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Re: [Xen-devel] [RFC] Hypercalls from HVM guests

On 9 Apr 2006, at 00:33, Nakajima, Jun wrote:

If eax is set to a value outside the recognized range of CPUID currently
defined, CPUID does not necessarily return all zero's on Intel. It's
"Reserved" (Information returned for highest basic information leaf).
Also "an unused index" can have conflicts in the future.

If we just need to tell on which CPU the current HVM guest is running, I
think "GeunineIntel" or "AuthenticAMD" is the best because it's been
used by native systems as well.

Then how do you tell whether you are running on a hypervisor without executing some instruction that might fault? We would like to avoid requiring that.

There is quite a lot of CPUID and MSR address space, and a random return value for CPUID when running natively is practically-speaking fine if we're looking for a 128-bit signature.

The only other option I think would be a BIOS table, probably below 1MB. I prefer the CPUID/MSR method.

 -- Keir

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