linux-kernel - Re: [RFC][PATCH 08/24] x86/entry: Add C version of SWAPGS and SWAPGS_UNSAFE

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Message-ID: <CALCETrXtqC9w5gAa=UcF=B4z7vKuUL2sutsCJOjaSCK4CX4q0Q@mail.gmail.com>
Date:   Mon, 9 Nov 2020 09:38:56 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 08/24] x86/entry: Add C version of SWAPGS and SWAPGS_UNSAFE_STACK

On Mon, Nov 9, 2020 at 3:22 AM Alexandre Chartre
<alexandre.chartre@...cle.com> wrote:
>
> SWAPGS and SWAPGS_UNSAFE_STACK are assembly macros. Add C versions
> of these macros (swapgs() and swapgs_unsafe_stack()).

This needs a very good justification.  It also needs some kind of
static verification that these helpers are only used by noinstr code,
and they need to be __always_inline.  And I cannot fathom how C code
could possibly use SWAPGS_UNSAFE_STACK in a meaningful way.