On Mon, Jun 10, 2019 at 06:45:52PM +0000, Nadav Amit wrote: > > On Jun 10, 2019, at 11:33 AM, Josh Poimboeuf <[email protected]> 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? Isn’t there an assumption that this > would be the case? Shouldn’t 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
