xen/evtchn: Add missing barriers when accessing/allocating an event channel

xen/evtchn: Add missing barriers when accessing/allocating an event channel

While the allocation of a bucket is always performed with the per-domain
lock, the bucket may be accessed without the lock taken (for instance, see
evtchn_send()).

Instead such sites relies on port_is_valid() to return a non-zero value
when the port has a struct evtchn associated to it. The function will
mostly check whether the port is less than d->valid_evtchns as all the
buckets/event channels should be allocated up to that point.

Unfortunately a compiler is free to re-order the assignment in
evtchn_allocate_port() so it would be possible to have d->valid_evtchns
updated before the new bucket has finish to allocate.

Additionally on Arm, even if this was compiled "correctly", the
processor can still re-order the memory access.

Add a write memory barrier in the allocation side and a read memory
barrier when the port is valid to prevent any re-ordering issue.

This is XSA-340.

Reported-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>