linux-kernel - Re: [PATCH v3 1/2] drm/virtio: Add window server support

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Date:   Mon, 12 Feb 2018 16:29:03 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Tomeu Vizoso <tomeu.vizoso@...labora.com>
Cc:     linux-kernel@...r.kernel.org, Zach Reizner <zachr@...gle.com>,
        kernel@...labora.com, dri-devel@...ts.freedesktop.org,
        virtualization@...ts.linux-foundation.org,
        "Michael S. Tsirkin" <mst@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        Jason Wang <jasowang@...hat.com>,
        Stefan Hajnoczi <stefanha@...il.com>
Subject: Re: [PATCH v3 1/2] drm/virtio: Add window server support

> > I was more thinking about a struct containing enough info to allow the
> > proxy on the host side find the buffer, something like:
> > 
> >     struct {
> >        enum type { stdvga, virtio-cpu, ... }
> >        pcislot device;
> >        union {
> >           int stdvga_pcibar_offset;
> >           int virtio_gpu_resource_id;
> >        }
> >     }
> > 
> > So when the guest proxy gets a message with a fd referencing a buffer it
> > would have to figure where the buffer is, rewrite the message into the
> > struct above for the host proxy.  The host proxy would rewrite the
> > message again for the server.
> 
> What I don't understand yet is how we can keep the buffer descriptions
> together with the protocol data that references them.
> 
> With SCM_RIGHTS, the FDs are placed in the ancillary data that "travels"
> together with the protocol data that references them.

Place the buffer description into the wayland extension protocol messages?

i.e. have some wl_virt_proxy protocol extension.  Then, for the stdvga case:

  (1) client sends wl_drm/create_prime_buffer request to the guest proxy
  (2) guest proxy rewrites this into wl_virt_proxy/create_buffer, or
      maybe a create_stdvga_buffer request, carrying all the information
      listed above, and sends it to the host proxy.
  (3) host proxy rewrites it again into a wl_shm_pool/create_buffer and
      forwards it to the server.

cheers,
  Gerd