Il 02/05/2014 21:51, Linus Torvalds ha scritto:
> Also, are you *really* sure that "popf" has the same one-instruction
> interrupt shadow that "sti" has? Because I'm not at all sure that is
> true, and it's not documented as far as I can tell. In contrast, the
> one-instruction shadow after "sti" very much _is_ documented.
Yeah, I'm pretty sure about this. The only instructions with an
interrupt shadow are "sti", "mov ss" and "pop ss".
Yep.
There may be specific microarchitectures that do it for a "popf" that
enables interrupts too, but that is not documented _anywhere_ I could
find.
Btw, on the "really easy to get wrong in emulation" note and looking
at the kernel sources: it looks like KVM gets "pop ss" wrong, and only
does the shadow on "mov ss".
Thanks, that's useful to know (and easy to fix). Note that in practice
arch/x86/kvm/emulate.c will only emulate POP SS in big real mode or if
the stack is in MMIO memory. The interrupt shadow will be handled by
the processor in all other cases, and Intel calls the bit "Blocking by
MOV SS" even if it also applies to POP SS.
Your suggested trick of splitting the return paths for IF=0/IF=1 can be
also done like this:
movq EFLAGS-ARGOFFSET(%rsp), %rdi
btrq $9, %rdi # Clear IF, save old value in CF
movq %rdi, (%rsi)
...
popfq
jnc 1f # If IF was 0, just return
sti # Using STI gets us an interrupt shadow
1f:
retq
Paolo
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