Re: [Qemu-devel] X support for QXL and SPICE

[Anthony Liguori]

Soeren Sandmann wrote:
> Hi,
>
> Here is an overview of what the current QXL driver does and does not
> do.  The parts of X rendering that are currently being used by cairo
> and Qt are:
>
> - Most of XRender 
>         - Image compositing
>         - Glyphs
>   

Does anything use Xrender for drawing glyphs these days?

Certainly, with a compositing window manager, nothing is getting 
rendered by X...

> The X driver for the QXL device is not yet very sophisticated. What it
> does is basically this:
>
>         - It keeps a separate memory framebuffer uptodate using
>           software
>
>         - Solid fills and CopyArea requests are turned into SPICE
>           commands.
>
>         - The cursor is drawn using cursor commands
>
>         - For other things, bitmaps are sent across the wire
>                 - It uses the hashing feature of SPICE to only send
>                   hashcodes for those bitmaps if it can get away with
>                   it.
>
> Even this simple support provides a better user experience than VNC
> because scrolling is accelerated and doesn't result in a huge bitmap
> getting sent across the wire.

Scrolling is accelerated in VNC.  In the Cirrus adapter, both X and 
Windows use a video-to-video bitblt, we use this to create a VNC 
CopyRect which makes scrolling and Window movement smooth.

> However, as things stand right now, there is not much point in adding
> this support, because X applications essentially always work like
> this:
>
>         - render to offscreen pixmap
>         - copy pixmap to screen
>
> There is not yet support for offscreen pixmaps in SPICE, so at the
> moment, solid fill and CopyArea are the two main things that actually
> make a difference.
>   

Okay, that's in line with what my expectations were.  So what's the 
future of Spice for X?  Anything clever or is Windows the only target 
right now?

Regards,

Anthony Liguori