linux-kernel - Re: [PATCH v3 0/6] Static calls

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Date:   Fri, 11 Jan 2019 14:04:20 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Nadav Amit <namit@...are.com>, Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH v3 0/6] Static calls

On Fri, Jan 11, 2019 at 11:03:30AM -0800, Linus Torvalds wrote:
> The we'd change the end of poke_int3_handler() to do something like
> this instead:
> 
>         void *newip = bp_int3_handler;
>         ..
>         if (new == magic_static_call_bp_int3_handler) {
>                 if (regs->flags &X86_FLAGS_IF) {
>                         newip = magic_static_call_bp_int3_handler_sti;
>                         regs->flags &= ~X86_FLAGS_IF;
>         }
>         regs->ip = (unsigned long) newip;
>         return 1;
> 
> AAND now we're *really* done.
> 
> Does anybody see any issues in this?

This sounds ok, with a possible tweak: instead of the sti tricks,
couldn't we just use synchronize_rcu_tasks() (as Jason suggested), to
make sure the stubs are no longer used by a preempted task?

But really, to me, having to create and manage all those custom
trampolines still feels a lot more complex than just making a gap on the
stack.

-- 
Josh