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

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Date:	Wed, 26 Jan 2011 13:45:04 -0200
From:	Glauber Costa <glommer@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	aliguori@...ibm.com
Subject: Re: [PATCH 05/16] KVM-HDR: Implement wallclock over KVM - KVM
 Virtual Memory

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. 

> > * 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.

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



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