linux-kernel - Re: [PATCH v2 3/3] x86/pti/64: Remove the SYSCALL64 entry trampoline

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Date:   Sat, 8 Sep 2018 08:36:03 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Joerg Roedel <joro@...tes.org>, Jiri Olsa <jolsa@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v2 3/3] x86/pti/64: Remove the SYSCALL64 entry
 trampoline

On Fri, 7 Sep 2018, Andy Lutomirski wrote:
> On Fri, Sep 7, 2018 at 5:04 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> > Virtual mapping tricks may be cool, but in the end, not having to use
> > them is better still, I think.
> >
> 
> If (and this is a *big* if) all the percpu data is within 2GB of the
> entry text, then we could avoid this extra TLB reference by accessing
> it directly instead of using an alias.
> 
> I suppose the summary is that the retpoline-free trampoline variant is
> even more complicated than the code I'm removing in this series, and
> that it would be at best a teeny tiny win.  Once all the Spectre dust
> settles and we get fixed CPUs, we could consider re-optimizing this.

That's going to be after my retirement ...