RFC: Petition Intel/AMD to add POPF_IF insn

[Denys Vlasenko]

Last year, a proposal was floated to avoid costly POPF.
In particular, each spin_unlock_irqrestore() does it,
a rather common operation.

https://lkml.org/lkml/2015/4/21/290
	[RFC PATCH] x86/asm/irq: Don't use POPF but STI

* Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > Another different approach would be to formally state that
> > pv_irq_ops.save_fl() needs to return all the flags, which would
> > make local_irq_save() safe to use in this circumstance, but that
> > makes a hotpath longer for the sake of a single boot time check.
>
> ...which reminds me:
>
> Why does native_restore_fl restore anything other than IF?  A branch
> and sti should be considerably faster than popf.


Ingo agreed:
====
Yes, this has come up in the past, something like the patch below?

Totally untested and not signed off yet: because we'd first have to
make sure (via irq flags debugging) that it's not used in reverse, to
re-disable interrupts:
	local_irq_save(flags);
	local_irq_enable();
	...
	local_irq_restore(flags); /* effective local_irq_disable() */
I don't think we have many (any?) such patterns left, but it has to be
checked first. If we have such cases then we'll have to use different
primitives there.
====

Linus replied:
=====
"popf" is fast for the "no changes to IF" case, and is a smaller
instruction anyway.
====


This basically shot down the proposal.

But in my measurements POPF is not fast even in the case where restored
flags are not changes at all:

                mov     $200*1000*1000, %eax
                pushf
                pop     %rbx
                .balign 64
loop:           push    %rbx
                popf
                dec     %eax
                jnz     loop

# perf stat -r20 ./popf_1g
     4,929,012,093      cycles                    #    3.412 GHz                      ( +-  0.02% )
       835,721,371      instructions              #    0.17  insn per cycle           ( +-  0.02% )
       1.446185359 seconds time elapsed                                          ( +-  0.46% )

If I replace POPF with a pop into an unused register, I get this:

loop:           push    %rbx
                pop     %rcx
                dec     %eax
                jnz     loop

       209,247,645      cycles                    #    3.209 GHz                      ( +-  0.11% )
       801,898,016      instructions              #    3.83  insn per cycle           ( +-  0.00% )
       0.066210725 seconds time elapsed                                          ( +-  0.59% )


IOW, POPF takes at least 6 cycles.


Linus does have a point that a "test+branch+STI" may end up not a clear win because of the branch.

But the need to restore IF flag exists, it is necessary not only for Linux, but for any OS
running on x86: they all have some sort of spinlock.

The addition of a POPF instruction variant which looks only at IF bit and changes
only that bit in EFLAGS may be a good idea, for all OSes.

I propose that we ask Intel / AMD to do that.

Maybe by the "ignored prefix" trick which was used when LZCNT insn was introduced
as REPZ-prefixed BSR?
Currently, REPZ POPF (f3 9d) insn does execute. Redefine this opcode as
POPF_IF. Then the same kernel will work on old and new CPUs.

CC'ing some @intel and @amd emails...