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

From: Avi Kivity
Date: Wed Jan 26 2011 - 10:17:13 EST

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,


* 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...


* 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.

error compiling committee.c: too many arguments to function

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