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

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Message-ID: <CA+55aFyv2tj1YQH3Ga4oAkNEZgsT2+KmuU_MYqN0aUUtkBygrQ@mail.gmail.com>
Date:	Wed, 17 Aug 2016 14:48:00 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
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 2:43 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> 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.

No, that would be horrible for the case where we had an irq spinlock
in an irq-region. Then we'd have a pointless "cli" there.

So I'd rather just make sure that only the spinlock code actually uses
native_restore_fl().

And yes, the patch works at least minimally, since I'm writing this
with a kernel compiled with that.

Of course, somebody really should do timings on modern CPU's (in cpl0,
comparing native_fl() that enables interrupts with a popf)

    Linus