Re: [Xen-devel] [PATCH] x86emul: minor cleanup

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.06.17 at 19:50, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 08/06/17 16:49, Jan Beulich wrote:
>> Drop a redundant input constraint, correct a comment, and (re)move
>> fix.insn_bytes adjustments (these aren't needed for custom stub
>> invocations when the instruction placed in the stub can't raise #XF)
> 
> I'm not sure these are wise to remove.  Even if we don't expect an
> exception, should one occur, fpu_handle_exception() will fail to step
> over the instruction, and will re-execute it.

Ah, perhaps I shouldn't have split this off the remaining
emulator series I have ready - you refer to a no longer
existing function (in my code base). Once there, do_trap()
will panic() as usual in that case, which I think it is sort of
appropriate if we receive an exception that shouldn't occur -
after all we then don't really know what to do with it. This
btw goes along the lines of me not really being happy about
us handling all sorts of exceptions once an .ex_table entry
is associated with an instruction, rather than just the ones
we really mean to recover from. You may recall such a
discussion from a few years back.

Would you be okay with temporarily adding a respective
BUG_ON(!fic->insn_bytes) to fpu_handle_exception() to
achieve the same effect?

As a side note, I'm removing these here since the further
SIMD emulation patches I have ready, but would prefer to
post only once 4.9 is out, do not add respective code in the
first place. Without knowing this in advance I'm not even
sure this would be reliably spottable during review.

Jan


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