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

To: Steve Ofsthun <sofsthun@xxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [RFC] Hypercalls from HVM guests
From: Anthony Liguori <aliguori@xxxxxxxxxx>
Date: Fri, 07 Apr 2006 15:22:27 -0500
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Mats@xxxxxxxxxx, Petersson@xxxxxxxxxxxxx
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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.


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?


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