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

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Date:   Wed, 7 Feb 2018 03:09:19 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        linux-kernel@...r.kernel.org, Zach Reizner <zachr@...gle.com>,
        kernel@...labora.com, dri-devel@...ts.freedesktop.org,
        virtualization@...ts.linux-foundation.org,
        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

On Tue, Feb 06, 2018 at 03:23:02PM +0100, Gerd Hoffmann wrote:
> > Creation of shareable buffer by guest
> > -------------------------------------------------
> > 
> > 1. Client requests virtio driver to create a buffer suitable for sharing
> > with host (DRM_VIRTGPU_RESOURCE_CREATE)
> 
> client or guest proxy?
> 
> > 4. QEMU maps that buffer to the guest's address space
> > (KVM_SET_USER_MEMORY_REGION), passes the guest PFN to the virtio driver
> 
> That part is problematic.  The host can't simply allocate something in
> the physical address space, because most physical address space
> management is done by the guest.  All pci bars are mapped by the guest
> firmware for example (or by the guest OS in case of hotplug).
> 
> > 4. QEMU pops data+buffers from the virtqueue, looks up shmem FD for each
> > resource, sends data + FDs to the compositor with SCM_RIGHTS

If you squint hard, this sounds a bit like a use-case for vhost-user-gpu, does it not?

> BTW: Is there a 1:1 relationship between buffers and shmem blocks?  Or
> does the wayland protocol allow for offsets in buffer meta data, so you
> can place multiple buffers in a single shmem block?
> 
> cheers,
>   Gerd