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 <l...@kernel.org> 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...
