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

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Date:   Wed, 26 Sep 2018 09:24:38 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Bryan O'Donoghue <pure.logic@...us-software.ie>,
        Denys Vlasenko <dvlasenk@...hat.com>, Ong@...utronix.de,
        Boon Leong <boon.leong.ong@...el.com>,
        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>
Subject: Re: [RFC PATCH 10/10] x86/fpu: defer FPU state load until return to userspace



> On Sep 26, 2018, at 8:32 AM, Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> 
> 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.

Certainly the early Quark CPUs were, um, crappy. They had so many terrifying errata that I’m surprised anyone is willing to use them.

I would *hope* that all supported Quark CPUs support FPU instructions even if they’re microcoded and run extremely slowly.