linux-kernel - Re: [PATCH 10/11] x86/fpu: prepare copy_fpstate_to_sigframe for TIF_LOAD

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Date:   Fri, 2 Nov 2018 16:44:16 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Rik van Riel <riel@...riel.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>
Subject: Re: [PATCH 10/11] x86/fpu: prepare copy_fpstate_to_sigframe for
 TIF_LOAD_FPU

On 2018-10-15 17:24:31 [+0200], Borislav Petkov wrote:
> On Fri, Oct 12, 2018 at 12:40:19PM -0700, Dave Hansen wrote:
> > > +	__fpregs_changes_end();
> > 
> > Do we really need the __fpregs_changes_*() abstraction for this single
> > call site?
> 
> Yap, I'm staring at those in patch 2, there's no documentation there what
> they're supposed to do, only the commit message of patch 11 says:
> 
> "The __fpregs_changes_{begin|end}() section ensures that the register
> remain unchanged. Otherwise a context switch or a BH could save the
> registers to its FPU context and processor's FPU register would remain
> random."
> 
> So I'd say we should drop that abstraction, use preempt_* and put that
> text above the single usage site.

There are more than one caller and this function disables preemption and
BH in the end. Therefore I would like to keep it. But as suggested in
the previous patch I will think about renaming it.

> Thx.

Sebastian