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Re: [libvirt] cpu flags



On Tue, Sep 16, 2008 at 04:45:09PM -0400, Ben Guthro wrote:
> Hi,
> 
> We're finding that we are going to be needing the cpu flags (as reported 
> in /proc/cpuinfo)
> ..specifically to find out if we are a vmx enabled machine.
> 
> So - off I went looking into this for a patch to submit upstream.
> Unfortunately, I ran into some questions which need answering before I 
> really proceed with this
> 
> It seems to me that this info would best be parsed in src/nodeinfo.c
> This is where other cpuinfo things are parsed...and stored in the 
> nodeinfo struct
> Perhaps we store this as a bitmask encoded int, as defined in 
> /usr/include/asm/cpufeature.h and tack this onto the end of sad struct.
> 
> My concern is that adding to the nodeinfo struct breaks the API - such 
> that the structs will be different sizes between versions.

Indeed - any struct or API in include/libvirt/libvirt.h is immutable
to preserve ABI.

> Also - this seems to be x86 specific. Are we primarily destined for x86? 
> Or would this type of change be unacceptable due to not working on PPC, 
> for example?

No,we need to work on every architecture - KVM for example is ported to
x86, ia64, ppc, and s390, Xen is on arm, x86, ia64. Now the set of flags
will likely be different across each processor - and some may not even
have a concept of flags, but we shouldn't be specifically x86 in API.

I think the most likely place for exposing CPU flags would be in the
capabilities XML format. We do in fact already expose 3 flags there,
PAE, VMX and SVM. 

As John mentions we also need to be careful to specify a naming that is
consistent across OS - though that may be as simple as defining that 
the naming is required to match Linux /proc/cpuinfo standards. The intent
is that if you have the same CPUs in 2 machines, one running Linux and 
the other Solaris then libvirt must report the same names.

Regards,
Daniel
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