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

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Message-ID: <f3a95fc0-2a98-5627-7cea-8847ee6496e7@redhat.com>
Date:   Wed, 5 Oct 2016 09:14:21 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     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 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.

Thanks,

Paolo