Re: [Qemu-devel] Ideas wiki for GSoC 2010

Jason
2010-03-16T00:25:05Z

Avi Kivity <avi <at> redhat.com> writes:
>
> On 03/15/2010 03:23 PM, Anthony Liguori wrote:
> > On 03/15/2010 08:11 AM, Avi Kivity wrote:
> >> Your guest wasn't doing a zillion VMREADs and VMWRITEs every exit.
> >>
> >> I plan to reduce VMREAD/VMWRITE overhead for kvm, but not much we can
> >> do for other guests.
> >
> > VMREAD/VMWRITEs are generally optimized by hypervisors as they tend to
> > be costly. KVM is a bit unusual in terms of how many times the
> > instructions are executed per exit.
>
> Do you know offhand of any unnecessary read/writes? There's
> update_cr8_intercept(), but on normal exits, I don't see what else we
> can remove.
>

In comparing KVM 2.6.31.6b to XenServer 5.5.0, it seems KVM has fewer overall
VMREADs and VMWRITEs, but there are a lot of VMWRITEs to Host FS_SEL, Host
GS_SEL, Host FS_BASE, and Host GS_BASE that don't appear in Xen. Also, KVM has a
lot of MSR accesses to 0xc0000081-0xc0000084 that Xen doesn't have.