Re: [PATCH] x86/memcpy: Introduce memcpy_mcsafe_fast

[Linus Torvalds]

On Fri, Apr 17, 2020 at 5:12 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> > @@ -106,12 +108,10 @@ static __always_inline __must_check unsigned long
> >  memcpy_mcsafe(void *dst, const void *src, size_t cnt)
> >  {
> >  #ifdef CONFIG_X86_MCE
> > -       if (static_branch_unlikely(&mcsafe_key))
> > -               return __memcpy_mcsafe(dst, src, cnt);
> > -       else
> > +       if (static_branch_unlikely(&mcsafe_slow_key))
> > +               return memcpy_mcsafe_slow(dst, src, cnt);
> >  #endif
> > -               memcpy(dst, src, cnt);
> > -       return 0;
> > +       return memcpy_mcsafe_fast(dst, src, cnt);
> >  }

It strikes me that I see no advantages to making this an inline function at all.

Even for the good case - where it turns into just a memcpy because MCE
is entirely disabled - it doesn't seem to matter.

The only case that really helps is when the memcpy can be turned into
a single access. Which - and I checked - does exist, with people doing

        r = memcpy_mcsafe(&sb_seq_count, &sb(wc)->seq_count, sizeof(uint64_t));

to read a single 64-bit field which looks aligned to me.

But that code is incredible garbage anyway, since even on a broken
machine, there's no actual reason to use the slow variant for that
whole access that I can tell. The macs-safe copy routines do not do
anything worthwhile for a single access.

So my reaction remains that a lot of this is just completely wrong and
incredibly mis-designed.

Yes, the hardware was buggy garbage. But why should we make things
worse with making the software be incomprehensibly bad too?

              Linus