Re: [RFC][PATCH 21/22] x86,word-at-a-time: Remove .fixup usage

Peter Zijlstra
2021-11-05T08:04:14Z

On Thu, Nov 04, 2021 at 04:33:10PM -0700, Josh Poimboeuf wrote:
> On Thu, Nov 04, 2021 at 05:47:50PM +0100, Peter Zijlstra wrote:
> > XXX: I'm not really happy with this patch
> >
> > static inline unsigned long load_unaligned_zeropad(const void *addr)
> > {
> > - unsigned long ret, dummy;
> > + unsigned long ret;
> > +
> > + asm("1:\tmov (%0),%0\n"
> > + "2:\n"
> > + _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_LOAD_UNALIGNED, %0)
> > + : "=&r" (ret)
> > + : "0" ((unsigned long)addr));
> >
> > - asm(
> > - "1:\tmov %2,%0\n"
> > - "2:\n"
> > - ".section .fixup,\"ax\"\n"
> > - "3:\t"
> > - "lea %2,%1\n\t"
> > - "and %3,%1\n\t"
> > - "mov (%1),%0\n\t"
> > - "leal %2,%%ecx\n\t"
> > - "andl %4,%%ecx\n\t"
> > - "shll $3,%%ecx\n\t"
> > - "shr %%cl,%0\n\t"
> > - "jmp 2b\n"
> > - ".previous\n"
> > - _ASM_EXTABLE(1b, 3b)
> > - :"=&r" (ret),"=&c" (dummy)
> > - :"m" (*(unsigned long *)addr),
> > - "i" (-sizeof(unsigned long)),
> > - "i" (sizeof(unsigned long)-1));
> > return ret;
> > }
>
> Yeah, it hurts code generation and I guess it's a hot path.
>
> Maybe put the fixup code in the function. In case of
> CONFIG_CC_HAS_ASM_GOTO_OUTPUT, it could be at a label at the end of the
> function. Otherwise it'd have to be inline.

Ooh, yes. If we use ASM_GOTO_OUTPUT then it'll work just right, and the
legacy code will get worse, but I can live with that (everybody is
already building their kernels with gcc-11/clang anyway :-))