Re: [PATCH 1/5] x86/xstate: Elide redundant writes in set_xcr0()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.05.2021 17:39, Andrew Cooper wrote:
> XSETBV is an expensive instruction as, amongst other things, it involves
> reconfiguring the instruction decode at the frontend of the pipeline.
> 
> We have several paths which reconfigure %xcr0 in quick succession (the context
> switch path has 5, including the fpu save/restore helpers), and only a single
> caller takes any care to try to skip redundant writes.
> 
> Update set_xcr0() to perform amortisation automatically, and simplify the
> __context_switch() path as a consequence.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>