From d3bb267bbdcba199568f1325743d9d501dea0560 Mon Sep 17 00:00:00 2001 From: Stefano Garzarella Date: Fri, 21 Jan 2022 16:31:08 +0100 Subject: vhost: cache avail index in vhost_enable_notify() In vhost_enable_notify() we enable the notifications and we read the avail index to check if new buffers have become available in the meantime. We are not caching the avail index, so when the device will call vhost_get_vq_desc(), it will find the old value in the cache and it will read the avail index again. It would be better to refresh the cache every time we read avail index, so let's change vhost_enable_notify() caching the value in `avail_idx` and compare it with `last_avail_idx` to check if there are new buffers available. We don't expect a significant performance boost because the above path is not very common, indeed vhost_enable_notify() is often called with unlikely(), expecting that avail index has not been updated. We ran virtio-test/vhost-test and noticed minimal improvement as expected. To stress the patch more, we modified vhost_test.ko to call vhost_enable_notify()/vhost_disable_notify() on every cycle when calling vhost_get_vq_desc(); in this case we observed a more evident improvement, with a reduction of the test execution time of about 3.7%. Signed-off-by: Stefano Garzarella Link: https://lore.kernel.org/r/20220121153108.187291-1-sgarzare@redhat.com Signed-off-by: Michael S. Tsirkin Reviewed-by: Stefan Hajnoczi --- drivers/vhost/vhost.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'drivers/vhost') diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index 1768362115c6..d02173fb290c 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -2550,8 +2550,9 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq) &vq->avail->idx, r); return false; } + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); - return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx; + return vq->avail_idx != vq->last_avail_idx; } EXPORT_SYMBOL_GPL(vhost_enable_notify); -- cgit v1.2.3