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Subject: Re: [PATCH] x86/fpu: Revert earlier patch of Disable AVX when
 eagerfpu is off
To: Ingo Molnar <mingo@kernel.org>, Yu-cheng Yu <yu-cheng.yu@intel.com>
References: <1453511438-25827-1-git-send-email-yu-cheng.yu@intel.com>
 <20160309124626.GA28660@gmail.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@suse.de>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Leonid Shatz <leonid.shatz@ravellosystems.com>,
        Fenghua Yu <fenghua.yu@intel.com>, Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
From: "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <56E070C1.6090302@zytor.com>
Date: Wed, 9 Mar 2016 10:51:45 -0800
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On 03/09/2016 04:46 AM, Ingo Molnar wrote:
> 
> * Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
> 
>> AVX was mistakenly believed to be dependent on eagerfpu switch.
>> This turns out to be false. The earlier patch should be reverted.
>>
>> Original patch:
>>     http://git.kernel.org/tip/394db20ca240741a08d472173db13d6f6a6e5a28
> 
> So the original patch had a whole host of explanations of why that change is 
> correct. This revert should explain where that argumentation was wrong.
> 
> Also note that eagerfpu=off is on the way out, on v4.6 we'll switch all CPUs to 
> eagerfpu:
> 
>   58122bf1d856 x86/fpu: Default eagerfpu=on on all CPUs
> 
> and in the not so distant future, if everything goes fine with the eager mode, I'd 
> like to drop the lazy FPU context switching code altogether - which will simplify 
> a lot of code. At that point the 'eagerfpu' boot option will go away as well.
> 

Last I heard the only use case which was seeing trouble with eagerfpu
was KVM during service of I/O events.  Might be worth checking into...

	-hpa