTGPU_PARAM_BLOB_ALIGNMENT 9 /* Device alignment 
> > requirements for blobs */
> > +#define VIRTGPU_PARAM_USERPTR 10 /* CREATE_BLOB userptr field is 
> > supported */
> > +#define VIRTGPU_PARAM_BLOB_READONLY 11 /* 
> > VIRTGPU_BLOB_FLAG_USE_READONLY is honored */
> >   struct drm_virtgpu_getparam {
> >       __u64 param;
> > @@ -186,6 +188,10 @@ struct drm_virtgpu_resource_create_blob {
> >   #define VIRTGPU_BLOB_FLAG_USE_MAPPABLE     0x0001
> >   #define VIRTGPU_BLOB_FLAG_USE_SHAREABLE    0x0002
> >   #define VIRTGPU_BLOB_FLAG_USE_CROSS_DEVICE 0x0004
> > +/* Wire flag: device MUST NOT write. Guest pins without FOLL_WRITE.
> > + * Rejected unless VIRTGPU_PARAM_BLOB_READONLY is 1.
> > + */
>
> According to Documentation/process/coding-style.rst, the preferred style 
> for long (multi-line) comments is:
>
>      /*
>       * This is the preferred style for multi-line
>       * comments in the Linux kernel source code.
>       * Please use it consistently.
>       *
>       * Description:  A column of asterisks on the left side,
>       * with beginning and ending almost-blank lines.
>       */
>
> So please avoid putting text on the same line as the opening /*.
>
> Furthermore, because this is a UAPI header (defining the interface 
> between userspace and the kernel, rather than the guest and the host), 
> it should use terminology appropriate for userspace. Kernel-internal 
> implementation details like FOLL_WRITE and terms like "guest" shouldn't 
> be mentioned here.
>
> That being said, I don't think this comment is necessary in the first 
> place. Anyone who needs to know the exact behavior will look at the 
> virtio specification. I recommend just dropping the comment entirely.

Thanks for the review, will remove this comment here.

> > +#define VIRTGPU_BLOB_FLAG_USE_READONLY     0x0008
> >       /* zero is invalid blob_mem */
> >       __u32 blob_mem;
> >       __u32 blob_flags;
> > @@ -205,6 +211,13 @@ struct drm_virtgpu_resource_create_blob {
> >   #define DRM_VIRTGPU_BLOB_FLAG_HINT_DEFER_MAPPING        0x0001
> >       __u32 blob_hints;
> >       __u32 pad2;
> > +
> > +    /*
> > +     * Guest VA to pin as blob backing. Non-zero selects the userptr
> > +     * path and is valid only with VIRTGPU_BLOB_MEM_GUEST. Must be 0
> > +     * otherwise. Probe VIRTGPU_PARAM_USERPTR before using this field.
> > +     */
>
> This comment is useful since it discusses UAPI specifics. However, 
> please avoid the term "Guest VA". Since guest userspace has no concept 
> of a "host" or "PA" (physical address), simply saying "address" is 
> sufficient and more concise.

Got it, will modify the comment in next version.

Regards,
Honglei

> Regards,
> Akihiko Odaki
>
> > +    __u64 userptr;
> >   };
> >   #define VIRTGPU_CONTEXT_PARAM_CAPSET_ID       0x0001
> > diff --git a/include/uapi/linux/virtio_gpu.h b/include/uapi/linux/ 
> > virtio_gpu.h
> > index 3d4dfadc9d..33f5332e59 100644
> > --- a/include/uapi/linux/virtio_gpu.h
> > +++ b/include/uapi/linux/virtio_gpu.h
> > @@ -72,6 +72,10 @@
> >    * must be aligned to that value.
> >    */
> >   #define VIRTIO_GPU_F_BLOB_ALIGNMENT      5
> > +/*
> > + * VIRTIO_GPU_BLOB_FLAG_USE_READONLY
> > + */
> > +#define VIRTIO_GPU_F_BLOB_READONLY       6
> >   enum virtio_gpu_ctrl_type {
> >       VIRTIO_GPU_UNDEFINED = 0,
> > @@ -415,6 +419,8 @@ struct virtio_gpu_resource_create_blob {
> >   #define VIRTIO_GPU_BLOB_FLAG_USE_MAPPABLE     0x0001
> >   #define VIRTIO_GPU_BLOB_FLAG_USE_SHAREABLE    0x0002
> >   #define VIRTIO_GPU_BLOB_FLAG_USE_CROSS_DEVICE 0x0004
> > +/* Device MUST NOT write. Driver may pin read-only guest pages. */
> > +#define VIRTIO_GPU_BLOB_FLAG_USE_READONLY     0x0008
> >       /* zero is invalid blob mem */
> >       __le32 blob_mem;
> >       __le32 blob_flags;