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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 29 Nov 2018 11:08:26 -0800
Message-ID: <CAHk-=wjN=sow0F-UtA6DtHHT_CNUV7wYVe3cQ4E8GPp4LN32+Q@mail.gmail.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Lutomirski <luto@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, mhiramat@kernel.org,
        jbaron@akamai.com, Jiri Kosina <jkosina@suse.cz>,
        David.Laight@aculab.com, bp@alien8.de, julia@ni.com,
        jeyu@kernel.org, Peter Anvin <hpa@zytor.com>
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On Thu, Nov 29, 2018 at 10:58 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> In contrast, if the call was wrapped in an inline asm, we'd *know* the
> compiler couldn't turn a "call wrapper(%rip)" into anything else.

Actually, I think I have a better model - if the caller is done with inline asm.

What you can do then is basically add a single-byte prefix to the
"call" instruction that does nothing (say, cs override), and then
replace *that* with a 'int3' instruction.

Boom. Done.

Now, the "int3" handler can just update the instruction in-place, but
leave the "int3" in place, and then return to the next instruction
byte (which is just the normal branch instruction without the prefix
byte).

The cross-CPU case continues to work, because the 'int3' remains in
place until after the IPI.

But that would require that we'd mark those call instruction with

                  Linus