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Re: [libvirt] [PATCH V2] Expose all CPU features in host definition

Hi Don,

I'm sorry for the delay...

On Fri, May 31, 2013 at 13:04:17 -0600, Don Dugger wrote:
> On Thu, May 30, 2013 at 01:48:32PM +0200, Jiri Denemark wrote:
> > On Tue, May 28, 2013 at 16:11:57 -0600, Don Dugger wrote:
> > > The virConnectGetCapabilities API describes the host capabilities
> > > by returning an XML description that includes the CPU model name
> > > and a set of CPU features.  The problem is that any features that
> > > are part of the CPU model are not explicitly listed, they are
> > > assumed to be part of the definition of that CPU model.  This
> > > makes it extremely difficult for the caller of this API to check
> > > for the presence of a specific CPU feature, the caller would have
> > > to know what features are part of which CPU models, a very
> > > daunting task.
> > >
> > > This patch solves this problem by having the API return a model
> > > name, as it currently does, but it will also explicitly list all
> > > of the CPU features that are present.  This would make it much  
> > > easier for a caller of this API to check for specific features.
> > 
> > It's actually the desired behavior and not a bug. We went that way for
> > several reasons. First, given how QEMU/KVM works, the fact that a
> > CPU feature is detected by libvirt in host CPU and reported in
> > capabilities XML does not mean that the feature will be available to
> > guests. This is the reason why we have virConnectCompareCPU. You can
> > create a guest CPU definition you would like to see in a guest and call
> > virConnectCompareCPU on it to check if that guest CPU can be run on that
> > particular host. And providing all features in capabilities XML would
> > just make the XML larger with no practical benefit.
> The fact that host CPU features at not necessarily available to a guest
> has no impact on whether or not we report all features that the host
> supports, that issue remains in either case.

I was just trying to say that it doesn't provide anything more than
"it's supported by the host CPU", which gives mostly no value in the
context of libvirt. Can you explain more what the use case is in which a
virt client would need to know what specific feature are supported by
host CPU? I feel like we should avoid people from being under the
impression that they can actually use the CPU capabilities for checking
whether a host can run guests that require specific CPU features.

Moreover, there's one thing that makes this issue even more complicated.
As the CPU model is the most useful part of host CPU capabilities (it
should be the best CPU model supported by the host), I wanted to extend
the probing code to select the right model even if it's missing some
features that we expect to be supported by that model. In other words,
if host CPU is, e.g., SandyBridge but has some basic feature disabled,
we would detect it as the best model which did not have the disabled
feature, which is not optimal. We'd ideally detect the CPU as
SandyBridge with just the feature disabled. That is, another reason for
having feature list relative to the CPU model. On the other hand, it
might be hard or even impossible to do without breaking compatibility
and perhaps even doubtful without involving emulator, which would make
it impossible to do within capabilities XML.


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