linux-kernel - Re: [patch V3 02/20] x86/process: Unify copy_thread

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Message-ID: <CAHk-=whNAEuNPU3Oy_-EpjOojpysWcCh4uqDgOt2RjBNx2xBSg@mail.gmail.com>
Date:   Wed, 13 Nov 2019 13:10:13 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Willy Tarreau <w@....eu>, Juergen Gross <jgross@...e.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch V3 02/20] x86/process: Unify copy_thread_tls()

On Wed, Nov 13, 2019 at 1:02 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> +int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
> +                   unsigned long arg, struct task_struct *p, unsigned long tls)
...
> +#ifdef CONFIG_X86_64
..
> +#else
> +       /* Clear all status flags including IF and set fixed bit. */
> +       frame->flags = X86_EFLAGS_FIXED;
> +#endif

Hmm. The unification I like, but it also shows these differences that
I don't remember the reason for.

Remind me why __switch_to_asm() on 32-bit safes eflags, but we don't
do it on x86-64?

The comment just talks about callee-saved registers, but flags isn't
callee-saved, so there's something else going on.

This patch clearly doesn't change anything, I'm not complaining about
the patch at all. I'm just wondering about the odd difference that the
patch exposes.

               Linus