Messages in this thread | | | From | Denys Vlasenko <> | Subject | RFC: Petition Intel/AMD to add POPF_IF insn | Date | Wed, 17 Aug 2016 19:20:37 +0200 |
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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@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...
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