Re: [PATCH] Add VIRTIO_F_DMB (Device Memory Buffer)

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Jul 02, 2026 at 04:00:06AM +0000, Alexander Graf wrote:
> * Shared memory region id: DMB needs a well-known shmid so the driver can
>   locate the region independent of device type. This draft reserves shmid 2
>   and adds a "Reserved Shared Memory Region IDs" registry. However shmid is
>   currently a per-device-type namespace, and at least one device type
>   (virtio-scmi) documents the entire shmid space as its own. Guidance
>   requested on the preferred mechanism: (a) a small transport-reserved shmid
>   range with device types adjusted to exclude it, or (b) identifying the DMB
>   region by a means other than a fixed shmid. The exact number is not fixed
>   pending this decision.

Hi Alex,
VIRTIO Transports could provide a way to fetch the DMB shmid. For
example, the MMIO transport would get a new read-only DMBSHMId register
that contains a valid shmid when VIRTIO_F_DMB has been negotiated.

Allowing the device to report an arbitrary shmid is flexible - it won't
interfere with device-specific Shared Memory Regions or future
Transport-specific Shared Memory Regions.

Stefan

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