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Date:   Sat, 12 Jan 2019 15:54:22 -0800
From:   Andy Lutomirski <>
To:     Josh Poimboeuf <>
Cc:     Linus Torvalds <>,
        Nadav Amit <>,
        Andy Lutomirski <>,
        Peter Zijlstra <>,
        "the arch/x86 maintainers" <>,
        Linux List Kernel Mailing <>,
        Ard Biesheuvel <>,
        Steven Rostedt <>,
        Ingo Molnar <>,
        Thomas Gleixner <>,
        Masami Hiramatsu <>,
        Jason Baron <>, Jiri Kosina <>,
        David Laight <>,
        Borislav Petkov <>,
        Julia Cartwright <>, Jessica Yu <>,
        "H. Peter Anvin" <>,
        Rasmus Villemoes <>,
        Edward Cree <>,
        Daniel Bristot de Oliveira <>
Subject: Re: [PATCH v3 0/6] Static calls

On Fri, Jan 11, 2019 at 1:22 PM Josh Poimboeuf <> 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 <> 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.

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