Re: [Qemu-devel] qvm86, kqemu and video speed

[Leonardo E. Reiter]

Just a couple of quick thoughts:

1. we too have discovered that the VGA routines are a huge bottleneck, 
even if KQEMU is used.  As Fabrice points out, this is clearly because 
there is virtually no blitting optimization going on, along with the 
memory copies, etc.

2. I think any solution has to be broad enough to be portable to other 
architectures, such as Solaris, or Linux on other platforms.  It would 
be nice to have it conditionally compile optimizations that only work on 
Linux, or only with KQEMU, or only with XFree86, etc.  But it would also 
be great if some basic optimization could be done on the pixel 
translation routines that would be portable to any platform/X server 
combination.

3. I think a really simple optimization may be to inline the glue 
functions in vga_template.h, cirrus_vga_rop.h, and cirrus_vga_rop2.h, 
which is very trivial.  We tried that a while back and it did improve 
performance a bit - for instance, it shaved 1.5 seconds off the boot 
time of a Windows 2000 session.  Windows 2000 likes to display heavy 
graphics, like marquees, etc., while booting in its status dialog boxes, 
which is why the improvement was there I think.  Maybe Fabrice or 
someone else can comment as to the possible consequences, other than the 
obvious code size increase of using inline functions (which is not much 
in this case) of inlining those functions.  We didn't notice anything 
adverse, but perhaps we weren't looking closely enough :)  We will keep 
testing over here, and if all goes well, post a patch that does this 
simple optimization.  That is, unless anyone can chime in with a good 
reason not to do this of course.

4. optimizations which require SDL should be conditionally configured in 
case SDL is not used.  For example, in the future there may be GTK-based 
GUIs, etc.  I think there is enough basic optimization that can be done 
in the VGA routines themselves to gain good performance even without 
SDL-specific hacks.  Of course SDL-specific hacks can (and probably 
should) still be used if SDL is configured in, since this is the default 
way to run QEMU.

My $0.02,

Leo Reiter

Hetz Ben Hamo wrote:
> Anyone thought about using XFree's Xv extension? last time I heard, it
> works with all the cards, supported well under XFree, and I think it
> will be better working rather with DGA..
>
> At least with my experience with regarding Video players, Xv was much
> better than DGA.
>
> Thanks,
> Hetz

--
Leonardo E. Reiter
Vice President of Engineering

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