linux-kernel - Re: [PATCH 3/3] x86_64: Use relative per-cpu offsets

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Message-ID: <20211114110305.GN174703@worktop.programming.kicks-ass.net>
Date:   Sun, 14 Nov 2021 12:03:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Brian Gerst <brgerst@...il.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 3/3] x86_64: Use relative per-cpu offsets

On Sat, Nov 13, 2021 at 11:54:19PM -0500, Brian Gerst wrote:
> On Sat, Nov 13, 2021 at 8:18 PM Andy Lutomirski <luto@...nel.org> wrote:
> >
> >
> >
> > On Sat, Nov 13, 2021, at 4:40 AM, Brian Gerst wrote:
> > > The per-cpu section is currently linked at virtual address 0, because
> > > older compilers hardcoded the stack protector canary value at a fixed
> > > offset from the start of the GS segment.  Use a standard relative offset
> > > as the GS base when the stack protector is disabled, or a newer compiler
> > > is used that supports a configurable location for the stack canary.
> >
> > Can you explain the benefit?  Also, I think we should consider dropping support for the fixed model like we did on x86_32.
> 
> This patch probably makes more sense if we drop the fixed model, as
> that gets rid of alot of code that works around having to link the
> percpu section differently.

Can someone spell out these benefits please? To me having per-cpu start
at 0 makes perfect sense, how does not having that make things better?