netdev - [RFC PATCH] vhost: cache avail index in vhost_enable

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Date:   Thu, 13 Jan 2022 15:56:42 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     virtualization@...ts.linux-foundation.org
Cc:     netdev@...r.kernel.org, stefanha@...hat.com,
        Jason Wang <jasowang@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>
Subject: [RFC PATCH] 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. 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@...hat.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