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

From: Josh Poimboeuf
Date: Thu Nov 29 2018 - 15:25:11 EST

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@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > 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.
> 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.

> Alternatively, we could actually emulate call instructions like this:
> void __noreturn jump_to_kernel_pt_regs(struct pt_regs *regs, ...)
> {
> struct pt_regs ptregs_copy = *regs;
> barrier();
> *(unsigned long *)(regs->sp - 8) = whatever; /* may clobber old
> regs, but so what? */
> asm volatile ("jmp return_to_alternate_ptregs");
> }
> where return_to_alternate_ptregs points rsp to the ptregs and goes
> through the normal return path. It's ugly, but we could have a test
> case for it, and it should work fine.

Is that really any better than my patch to create a gap in the stack
(modified for kernel space #BP only)?