Re: RFC: Petition Intel/AMD to add POPF_IF insn

[Linus Torvalds]

On Wed, Aug 17, 2016 at 2:43 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
> It shouldn't be *too* bad, since xen_restore_fl only affects "IF".
> And even if native_restore_fl needs to be able to turn IRQs off as
> well as on, we can just do:
>
> if (likely(flags & X86_EFLAGS_IF))
>   sti();
> else
>   cli();
>
> at some cost to code size but hopefully little to no runtime cost for
> the sane cases.

No, that would be horrible for the case where we had an irq spinlock
in an irq-region. Then we'd have a pointless "cli" there.

So I'd rather just make sure that only the spinlock code actually uses
native_restore_fl().

And yes, the patch works at least minimally, since I'm writing this
with a kernel compiled with that.

Of course, somebody really should do timings on modern CPU's (in cpl0,
comparing native_fl() that enables interrupts with a popf)

    Linus