Re: xen-unstable, winxp32 very poor performance on AMD FX-8150, I bisected and changeset is 24770:7f79475d3de7

[Tim Deegan <tim@xen.org>]

At 19:21 +0200 on 20 Oct (1350760876), Peter Maloney wrote:
> I ran a bisect to find out when Windows XP 32 bit becomes unusably slow.
> And I found the changeset that caused it.
> 
> ==========
> The problem:
> ==========
> 
> Windows 8 64 bit and 32 bit run fast and fine in the newest xen versions.
> 
> Windows XP 32 bit runs unusably slow in anything new that I built from
> xen-unstable, but runs fast in 4.1.2 and 4.1.3 stable. While it is
> running slow, "xm top" or "xl top" show cpu usage around 650% for the domu.
> 
> The bug might be AMD specific. I'm running an AMD FX-8150.

The bug does seem to be AMD-specific, and NPT-specific; with
'hap=0' it goes much faster. 

> ==========
> The result:
> ==========
> 
> good: 24769:730f6ed72d70
> bad: 24770:7f79475d3de7
> 
> The change was 8 months ago
> 
> changeset:   24770:7f79475d3de7
> user:        Andres Lagar-Cavilla <andres@lagarcavilla.org>
> date:        Fri Feb 10 16:07:07 2012 +0000
> summary:     x86/mm: Make p2m lookups fully synchronized wrt modifications

This change was bad for performnace across the board and most of it has
since been either reverted or amended, but clearly we missed something
here. 

It's interesting that Win8 isn't slowed down.  I wonder whether that's to
do with the way it drives the VGA card -- IIRC it uses a generic VESA
driver rather than a Cirrus one. 

Tim.