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Date:   Mon, 25 Oct 2021 17:03:00 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Frederic Weisbecker <frederic@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        James Morse <james.morse@arm.com>,
        David Laight <David.Laight@aculab.com>,
        Quentin Perret <qperret@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH 2/4] arm64: implement support for static call trampolines
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On Mon, Oct 25, 2021 at 04:55:17PM +0200, Ard Biesheuvel wrote:
> On Mon, 25 Oct 2021 at 16:47, Peter Zijlstra <peterz@infradead.org> wrote:

> > Perhaps a little something like so.. Shaves 2 instructions off each
> > trampoline.
> >
> > --- a/arch/arm64/include/asm/static_call.h
> > +++ b/arch/arm64/include/asm/static_call.h
> > @@ -11,9 +11,7 @@
> >             "   hint    34      /* BTI C */                             \n" \
> >                 insn "                                                  \n" \
> >             "   ldr     x16, 0b                                         \n" \
> > -           "   cbz     x16, 1f                                         \n" \
> >             "   br      x16                                             \n" \
> > -           "1: ret                                                     \n" \
> >             "   .popsection                                             \n")
> >
> >  #define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func)                      \
> > --- a/arch/arm64/kernel/patching.c
> > +++ b/arch/arm64/kernel/patching.c
> > @@ -90,6 +90,11 @@ int __kprobes aarch64_insn_write(void *a
> >         return __aarch64_insn_write(addr, &i, AARCH64_INSN_SIZE);
> >  }
> >
> > +asm("__static_call_ret:                \n"
> > +    "  ret                     \n")
> > +
> 
> This breaks BTI as it lacks the landing pad, and it will be called indirectly.

Argh!

> > +extern void __static_call_ret(void);
> > +
> 
> Better to have an ordinary C function here (with consistent linkage),
> but we need to take the address in a way that works with Clang CFI.

There is that.

> As the two additional instructions are on an ice cold path anyway, I'm
> not sure this is an obvious improvement tbh.

For me it's both simpler -- by virtue of being more consistent, and
smaller. So double win :-)

That is; you're already relying on the literal being unconditionally
updated for the normal B foo -> NOP path, and having the RET -> NOP path
be handled differently is just confusing.

At least, that's how I'm seeing it today...