Messages in this thread | | | From | Jann Horn <> | Date | Mon, 17 Feb 2020 22:10:27 +0100 | Subject | Re: [PATCH v3 0/6] Static calls |
| |
On Thu, Jan 10, 2019 at 9:52 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote: > 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.
Silly suggestion from someone who has skimmed the thread:
Wouldn't a retpoline-style trampoline solve this without needing memory allocations? Let the interrupt handler stash the destination in a percpu variable and clear IF in regs->flags. Something like:
void simulate_call(unsigned long target) { __this_cpu_write(static_call_restore_if, (regs->flags & X86_EFLAGS_IF) != 0); regs->flags &= ~X86_EFLAGS_IF; __this_cpu_write(static_call_trampoline_source, regs->ip + 5); __this_cpu_write(static_call_trampoline_target, target); regs->ip = magic_static_call_trampoline; }
magic_static_call_trampoline: ; set up return address for returning from target function pushl PER_CPU_VAR(static_call_trampoline_source) ; set up retpoline-style return address pushl PER_CPU_VAR(static_call_trampoline_target) ; restore flags if needed cmp PER_CPU_VAR(static_call_restore_if), 0 je 1f sti ; NOTE: percpu data must not be accessed past this point 1: ret ; "return" to the call target
By using a return to implement the call, we don't need any scratch registers for the call.
| |