From:   Stefano Garzarella <sgarzare@redhat.com>
To:     virtualization@lists.linux-foundation.org
Cc:     netdev@vger.kernel.org, stefanha@redhat.com,
        Jason Wang <jasowang@redhat.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>
Subject: [RFC PATCH] vhost: cache avail index in vhost_enable_notify()
Date:   Thu, 13 Jan 2022 15:56:42 +0100
Message-Id: <20220113145642.205388-1-sgarzare@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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. In this case, the device would go to re-read avail
index to access the descriptor.

As we already do in other place, we can cache the value in `avail_idx`
and compare it with `last_avail_idx` to check if there are new
buffers available.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
 drivers/vhost/vhost.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 59edb5a1ffe2..07363dff559e 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2543,8 +2543,9 @@ 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);
 
-- 
2.31.1