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Re: [Xen-devel] [PATCH v10 05/11] x86/entry: Organise the use of MSR_SPEC_CTRL at each entry/exit point

On 25/01/18 14:36, Jan Beulich wrote:
>>>> On 25.01.18 at 15:12, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 25/01/18 13:08, Jan Beulich wrote:
>>> It may also be worthwhile again to pull up the zeroing of %edx,
>>> using %dl instead of $0 in the movb (and maybe then also
>>> similarly in DO_SPEC_CTRL_EXIT_TO_XEN, but there I'm less
>>> certain it couldn't have a negative effect).
>> What negative effects are you worried about?  These macros are self
>> contained.
> The result of the xor then is an input to the cmp, which may be
> one cycle more latency than with the immediate zero.

Why?  Cmp writes all flags and reads none of them, so doesn't have any
flags dependency on earlier instructions.

It is only instructions which partially preserve older flag bits (and
where undefined behaviour is complicated) which suffer flags-merge


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