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

[Nadav Amit]

> 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?