[Qemu-devel] Re: [PATCH] Reducing X communication bandwidth, take 2

[Brian Johnson <bjj4@charter.net>]

Paul Brook wrote:
>>> Will this work also for the CL542x adaptor?  (Does that fall in the
>>> category of vga?)  My current hack works for with/without -std-vga and I
>>> think that's because it lives "underneath" both, in the connection to
>>> SDL.
>> Each adapter will have to do it's own minimization but that's sort of
>> the write thing anyway IMHO.  How granular each update is really only
>> depends on the adapter.  For instance, the VMware adapter really
>> shouldn't need to do any minimization at all.
> 
> It would be nice if we could share the framebuffer blitting routines. We've 
> currently got 3 different implementations (vga/cirrus, tcx and pl110) of 
> basically the same framebuffer rendering routines.

Take a look at the video code in BasiliskII / SheepShaver, a 68k/PPC 
classic MacOS emulator written by Gwenolé Beauchesne:

http://gwenole.beauchesne.info/projects/sheepshaver/

It contains optimized code (source level) for blitting between various 
bit depths and endiannesses.  See 
SheepShaver-2.3/src/Unix/video_blit.{h,cpp} in the sources.

It also uses a technique called "video on segfault" (VOSF) to improve 
performance on platforms which support it:  rather than testing each 
store to see if it modifies the framebuffer, it keeps the framebuffer 
write-protected (via mprotect(), or the equivalent on non-POSIX systems) 
and uses a SIGSEGV handler to catch stores to the buffer.  When a page 
receives a store, the handler unprotects the page and updates a bitmap 
of modified pages.  Every so often a display update thread wakes up, 
consults the bitmap, calculates the updated region, blits it to the 
screen (using the optimized blitters), and clears the bitmap.  See 
SheepShaver-2.3/src/Unix/video_vosf.h, 
SheepShaver-2.3/src/Unix/video_x.cpp, and other files.

The emulators also have alternative techniques for tracking update 
regions on systems for which VOSF is not supported.  But VOSF is almost 
always a big win.  And most modern OSes can support it without trouble.

The code is in C++ so it can't be dropped in directly, but Some of the 
techniques may be useful in qemu.


Brian J. Johnson