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?
And this version of memory_create is probably the most special one among
its other potential variants, because it is the only(?) one who imports the
pre-allocated guest pages.
Do we expect these new commands to be supported by OpenGL, which does not
separate resources and memories?
On Thu, Apr 11, 2019 at 10:49 PM Gerd Hoffmann <kraxel at redhat.com>
wrote:
> On Thu, Apr 11, 2019 at 06:36:15PM -0700, Gurchetan Singh wrote:
> > On Wed, Apr 10, 2019 at 10:03 PM Gerd Hoffmann <kraxel at
redhat.com>
> wrote:
> > >
> > > > > +/* VIRTIO_GPU_CMD_RESOURCE_CREATE_V2 */
> > > > > +struct virtio_gpu_cmd_resource_create_v2 {
> > > > > + struct virtio_gpu_ctrl_hdr hdr;
> > > > > + __le32 resource_id;
> > > > > + __le32 format;
> > > > > + __le32 width;
> > > > > + __le32 height;
> > > > > + /* 3d only */
> > > > > + __le32 target;
> > > > > + __le32 bind;
> > > > > + __le32 depth;
> > > > > + __le32 array_size;
> > > > > + __le32 last_level;
> > > > > + __le32 nr_samples;
> > > > > + __le32 flags;
> > > > > +};
> > > >
> > > >
> > > > I assume this is always backed by some host side allocation,
without
> any
> > > > guest side pages associated with it?
> > >
> > > No. It is not backed at all yet. Workflow would be like this:
> > >
> > > (1) VIRTIO_GPU_CMD_RESOURCE_CREATE_V2
> > > (2) VIRTIO_GPU_CMD_MEMORY_CREATE (see patch 2)
> > > (3) VIRTIO_GPU_CMD_RESOURCE_MEMORY_ATTACH (see patch 2)
> >
> > Thanks for the clarification.
> >
> > >
> > > You could also create a larger pool with
VIRTIO_GPU_CMD_MEMORY_CREATE,
> > > then go attach multiple resources to it.
> > >
> > > > If so, do we want the option for the guest allocate?
> > >
> > > Allocation options are handled by VIRTIO_GPU_CMD_MEMORY_CREATE
> > > (initially guest allocated only, i.e. what virtio-gpu supports
today,
> > > the plan is to add other allocation types later on).
> >
> > You want to cover Vulkan, host-allocated dma-bufs, and guest-allocated
> > dma-bufs with this, correct? Let me know if it's a non-goal :-)
>
> 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?
>
> > If so, we might want to distinguish between memory types (kind of like
> > memoryTypeIndex in Vulkan). [Assuming memory_id is like resource_id]
>
> For the host-allocated buffers we surely want that, yes.
> For guest-allocated memory regions it isn't useful I think ...
>
Guest-allocated memory regions can be just another memory type.
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.
>
> > 1) Vulkan seems the most straightforward
> >
> > virtio_gpu_cmd_memory_create --> create kernel data structure,
> > vkAllocateMemory on the host or import guest memory into Vulkan,
> > depending on the memory type
> > virtio_gpu_cmd_resource_create_v2 --> vkCreateImage +
> > vkGetImageMemoryRequirements on host
> > virtio_gpu_cmd_resource_attach_memory --> vkBindImageMemory on
host
>
> Yes.
>
> Note 1: virtio_gpu_cmd_memory_create + virtio_gpu_cmd_resource_create_v2
> ordering doesn't matter, so you can virtio_gpu_cmd_resource_create_v2
> first to figure stride and size, then adjust memory size accordingly.
>
> Note 2: The old virtio_gpu_cmd_resource_create variants can be used
> too if you don't need the _v2 features.
>
> Note 3: If I understand things correctly it would be valid to create a
> memory pool (allocate one big chunk of memory) with vkAllocateMemory,
> then bind multiple images at different offsets to it.
>
> > 2) With a guest allocated dma-buf using some new allocation library,
> >
> > virtio_gpu_cmd_resource_create_v2 --> host returns metadata
describing
> > optimal allocation
> > virtio_gpu_cmd_memory_create --> allocate guest memory pages since
> > it's guest memory type
> > virtio_gpu_cmd_resource_attach_memory --> associate guest pages
with
> > resource in kernel, send iovecs to host for bookkeeping
>
> virtio_gpu_cmd_memory_create sends the iovecs. Otherwise correct.
>
> > 3) With gbm it's a little trickier,
> >
> > virtio_gpu_cmd_resource_create_v2 --> gbm_bo_create_with_modifiers,
> > get metadata in return
>
> Only get metadata in return.
>
> > virtio_gpu_cmd_memory_create --> create kernel data structure, but
> > don't allocate pages, nothing on the host
>
> Memory allocation happens here. Probably makes sense to have a
> virtio_gpu_cmd_memory_create_host command here, because the parameters
> we need are quite different from the guest-allocated case.
>
If we follow Vulkan, we only need the size and the memory type in most
cases. The current version of memory_create is a special case because it
is an import operation and needs the guest mem_entry. Perhaps
memory_create (for host) and memory_import_guest (to replace the current
version)?
>
> Maybe we even need a virtio_gpu_cmd_memory_create_host_for_resource
> variant, given that gbm doesn't have raw memory buffers without any
> format attached to it.
>
And the memory will only be attachable to the given (or compatible)
resource, right?
Vulkan is much more explicit than any pre-existing API. I guess we will
have to add this to cover APIs beyond Vulkan.
>
> > > > > +/* VIRTIO_GPU_RESP_OK_RESOURCE_INFO */
> > > > > +struct virtio_gpu_resp_resource_info {
> > > > > + struct virtio_gpu_ctrl_hdr hdr;
> > > > > + __le32 stride[4];
> > > > > + __le32 size[4];
> > > > > +};
> > > >
> > > > offsets[4] needed too
> > >
> > > That is in VIRTIO_GPU_CMD_RESOURCE_MEMORY_ATTACH ...
> >
> > I assume the offsets aren't returned by
> > VIRTIO_GPU_CMD_RESOURCE_MEMORY_ATTACH.
>
> Yes, they are send by the guest.
>
Gurchetan probably means alignment[4].
>
> > How does the guest know at which offsets in memory will be compatible
> > to share with display, camera, etc?
>
> Is is good enough to align offsets to page boundaries?
>
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.
>
> > Also, do you want to cover the case where the resource is backed by
> > three separate memory regions (VK_IMAGE_CREATE_DISJOINT_BIT)?
>
> Good point. I guess we should make memory_id in
> virtio_gpu_cmd_resource_attach_memory an array then,
> so you can specify a different memory region for each plane.
>
> cheers,
> Gerd
>
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