Re: [Xen-devel] AMD VMMCALL and VM86 mode

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 12/09/2016 02:01 PM, Andrew Cooper wrote:
> Hello,
>
> While working on XSA-192, I found a curious thing.  On AMD hardware, the
> VMMCALL instruction appears to behave like a nop if executed in VM86
> mode.  All other processor modes work fine.
>
> The documentation suggests it should be valid in any situation, but I
> never get a #VMEXIT from it. 

And I assume GENERAL2_INTERCEPT_VMMCALL is set (which is what we have in
Xen by default)?

What happens if you don't set it?

-boris


>  Thus, I would have thought it would fall
> into the un-intercepted category and raise a #UD fault, but I don't get
> that either.
>
> Is this behaviour expected?  The documentation would certainly seem to
> indicate not.




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