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SubjectRe: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64
On Thu, 29 Nov 2018 11:08:26 -0800
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> 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
>

In my original proof of concept, I tried to to implement the callers
with asm, but then the way to handle parameters became a nightmare.

The goal of this (for me) was to replace the tracepoint indirect calls
with static calls, and tracepoints can have any number of parameters to
pass. I ended up needing the compiler to help me with the passing of
parameters.

-- Steve

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