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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 8 Apr 2006, at 15:12, Andi Kleen wrote:

> > > This sounds encouraging, but is CPUID always trapped by the HVM code?
> > It can be, and in practise yes it is so this could work.
> CPUID doesn't have any advantage over MSRs for this purpose because
> for custom CPUIDs like 0xb... you can't use the normal "max count" 
> mechanism
> of determining if a CPUID is supported.  All that would work is to try
> it and handle the GPF if it didn't work. That would give the same ugly
> implementation as with MSRs.
CPUID never faults. Well, unless the processor doesn't support the 
instruction, but you find that out from EFLAGS.
> Using the MSR would have the advantage of it being trappable in a para 
> virtual
> kernel too.
I would definitely prefer to use MSRs for gathering hypervisor 
signature and other information, but because of the possible hassle of 
catching faults I'd also support a signature return (and maybe 
identifying some hypervisor features) via a special CPUID index. The 
index could be greater than the normal "max count" and you'd determine 
if the index(es) were supported by checking for a well-known signature 
in EAX,EBX,ECX,EDX for the first of the hypervisor indexes. Running on 
native hardware would not fault and you'd expect it to be vastly 
unlikely that the final values of EAX thru EDX would  coincidentally 
match a 128-bit signature.
(Cue attempts to think up a 16-character signature string. :-)

 -- Keir


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