tree 697c362d07b7e803f09a10293c4064bd56be40d5
parent 63985c224d4f40572613479482d2dacdbfcdead9
author Venkatesh Srinivas <venkateshs@google.com> 1548804623 -0800
committer CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org> 1548812289 +0000

[virtio] ring: Barriers before updating avail->idx/after reading used->idx

virtio::ring needs to have a write memory barrier with the DMA domain
after writing descriptor chains, before updating avail->idx, to ensure
the device sees up-to-date descriptor chains.

virtio::ring needs to have a read memory barrier with the DMA domain
after reading used->idx and before reading completed descriptor chains
from the device.

virtio::ring needs a write barrier after updating avail->idx but before
notifying the device, so that the device sees the newest avail->idx.
This barrier could be fused into PIO/MMIO notification writes in the
future - on some platforms UC stores may imply the required barrier.

Tested:
* Used virtio-net and virtio-blk on QEMU

Change-Id: I0bd1529022625a1509b657c443b6259c0a061d5b