[RFC please help] membarrier: Rewrite sync_core_before_usermode()

[Peter Zijlstra]

On Tue, Jan 05, 2021 at 08:20:51AM -0800, Andy Lutomirski wrote:
> >     Interestingly, the architecture recently added a control bit to remove
> >     this synchronisation from exception return, so if we set that then we'd
> >     have a problem with SYNC_CORE and adding an ISB would be necessary (and
> >     we could probable then make kernel->kernel returns cheaper, but I
> >     suspect we're relying on this implicit synchronisation in other places
> >     too).
> > 
> 
> Is ISB just a context synchronization event or does it do more?

IIRC it just the instruction sync (like power ISYNC).

> On x86, it’s very hard to tell that MFENCE does any more than LOCK,
> but it’s much slower.  And we have LFENCE, which, as documented,
> doesn’t appear to have any semantics at all.  (Or at least it didn’t
> before Spectre.)

AFAIU MFENCE is a completion barrier, while LOCK prefix is not. A bit
like ARM's DSB vs DMB.

It is for this reason that mb() is still MFENCE, while our smp_mb() is a
LOCK prefixed NO-OP.

And yes, LFENCE used to be poorly defined and it was sometimes
understood to be a completion barrier relative to prior LOADs, while it
is now a completion barrier for any prior instruction, and really should
be renamed to IFENCE.