linux-kernel - Re: [PATCH 15/22] x86/entry: Add TIF_NEED_FPU

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Message-ID: <20190130115507.GE18383@zn.tnic>
Date:   Wed, 30 Jan 2019 12:55:07 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 15/22] x86/entry: Add TIF_NEED_FPU_LOAD

On Wed, Jan 09, 2019 at 12:47:37PM +0100, Sebastian Andrzej Siewior wrote:
> Add TIF_NEED_FPU_LOAD. This is reserved for loading the FPU registers
> before returning to userland. This flag must not be set for systems
> without a FPU.
> If this flag is cleared, the CPU's FPU register hold the current content
> of current()'s FPU register. The in-memory copy (union fpregs_state) is
> not valid.
> If this flag is set, then all of CPU's FPU register may hold a random
> value (except for PKRU) and it is required to load the content of the
> FPU register on return to userland.

This definitely needs to be written somewhere in

arch/x86/include/asm/fpu/internal.h

or where we decide to put the FPU handling rules.

-- 
Regards/Gruss,
    Boris.

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