Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 29 Nov 2018 12:37:12 -0600
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>, julia@ni.com, jeyu@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64
Message-ID: <20181129183712.s5eocp5hf4ta4zks@treble>
References: <20181126212628.4apztfazichxnt7r@treble>
 <20181127084330.GX2113@hirez.programming.kicks-ass.net>
 <CALCETrXam8JBtXE3S97Vyb_=+pmjgr23SoZ-Dm5pmYtwZYA+7Q@mail.gmail.com>
 <20181129094210.GC2131@hirez.programming.kicks-ass.net>
 <ECF632D9-581B-40B7-9F0A-6B5607A4D0E3@amacapital.net>
 <20181129143853.GO2131@hirez.programming.kicks-ass.net>
 <20181129163342.tp5wlfcyiazwwyoh@treble>
 <20181129164914.GA9027@hirez.programming.kicks-ass.net>
 <0FDA053D-7ADC-4F42-AEA5-99DA155FCED0@amacapital.net>
 <20181129171539.GD9027@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <20181129171539.GD9027@hirez.programming.kicks-ass.net>
User-Agent: NeoMutt/20180716
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Nov 29, 2018 at 06:15:39PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 29, 2018 at 08:59:31AM -0800, Andy Lutomirski wrote:
> 
> > If you make it conditional on CPL, do it for 32-bit as well, add
> > comments, 
> 
> > and convince yourself that there isn’t a better solution
> > (like pointing IP at a stub that retpolines to the target by reading
> > the function pointer, a la the unoptimizable version), then okay, I
> > guess, with only a small amount of grumbling.
> 
> Right; so we _could_ grow the trampoline with a retpoline indirect call
> and ret. It just makes the trampoline a whole lot bigger, but it could
> work.

I'm trying to envision how this would work.  How would the function (or
stub) know how to return back to the call site?

-- 
Josh