Re: [Xen-devel] [PATCH 1/4] hvm/fep: Allow testing of instructions crossing the -1 -> 0 virtual boundary

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.09.16 at 16:11, <andrew.cooper3@xxxxxxxxxx> wrote:
> The Force Emulation Prefix is named to follow its PV counterpart for cpuid or
> rdtsc, but isn't really an instruction prefix.  It behaves as a break-out into
> Xen, with the purpose of emulating the next instruction in the current state.
> 
> It is important to be able to test legal situations which occur in real
> hardware, including instruction which cross certain boundaries, and
> instructions starting at 0.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

While you did mostly convince me at that time, I've got some more
concerns here: What if the instruction to be emulated causes a
fault that then needs to be propagated to and handled by the
guest, before it can be restarted? Such a fault would be raised
with rIP pointing past the forced emulation prefix, and hence the
restarted instruction then wouldn't get emulated.

Along those line, if you don't want to treat this as an instruction
prefix, there ought to be two #DB due to instruction breakpoint
match (if set for both places, of course), yet that's impossible to
implement together with the desire to emulate the insn.

Jan


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