On Tue, Sep 28, 2021 at 06:36:37PM -0700, Josh Poimboeuf wrote: > On Tue, Sep 28, 2021 at 11:35:43AM +0100, Mark Rutland wrote: > > > In the other x86 thread Josh Poimboeuf suggested to use asm goto to a > > > cold part of the function instead of .fixup: > > > https://lore.kernel.org/lkml/20210927234543.6waods7rraxseind@treble/ > > > This sounds like a more reliable solution that will cause less > > > maintenance burden. Would it work for arm64 as well? > > > > Maybe we can use that when CC_HAS_ASM_GOTO_OUTPUT is avaiable, but in > > general we can't rely on asm goto supporting output arguments (and IIRC > > GCC doesn't support that at all), so we'd still have to support the > > current fixup scheme. gcc-11 has it > Even without CC_HAS_ASM_GOTO_OUTPUT it should still be possible to hack > something together if you split the original insn asm and the extable > asm into separate statements, like: > > diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h > index 6b52182e178a..8f62469f2027 100644 > --- a/arch/x86/include/asm/msr.h > +++ b/arch/x86/include/asm/msr.h > @@ -137,20 +139,21 @@ static inline unsigned long long native_read_msr_safe(unsigned int msr, > { > DECLARE_ARGS(val, low, high); > > + *err = 0; > + asm volatile("417: rdmsr\n" > + : EAX_EDX_RET(val, low, high) > + : "c" (msr)); > + asm_volatile_goto(_ASM_EXTABLE(417b, %l[Efault]) :::: Efault); That's terrible :-) Could probably do with a comment, but might just work.. > + > +done: > if (tracepoint_enabled(read_msr)) > do_trace_read_msr(msr, EAX_EDX_VAL(val, low, high), *err); > return EAX_EDX_VAL(val, low, high); > + > +Efault: > + *err = -EIO; > + ZERO_ARGS(val, low, high); > + goto done; > } > > /* Can be uninlined because referenced by paravirt */ >
