linux-kernel - Re: [RFC PATCH 10/10] x86/fpu: defer FPU state load until return to userspace

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Message-ID: <20180926153229.n5rwlfhuqu42ds6v@linutronix.de>
Date:   Wed, 26 Sep 2018 17:32:30 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Bryan O'Donoghue <pure.logic@...us-software.ie>,
        Denys Vlasenko <dvlasenk@...hat.com>, Ong@...utronix.de,
        Boon Leong <boon.leong.ong@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [RFC PATCH 10/10] x86/fpu: defer FPU state load until return to
 userspace

On 2018-09-26 07:34:09 [-0700], Andy Lutomirski wrote:
> > So I *think* nobody relies on FPU-emulation anymore. I would suggest to
> > get this patch set into shape and then getting rid of
> > CONFIG_MATH_EMULATION?

Bryan, Denys, does anyone of you rely on CONFIG_MATH_EMULATION?
The manual for Quark claims that the CPU has no FPU and yet the current
kernel (or the yocto v3.14) does not select this option.
Denys added support for some opcodes in 2015 but I didn't figure out
*why* or which CPU in particular requires this.

> I don’t think you can boot a kernel without math emulation on a no-FPU CPU. So maybe we should table this particular idea.  I didn’t realize there were Quark CPUs without FPU.

Okay.

Sebastian