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

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Date:   Thu, 24 Mar 2022 12:03:07 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, maz@...nel.org, tglx@...utronix.de,
        peterz@...radead.org, keirf@...gle.com
Subject: Re: [PATCH 1/3] virtio: use virtio_device_ready() in
 virtio_device_restore()

On Thu, Mar 24, 2022 at 06:48:05AM -0400, Michael S. Tsirkin wrote:
>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?

Nope, I mean, when I wrote the original version, I meant to do the same 
things that we do in virtio_dev_probe() where we called 
virtio_device_ready() which not only set the state, but also called 
.enable_cbs callback.

Was this a side effect and maybe more compliant with the spec?

>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);

Could there be a case where we want to set the status again even though 
the device tells us it's already set?

I think not, so I guess it's okay.

>
>
>> 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.

I agree, copy & paste from virtio_dev_probe().

Jason can you fix this patch or should I resend?

Thanks,
Stefano