Re: [RFC][PATCH 02/22] x86,mmx_32: Remove .fixup usage

Peter Zijlstra
2021-11-05T08:05:58Z

On Thu, Nov 04, 2021 at 01:22:20PM -0700, Josh Poimboeuf wrote:
> On Thu, Nov 04, 2021 at 05:47:31PM +0100, Peter Zijlstra wrote:
> > This code puts an exception table entry on the "PREFIX" instruction to
> > overwrite it with a jmp.d8 when it triggers an exception. Except of
> > course, our code is no longer writable, also SMP.
> >
> > Replace it with ALTERNATIVE, the novel
> >
> > XXX: arguably we should just delete this code
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > ---
> > arch/x86/lib/mmx_32.c | 83 ++++++++++++++++----------------------------------
> > 1 file changed, 27 insertions(+), 56 deletions(-)
> >
> > --- a/arch/x86/lib/mmx_32.c
> > +++ b/arch/x86/lib/mmx_32.c
> > @@ -50,23 +50,17 @@ void *_mmx_memcpy(void *to, const void *
> > kernel_fpu_begin_mask(KFPU_387);
> >
> > __asm__ __volatile__ (
> > - "1: prefetch (%0)\n" /* This set is 28 bytes */
> > - " prefetch 64(%0)\n"
> > - " prefetch 128(%0)\n"
> > - " prefetch 192(%0)\n"
> > - " prefetch 256(%0)\n"
> > - "2: \n"
> > - ".section .fixup, \"ax\"\n"
> > - "3: movw $0x1AEB, 1b\n" /* jmp on 26 bytes */
> > - " jmp 2b\n"
> > - ".previous\n"
> > - _ASM_EXTABLE(1b, 3b)
> > - : : "r" (from));
> > + ALTERNATIVE "",
> > + "prefetch (%0)\n"
> > + "prefetch 64(%0)\n"
> > + "prefetch 128(%0)\n"
> > + "prefetch 192(%0)\n"
> > + "prefetch 256(%0)\n", X86_FEATURE_3DNOW
>
> I think this should instead be X86_FEATURE_3DNOWPREFETCH (which isn't
> 3DNow-specific and should really just be called X86_FEATURE_PREFETCH
> anyway)

Ack.

> > + : : "r" (from));
> >
> > for ( ; i > 5; i--) {
> > __asm__ __volatile__ (
> > - "1: prefetch 320(%0)\n"
> > - "2: movq (%0), %%mm0\n"
> > + " movq (%0), %%mm0\n"
>
> Not sure why this prefetch was removed? It can also be behind an
> alternative just like the first one.

Urgh, that was me just plain killing all the prefetch in there and
forgetting to put it back :/ Will fix.