On March 21, 2019 10:25:05 AM PDT, Denys Vlasenko <dvlas...@redhat.com> wrote: >On 3/18/19 7:10 PM, Linus Torvalds wrote: >> On Mon, Mar 18, 2019 at 10:51 AM Peter Zijlstra ><pet...@infradead.org> wrote: >>> >>> How about I do a patch that schedules EFLAGS for both 32bit and >64bit, >>> mark this for backporting to infinity. >>> >>> And then at the end, after the objtool-ac bits land, I do a patch >>> removing the EFLAGS scheduling for x86_64. >> >> Sounds sane to me. >> >> And we can make it AC-conditional if it's actually shown to be >visible >> from a performance standpoint. >> >> But iirc pushf/popf isn't really that expensive - in fact I think >it's >> pretty cheap when system flags don't change. > >I did not see evidence of this. In my testing, >POPF is always ~20 cycles, even if popped flags are identical to >current >state of flags.
I think you will find that if you change system flags it is much slower. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
