Re: [PATCH 13/15] x86/static_call: Add inline static call implementation for x86-64

[Josh Poimboeuf]

On Mon, Jun 10, 2019 at 06:45:52PM +0000, Nadav Amit wrote:
> > On Jun 10, 2019, at 11:33 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > 
> > On Wed, Jun 05, 2019 at 03:08:06PM +0200, Peter Zijlstra wrote:
> >> --- a/arch/x86/include/asm/static_call.h
> >> +++ b/arch/x86/include/asm/static_call.h
> >> @@ -2,6 +2,20 @@
> >> #ifndef _ASM_STATIC_CALL_H
> >> #define _ASM_STATIC_CALL_H
> >> 
> >> +#include <asm/asm-offsets.h>
> >> +
> >> +#ifdef CONFIG_HAVE_STATIC_CALL_INLINE
> >> +
> >> +/*
> >> + * This trampoline is only used during boot / module init, so it's safe to use
> >> + * the indirect branch without a retpoline.
> >> + */
> >> +#define __ARCH_STATIC_CALL_TRAMP_JMP(key, func)				\
> >> +	ANNOTATE_RETPOLINE_SAFE						\
> >> +	"jmpq *" __stringify(key) "+" __stringify(SC_KEY_func) "(%rip) \n"
> >> +
> >> +#else /* !CONFIG_HAVE_STATIC_CALL_INLINE */
> > 
> > I wonder if we can simplify this (and drop the indirect branch) by
> > getting rid of the above cruft, and instead just use the out-of-line
> > trampoline as the default for inline as well.
> > 
> > Then the inline case could fall back to the out-of-line implementation
> > (by patching the trampoline's jmp dest) before static_call_initialized
> > is set.
> 
> I must be missing some context - but what guarantees that this indirect
> branch would be exactly 5 bytes long? Isnât there an assumption that this
> would be the case? Shouldnât there be some handling of the padding?

We don't patch the indirect branch.  It's just part of a temporary
trampoline which is called by the call site, and which does "jmp
key->func" during boot until static call initialization is done.

(Though I'm suggesting removing that.)

-- 
Josh