linux-kernel - Re: [PATCH 1/3] virtio: use virtio_device_ready() in virtio_device

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Date:   Thu, 24 Mar 2022 06:48:05 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, maz@...nel.org, tglx@...utronix.de,
        peterz@...radead.org, sgarzare@...hat.com, keirf@...gle.com
Subject: Re: [PATCH 1/3] virtio: use virtio_device_ready() in
 virtio_device_restore()

On Thu, Mar 24, 2022 at 04:40:02PM +0800, Jason Wang wrote:
> From: Stefano Garzarella <sgarzare@...hat.com>
> 
> This avoids setting DRIVER_OK twice for those drivers that call
> virtio_device_ready() in the .restore

Is this trying to say it's faster?
If yes this one looks like a red herring. Yes we skip a write but we
replace it with a read which is not better performance-wise.
If we want to optimize this, it is better to just do that inside
virtio_add_status:



diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index 75c8d560bbd3..cd943c31bdbb 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -161,8 +161,14 @@ static void virtio_config_enable(struct virtio_device *dev)
 
 void virtio_add_status(struct virtio_device *dev, unsigned int status)
 {
+	unsigned int device_status;
+
 	might_sleep();
-	dev->config->set_status(dev, dev->config->get_status(dev) | status);
+
+	device_status = dev->config->get_status(dev);
+
+	if (status & ~device_status)
+		dev->config->set_status(dev, device_status | status);
 }
 EXPORT_SYMBOL_GPL(virtio_add_status);
 

> and it will allows us to do
> extension on virtio_device_ready() without duplicating codes.
> 
> Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
>  drivers/virtio/virtio.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index 22f15f444f75..75c8d560bbd3 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -526,8 +526,9 @@ int virtio_device_restore(struct virtio_device *dev)
>  			goto err;
>  	}
>  
> -	/* Finally, tell the device we're all set */
> -	virtio_add_status(dev, VIRTIO_CONFIG_S_DRIVER_OK);
> +	/* If restore didn't do it, mark device DRIVER_OK ourselves. */

I preferred the original comment, it said why we are doing this,
new one repeats what code is doing.

> +	if (!(dev->config->get_status(dev) & VIRTIO_CONFIG_S_DRIVER_OK))
> +		virtio_device_ready(dev);
>  
>  	virtio_config_enable(dev);
>  
> -- 
> 2.25.1