vmm: remove a wmb() call

[avg]

Summary

After removing wmb(), vm_set_rendezvous_func() became super trivial, so
there was no point in keeping it.

The wmb (sfence on amd64, lock nop on i386) was not needed. This can be
explained from several points of view.

First, wmb() is used for store-store ordering (although, the primitive is
undocumented). There was no obvious subsequent store that needed the
barrier.

Second, x86 has a memory model with strong ordering including total store
order. An explicit store barrier may be needed only when working with
special memory (device, special caching mode) or using special instructions
(non-temporal stores). That was not the case for this code.

Third, I believe that there is a misconception that sfence "flushes" the
store buffer in a sense that it speeds up the propagation of stores from the
store buffer to the global visibility. I think that such propagation always
happens as fast as possible. sfence only makes subsequent stores wait for
that propagation to complete. So, sfence is only useful for ordering of
stores and only in the situations described above.

[kib]

It is relative, of course. SFENCE might not flush store buffers faster than possbile, but it does guarantee that code after it sees store buffers flushed.

Anyway, I think that for formal correctness stores to vm->rendezvous_func should be _rel, and loads _acq, to properly express what the removed comment tried to state.

[avg]

I think that the formal correctness is achieved by rendezvous_mtx.
That wmb() was for the benefit of code like:

if (rendezvous_func != NULL) {
  ...
  mtx_lock(rendezvous_mtx);
  // do real things with rendezvous_func
  mtx_unlock(rendezvous_mtx);
}

That is, rendezvous_func is assigned and used always under rendezvous_mtx.
Access outside of the lock are only to check it for being set.

[avg]

I plan on committing this tomorrow.

[jhb]

The sfence does nothing for checks outside the lock, so this does indeed seem pointless. The removed code also used KASSERT(mtx_owned...) instead of mtx_assert() which seems sub-par.

Checking the function pointer outside of the lock is racy regardless. I think though it doesn't matter as bhyve just needs the vCPU to see the function pointer "eventually" and invoke it. I believe a rendezvous will block until all vCPUs have invoked the handler, so there isn't an issue with the function pointer being overwritten and a rendezvous being "missed" AFAIK.