Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

[Linus Torvalds]

On Thu, Nov 29, 2018 at 10:58 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> 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