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

[Avi Kivity <avi@redhat.com>]

On 01/26/2011 05:45 PM, Glauber Costa wrote:
> On Wed, 2011-01-26 at 17:17 +0200, Avi Kivity wrote:
> >  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.
>
> I don't see why.
> I's about feature enablement, not feature discovery.

Well, "zero userspace coordination" would be a bug, since it would 
remove userspace-controlled discovery.  Since the userspace patches for 
these types of features are usually very small, "less coordination" 
doesn't buy us much.

> >
> >  >  * 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.
>
> Which structure? For msrs, we're usually going for just an u64, but of
> course we could change that when needed.

It's usually a physical address of a structure (together with an enable 
bit).

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