linux-kernel - Re: [PATCH 2/9] x86/fpu: Hard-disable lazy fpu mode

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Message-ID: <cf59408b-7b30-4eae-874f-d4be9b3484be@redhat.com>
Date:   Wed, 5 Oct 2016 16:03:49 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org
Cc:     dave.hansen@...ux.intel.com, x86@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, luto@...nel.org, pa@...or.com, bp@...e.de
Subject: Re: [PATCH 2/9] x86/fpu: Hard-disable lazy fpu mode



On 05/10/2016 15:57, Rik van Riel wrote:
> On Wed, 2016-10-05 at 09:14 +0200, Paolo Bonzini wrote:
>>
>> On 05/10/2016 02:34, riel@...hat.com wrote:
>>>
>>> From: Andy Lutomirski <luto@...nel.org>
>>>
>>> Since commit 58122bf1d856 ("x86/fpu: Default eagerfpu=on on all
>>> CPUs") in Linux 4.6, eager FPU mode has been the default on all x86
>>> systems, and no one has reported any regressions.
>>>
>>> This patch removes the ability to enable lazy mode: use_eager_fpu()
>>> becomes "return true" and all of the FPU mode selection machinery
>>> is
>>> removed.
>>
>> I haven't quite followed up on my promise to benchmark lazy vs. eager
>> FPU, but I probably should do that now...
>>
>> I see two possible issues with this.  First, AMD as far as I know does
>> not have XSAVEOPT.  Second, when using virtualization, depending on
>> how you configure your cluster it's enough to have one pre-SandyBridge
>> Intel machine to force no XSAVE on all machines.
> 
> The "OPT" part of XSAVEOPT does not work across the
> host/guest boundary, anyway.

Yes, but it works for bare metal (and in fact eager FPU was keyed on
XSAVEOPT before 58122bf1d856, not XSAVE).

I'm not talking about KVM here; I am just saying that the lazy FPU code
might be used more than we'd like to, because of AMD machines and of
cases where XSAVE is hidden altogether from guests.  Of course it is
quite unlikely that it be reported as a regression, since things just
work.  But as far as I know 58122bf1d856 went in without any substantial
(or not-so-substantial) benchmarking.

Paolo