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

On 7 Apr 2006, at 18:24, Petersson, Mats wrote:

Good question - the way I'd say is to look at CPUID to see if it's
"GeunineIntel" or "AuthenticAMD", but I'm not sure if that's the BEST.
Of course, this assumes the code is already aware that it's in a HVM
environment, which I'm not sure if you know that or not at the point you need to know if it's AMD or Intel... Of course, if CPUID is intercepted,
it may return other things (but it's against the "rules" to lie about
the brand of the CPU!)

I like the idea of stealing some MSR space for this, and doing some initial interaction with the underlying hypervisor platform via RDMSR/WRMSR to known MSRs. We could 'read' an 'MSR' that would tell us the correct instruction sequence to do a hypercall (either directly, or maybe tell us a 'physical address' to read the hypercall transport information from -- then we could have a hypercall transfer page just as we already do for paravirtualised guests).

We just need to pick some MSRs that won't get used by Intel or AMD in the future. There's quite a lot of addressing space to carve up though. :-)

 -- Keir

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