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. 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); - asm volatile("2: rdmsr ; xor %[err],%[err]\n" - "1:\n\t" - ".section .fixup,\"ax\"\n\t" - "3: mov %[fault],%[err]\n\t" - "xorl %%eax, %%eax\n\t" - "xorl %%edx, %%edx\n\t" - "jmp 1b\n\t" - ".previous\n\t" - _ASM_EXTABLE(2b, 3b) - : [err] "=r" (*err), EAX_EDX_RET(val, low, high) - : "c" (msr), [fault] "i" (-EIO)); + *err = 0; + asm volatile("417: rdmsr\n" + : EAX_EDX_RET(val, low, high) + : "c" (msr)); + asm_volatile_goto(_ASM_EXTABLE(417b, %l[Efault]) :::: Efault); + +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 */
