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

On Thu, Jun 27, 2013 at 10:35:58AM -0600, Don Dugger wrote:
> On Mon, Jun 17, 2013 at 10:27:36AM +0200, Jiri Denemark wrote:
> > On Fri, Jun 14, 2013 at 12:32:35 -0600, Don Dugger wrote:
> > > On Fri, Jun 14, 2013 at 03:06:52PM +0200, Jiri Denemark wrote:
> > > > 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.
> > > 
> > > The specific use case I'm trying to address is a cloud environment where,
> > > with hundreds of hosts, you might want to schedule an instance to a host
> > > that supports a particular HW acceleration, like AES/NI.  I agree, what
> > > I `really` want is an API that shows the capabilities of a specific guest
> > > that could be created on the host but, absent that API, at least knowing
> > > that a host doesn't support the feature is more information than I can get
> > > right now.
> > 
> > Hmm, fair enough. Let's wait a few days for Daniel to return from
> > vacation in case he wants to express his opinion here.
> So, any progress here?

I believe the cloud use case above is approaching the problem in the wrong
way. We designed our APIs such that apps should never need to write logic
for comparing CPU features directly. If an application has a set of CPU
features it requires from the host, then it should use a libvirt API to
query whether those features are available, not try to write the CPU
fetaure comparison logic itself.

You can already pretty much do this with te virConnectCompareCPU()
method, by passing an XML document which specifies the AES/NI feature
flag that you want to check for support of. Then libvirt will tell
you whether the host CPU can support it. It is entirely possible to
make use of this capability as is in OpenStack.

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