Re: [RFC] Hypercalls from HVM guests

[Anthony Liguori <aliguori@us.ibm.com>]

Steve Ofsthun wrote:
> Petersson, Mats wrote:
>
>
> This sounds like a simple to use method for communicating with the HVM 
> code,
> but I would like to gracefully detect native execution and print a useful
> error message at module load time.  Recovering from a native mode 
> exception
> will be very O/S specific (if allowed at all).

PIO is probably a bit safer.  PIO probing, while a bit ugly, is 
certainly frequently used.

Plus, PIO instructions are not sensitive (whereas CPUID is) which means 
that it can also be used as a mechanism for non-HVM guests.

Regards,

Anthony Liguori

>> Actually, maybe using an unused index for CPUID (e.g. 0xb0000000) would
>> be better? As that's defined to return all zero's, and not cause any
>> traps whatever value you use (unless the CPU is so old that it doesn't
>> support CPUID, of course).
>
> This sounds encouraging, but is CPUID always trapped by the HVM code?
>
> Steve