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
