linux-kernel - Re: [RFC PATCH] x86/fpu: Don't unconditionally add XFEATURE_MASK

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Date:   Fri, 26 Apr 2019 09:33:28 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, jannh@...gle.com,
        riel@...riel.com, mingo@...hat.com, bp@...e.de, Jason@...c4.com,
        luto@...nel.org, tglx@...utronix.de, rkrcmar@...hat.com,
        mingo@...nel.org, hpa@...or.com, kvm@...r.kernel.org,
        pbonzini@...hat.com, kurt.kanzenbach@...utronix.de
Subject: Re: [RFC PATCH] x86/fpu: Don't unconditionally add
 XFEATURE_MASK_FPSSE on sigentry

On 4/26/19 12:26 AM, Sebastian Andrzej Siewior wrote:
>> That's just a guess, though.
>>
>> If we care, I think we should just use XSAVE instead of XSAVEOPT and
>> trying to reconstruct the init state in software.
> We can't use XSAVE directly in the slowpath. We need to reconstruct the
> init state. We have the mxcsr quirk. We would need just to extend it and
> set the FP area to init state if the FP state is missing like we do in
> fpstate_sanitize_xstate().

Can you remind me why we can't use XSAVE directly in the slow path?