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

[Tim Deegan <tim@xen.org>]

Hi,

At 14:59 +0100 on 22 Oct (1350917960), Tim Deegan wrote:
> At 19:21 +0200 on 20 Oct (1350760876), Peter Maloney wrote:
> > 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. 

In fact this is to do with the APIC.  On my test system, a busy 2-vcpu
VM is making about 300k/s accesses to the APIC TPR.  These accesses are
all trapped and emulated by Xen, and that emulation has got more
expensive as part of this change.

Later Windows OSes have a feature called 'lazy IRQL' which makes those
accesses go away, but sadly that's not been done for WinXP.  On modern
Intel CPUs, the hardware acceleration for TPR accesses works for XP; on
AMD it requires the OS to use 'MOV reg32, CR8' to access the TPR instead
of MMIO, which XP is clearly not doing. :(

Peter: if you have the option, you might find that installing the PV
drivers that ship with Citrix XenServer 6.0 makes things work better.

Andres: even though this load of APIC emulations is pretty extreme, it's
surprising that the VM runs faster on shadow pagetables!  Any ideas for
where this slowdown is coming from?

Cheers,

Tim.