Re: [Xen-devel] [PATCH] x86/entry: Use 32bit xors rater than 64bit xors for clearing GPRs

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 14.02.18 at 14:14, <andrew.cooper3@xxxxxxxxxx> wrote:
> Intel's Silvermont/Knights Landing architecture treats them as full ALU
> operations, rather than zeroing idoms.
> 
> No functional change, and no change in code volume (only changing the bit
> selection in the REX prefix).
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> 
> If anyone is interested, <20180211104949.12992-5-linux@xxxxxxxxxxxxxxxxxxxx>
> is the LKML discusson on the subject.  It is most likely that this is a
> deliberate simplification in the Knights* architecture because compilers
> follow optimisation instructions of "use xors for zeroing" and "use 32bit
> operations in preference to 64bit ones wherever possible".

Hmm, we can only trust what is being said there, which is not very
nice. Or wait, no, the ORM actually says so for Silvermont.

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan


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