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Subject: Re: RFC: Petition Intel/AMD to add POPF_IF insn
To: Linus Torvalds <torvalds@linux-foundation.org>
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Cc: Andy Lutomirski <luto@amacapital.net>, Sara Sharon <sara.sharon@intel.com>,
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        =?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>
From: Denys Vlasenko <dvlasenk@redhat.com>
Message-ID: <4c31334b-4424-7660-5513-a601bfa763f3@redhat.com>
Date: Wed, 17 Aug 2016 21:35:59 +0200
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On 08/17/2016 09:32 PM, Linus Torvalds wrote:
> On Wed, Aug 17, 2016 at 12:26 PM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>>
>> Exactly. And more:
>
> All of which is ENTIRELY IRRELEVANT.
>
> The 2-page pseudo-code is about behavior. It's trivial to
> short-circuit. There are obvious optimizations, which involve just
> making the rare cases go slow and have a trivial "if those bits didn't
> change, don't go through the expense of testing them".
>
> The problem is that IF is almost certainly right now in that rare case
> mask, and so popf is stupidly slow for IF.

I ran the test, see the first email in the thread.

Experimentally, POPF is stupidly slow _always_. 6 cycles
even if none of the "scary" flags are changed.
Either:
*  its microcode has no rare case mask
or
* its microcode is so slow that even fast path is slow,
   and slow path is worse