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virtio: harsher barriers for rpmsg.

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We were cheating with our barriers; using the smp ones rather than the
real device ones.  That was fine, until rpmsg came along, which is
used to talk to a real device (a non-SMP CPU).

Unfortunately, just putting back the real barriers (reverting
d57ed95d) causes a performance regression on virtio-pci.  In
particular, Amos reports netbench's TCP_RR over virtio_net CPU
utilization increased up to 35% while throughput went down by up to
14%.

By comparison, this branch is in the noise.

Reference: https://lkml.org/lkml/2011/12/11/22

Signed-off-by: Rusty Russell <[email protected]>
This commit is contained in:
Rusty Russell 2012-01-12 15:44:42 +10:30
commit 7b21e34fd1
8 changed files with 35 additions and 22 deletions
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8
drivers/lguest/lguest_device.c
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@ -292,10 +292,12 @@ static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
/*
* OK, tell virtio_ring.c to set up a virtqueue now we know its size
* and we've got a pointer to its pages.
* and we've got a pointer to its pages. Note that we set weak_barriers
* to 'true': the host just a(nother) SMP CPU, so we only need inter-cpu
* barriers.
*/
vq = vring_new_virtqueue(lvq->config.num, LGUEST_VRING_ALIGN,
vdev, lvq->pages, lg_notify, callback, name);
vq = vring_new_virtqueue(lvq->config.num, LGUEST_VRING_ALIGN, vdev,
true, lvq->pages, lg_notify, callback, name);
if (!vq) {
err = -ENOMEM;
goto unmap;