linux-kernel - Re: [PATCH 3/3] virtio-gpu api: VIRTIO_GPU_F_RESSOURCE

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Date:   Wed, 17 Apr 2019 11:57:50 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Chia-I Wu <olvaffe@...il.com>
Cc:     Gurchetan Singh <gurchetansingh@...omium.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        Jason Wang <jasowang@...hat.com>,
        open list <linux-kernel@...r.kernel.org>,
        ML dri-devel <dri-devel@...ts.freedesktop.org>,
        "open list:VIRTIO CORE, NET AND BLOCK DRIVERS" 
        <virtualization@...ts.linux-foundation.org>,
        David Airlie <airlied@...hat.com>, virtio@...ts.oasis-open.org
Subject: Re: [PATCH 3/3] virtio-gpu api: VIRTIO_GPU_F_RESSOURCE_V2

On Fri, Apr 12, 2019 at 04:34:20PM -0700, Chia-I Wu wrote:
> Hi,
> 
> I am still new to virgl, and missed the last round of discussion about
> resource_create_v2.
> 
> From the discussion below, semantically resource_create_v2 creates a host
> resource object _without_ any storage; memory_create creates a host memory
> object which provides the storage.  Is that correct?

Right now all resource_create_* variants create a resource object with
host storage.  memory_create creates guest storage, and
resource_attach_memory binds things together.  Then you have to transfer
the data.

Hmm, maybe we need a flag indicating that host storage is not needed,
for resources where we want establish some kind of shared mapping later
on.

> Do we expect these new commands to be supported by OpenGL, which does not
> separate resources and memories?

Well, for opengl you need a 1:1 relationship between memory region and
resource.

> > Yes, even though it is not clear yet how we are going to handle
> > host-allocated buffers in the vhost-user case ...
> 
> This might be another dumb question, but is this only an issue for
> vhost-user(-gpu) case?  What mechanisms are used to map host dma-buf into
> the guest address space?

qemu can change the address space, that includes mmap()ing stuff there.
An external vhost-user process can't do this, it can only read the
address space layout, and read/write from/to guest memory.

> But one needs to create the resource first to know which memory types can
> be attached to it.  I think the metadata needs to be returned with
> resource_create_v2.

There is a resource_info reply for that.

> That should be good enough.  But by returning alignments, we can minimize
> the gaps when attaching multiple resources, especially when the resources
> are only used by GPU.

We can add alignments to the resource_info reply.

cheers,
  Gerd