Re: [PATCH 05/16] KVM-HDR: Implement wallclock over KVM - KVM Virtual Memory

[Avi Kivity <avi@redhat.com>]

On 01/26/2011 02:20 PM, Glauber Costa wrote:
> On Wed, 2011-01-26 at 13:13 +0200, Avi Kivity wrote:
> >  On 01/24/2011 08:06 PM, Glauber Costa wrote:
> >  >  As a proof of concept to KVM - Kernel Virtual Memory, this patch
> >  >  implements wallclock grabbing on top of it. At first, it may seem
> >  >  as a waste of work to just redo it, since it is working well. But over the
> >  >  time, other MSRs were added - think ASYNC_PF - and more will probably come.
> >  >  After this patch, we won't need to ever add another virtual MSR to KVM.
> >  >
> >
> >  So instead of adding MSRs, we're adding area identifiers.  What did we gain?
>
> * No risk of namespace clashes of any kind,
> * less need for userspace coordination for feature enablement,

That's a bug, not a feature.

> * more robust mechanism that can do discovery even on early boot,

cpuid/wrmsr should be robust enough.

> * more informative result values can be passed on to guest kernel,

True.

> * more flexibility, since we go back to userspace if we can't handle
> some request. Also, some requests are better handled by userspace
> anyway. But again, maybe this is a separate issue here...

Yes.

> * size information goes together with base, allowing for extending
> structures (well, maybe I should add versioning explicitly?)
>

We could do that as well with wrmsr, by having the size as the first 
field of the structure.  Usually the size isn't really interesting, 
though, since you need to discover/enable the new features independently.

-- 
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