Drivers: hv: vmbus: Prevent load re-ordering when reading ring buffer

When reading a packet from a host-to-guest ring buffer, there is no
memory barrier between reading the write index (to see if there is
a packet to read) and reading the contents of the packet. The Hyper-V
host uses store-release when updating the write index to ensure that
writes of the packet data are completed first. On the guest side,
the processor can reorder and read the packet data before the write
index, and sometimes get stale packet data. Getting such stale packet
data has been observed in a reproducible case in a VM on ARM64.

Fix this by using virt_load_acquire() to read the write index,
ensuring that reads of the packet data cannot be reordered
before it. Preventing such reordering is logically correct, and
with this change, getting stale data can no longer be reproduced.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Link: https://lore.kernel.org/r/1648394710-33480-1-git-send-email-mikelley@microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>