Re: [PATCH v3 0/6] Static calls

[Josh Poimboeuf]

On Thu, Jan 10, 2019 at 09:30:23PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 09, 2019 at 04:59:35PM -0600, Josh Poimboeuf wrote:
> > With this version, I stopped trying to use text_poke_bp(), and instead
> > went with a different approach: if the call site destination doesn't
> > cross a cacheline boundary, just do an atomic write.  Otherwise, keep
> > using the trampoline indefinitely.
> 
> > - Get rid of the use of text_poke_bp(), in favor of atomic writes.
> >   Out-of-line calls will be promoted to inline only if the call sites
> >   don't cross cache line boundaries. [Linus/Andy]
> 
> Can we perserve why text_poke_bp() didn't work? I seem to have forgotten
> again. The problem was poking the return address onto the stack from the
> int3 handler, or something along those lines?

Right, emulating a call instruction from the #BP handler is ugly,
because you have to somehow grow the stack to make room for the return
address.  Personally I liked the idea of shifting the iret frame by 16
bytes in the #DB entry code, but others hated it.

So many bad-but-not-completely-unacceptable options to choose from.

-- 
Josh