Re: [Qemu-devel] [Spice-devel] [PATCH Qemu] Change spice-server protocol for GL texture passing

[Marc-André Lureau]

Hi

----- Original Message -----
> > 
> > Hi
> > 
> > ----- Original Message -----
> > > Forgot to add RFC to the subject
> > > 
> > 
> > What's the rationale? if you share the texture id, you must share the GL
> > context too, right? Why not use a lower level dmabuf fd that can be
> > imported
> > by the server gl context (which is also what the protocol require anyway)?
> > 
> 
> Yes, the display and context are shared using spice_qxl_gl_init.
> Importing again into a gl context would mean that you have to export the
> DRM prime and import again in a separate (not shared) context.
> It's also doable, just add 2 system call and wrapping/unwrapping.
> Would be good to pass the EGLDisplay then so spice-server don't have to
> initialize again possibly using another physical card.
> 
> We have 4 cases:
> - client not connected;
> - local client;
> - remote client, software encoding;
> - remote client, hardware encoding.
> 

Before optimizing those syscalls and changing API etc, I would like to know if 
they are expensive (it's not my feeling)

Also, it is possible virglrenderer could be optimized to avoid exporting the 
prime fd for each scanout, if the backing image is always the same.

Sharing a GL context brings new issues. If spice server could use its own 
context, we have some context isolation (gl is still bad at MT iirc).

> Client not connected
> Passing the texture is a no-operation, passing DRM prime require to
> extract the handle and close every frame.
> 
> Local client
> In this case there is no overhear, DRM prime is always extracted and
> passed to the client
> 
> Remote client, software encoding
> Due to different problems (DRM prime not mmap-able or data not portably
> extractable) we'll need to import the DRM prime into a different EGL
> context (not shared with the original one), create another texture,
> extract data and free all texture/DRM prime.

I don't think we have strong reasons to support software encoding, video 
encoding is really expensive, and that mmap/copy is not going to be marginal, 
so even less these 2 syscalls.

> 
> Remote client, hardware encoding
> It's not clear if it's better to pass the DRM prime or the texture,
> some API pass the texture. I got confirmation that gst_dmabuf_allocator_new
> could try to use mmap in some cases so we should check this somehow
> to make sure it does not.
> 

We definitely don't want any mmap/copy to take place for hw encoding.

> Taking into account that DRM prime came with "free" reference counting
> creating the DRM prime from texture basically increase a counter which is
> used by our implementation to make sure texture is still existing so
> possibly passing texture instead of DRM prime just save a system call
> in the normal case. I don't know what happens to the DRM object handle when
> the texture is destroyed (in Qemu) with glDeleteTextures if bindings keep
> texture "alive" or are all reset.
> 
> 
> Could be that keeping qemu_spice_gl_scanout and spice_qxl_gl_scanout_texture
> as current implementation and adding a spice_qxl_gl_init/spice_qxl_gl_setup
> passing just QXLInstance and EGLDisplay is a better solution.
> 
> Does is sound reasonable?

I wouldn't rush with API changes before we have a better idea how hw encoding 
can be done without mmap and wether its really worth it (I would rather see 
spice spawning a seperate gl context and process for the encoding than sharing 
it)