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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 17 Aug 2016 14:48:00 -0700
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Subject: Re: RFC: Petition Intel/AMD to add POPF_IF insn
To: Andy Lutomirski <luto@amacapital.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>, Sara Sharon <sara.sharon@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>,
        Vinod Koul <vinod.koul@intel.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        Johannes Berg <johannes.berg@intel.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Ingo Molnar <mingo@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Adrian Hunter <adrian.hunter@intel.com>
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On Wed, Aug 17, 2016 at 2:43 PM, Andy Lutomirski <luto@amacapital.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