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



On Fri, Jun 28, 2013 at 10:24:48AM +0100, Daniel P. Berrange wrote:
> 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.

I don't think this would work with the way scheduling in OpenStack works.
The problem is that the OpenStack scheduler doesn't want to query each node
in the system on every schedule request (with 100s if not 1,000s of compute
nodes this would not be practical).  Instead the scheduler maintains info
about all of the compute nodes and, when a request comes in, the scheduler
identifies the best compute node for the request and then causes the VM
to be started on that node.  Apriori the scheduler doesn't even know which
CPU features users are interested in, that information only becomes available
when a schedule request comes in so trying to do a `virConnectCompareCPU()'
call at that point in time is too late.

Having said all of that I still think that conceptually the current
`virConnectGetCapabilities()' is wrong.  If the API is going to list any
CPU features is should list all of them.  Any user of the API that cares
about CPU features probably cares about all of the features and a user that
doesn't care about CPU features will just ignore that part of the definition
anyway.

> 
> Daniel
> -- 
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> 

-- 
Don Dugger
"Censeo Toto nos in Kansa esse decisse." - D. Gale
n0ano n0ano com
Ph: 303/443-3786


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