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From:   Andy Lutomirski <luto@kernel.org>
Date:   Sat, 12 Jan 2019 15:54:22 -0800
Message-ID: <CALCETrU4BLNRxHWv3+8_4+0d1XQmRxhhRfWpn0jNnXBn+YJedQ@mail.gmail.com>
Subject: Re: [PATCH v3 0/6] Static calls
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Nadav Amit <namit@vmware.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux List Kernel Mailing <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>,
        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 Cartwright <julia@ni.com>, Jessica Yu <jeyu@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Edward Cree <ecree@solarflare.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
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On Fri, Jan 11, 2019 at 1:22 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> On Fri, Jan 11, 2019 at 12:46:39PM -0800, Linus Torvalds wrote:
> > On Fri, Jan 11, 2019 at 12:31 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > >
> > > I was referring to the fact that a single static call key update will
> > > usually result in patching multiple call sites.  But you're right, it's
> > > only 1-2 trampolines per text_poke_bp() invocation.  Though eventually
> > > we may want to batch all the writes like what Daniel has proposed for
> > > jump labels, to reduce IPIs.
> >
> > Yeah, my suggestion doesn't allow for batching, since it would
> > basically generate one trampoline for every rewritten instruction.
>
> As Andy said, I think batching would still be possible, it's just that
> we'd have to create multiple trampolines at a time.
>
> Or... we could do a hybrid approach: create a single custom trampoline
> which has the call destination patched in, but put the return address in
> %rax -- which is always clobbered, even for callee-saved PV ops.  Like:
>

One think I particularly like about the current design is that there
are no requirements at all on the calling convention.  I think it
seems fragile to add a calling convention constraint that only applies
when there's a race.  I'd rather do a longjmp-like hack or a stack gap
adding hack than make the actual static calls more fragile.