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

Josh Poimboeuf
2018-11-29T22:17:52Z

On Thu, Nov 29, 2018 at 02:24:52PM -0600, Josh Poimboeuf wrote:
> On Thu, Nov 29, 2018 at 11:27:00AM -0800, Andy Lutomirski wrote:
> > On Thu, Nov 29, 2018 at 11:08 AM 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.
> >
> > Hmm, cute. But then the calls are in inline asm, which results in
> > giant turds like we have for the pvop vcalls. And, if they start
> > being used more generally, we potentially have ABI issues where the
> > calling convention isn't quite what the asm expects, and we explode.
> >
> > I propose a different solution:
> >
> > As in this patch set, we have a direct and an indirect version. The
> > indirect version remains exactly the same as in this patch set. The
> > direct version just only does the patching when all seems well: the
> > call instruction needs to be 0xe8, and we only do it when the thing
> > doesn't cross a cache line. Does that work? In the rare case where
> > the compiler generates something other than 0xe8 or crosses a cache
> > line, then the thing just remains as a call to the out of line jmp
> > trampoline. Does that seem reasonable? It's a very minor change to
> > the patch set.
>
> Maybe that would be ok. If my math is right, we would use the
> out-of-line version almost 5% of the time due to cache misalignment of
> the address.

BTW, this means that if any of a trampoline's callers crosses cache
boundaries then we won't be able to poison the trampoline. Which is
kind of sad.

--
Josh