linux-kernel - Re: RFC: Petition Intel/AMD to add POPF

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Date:	Wed, 17 Aug 2016 14:43:27 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Sara Sharon <sara.sharon@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Christian König <christian.koenig@....com>,
	Vinod Koul <vinod.koul@...el.com>,
	Alex Deucher <alexander.deucher@....com>,
	Johannes Berg <johannes.berg@...el.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Andy Lutomirski <luto@...nel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: RFC: Petition Intel/AMD to add POPF_IF insn

On Wed, Aug 17, 2016 at 2:35 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Wed, Aug 17, 2016 at 2:26 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> Of course, if somebody uses native_restore_fl() to actually *disable*
>> interrupts (when they weren't already disabled), then this untested
>> patch will just not work. But why would you do somethign so stupid?
>> Famous last words...
>
> Looking around, the vsmp code actually uses "native_restore_fl()" to
> not just set the interrupt flag, but AC as well.
>
> And the PV spinlock case has that "push;popf" sequence encoded in an alternate.
>
> So that trivial patch may (or may not - still not tested) work for
> some quick testing, but needs more effort for any *real* use.
>

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.