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

Avi Kivity
2010-03-16T06:12:05Z

On 03/16/2010 02:20 AM, Jason wrote:
>
> 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.

Ugh, these should definitely be eliminated, they keep writing the same
value most of the time.

> Also, KVM has a
> lot of MSR accesses to 0xc0000081-0xc0000084 that Xen doesn't have.
>

These are unavoidable. Those msrs are used for system calls and we need
them to keep ordinary userspace going.

2.6.33 should reduce their frequency though.

Usually it doesn't make sense to pass them through, but if we detect the
guest is writing them often, we can do so and eliminate the exits.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.