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

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Date:   Thu, 24 Mar 2022 07:07:09 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Stefano Garzarella <sgarzare@...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 12:03:07PM +0100, Stefano Garzarella wrote:
> 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?


Sorry I don't understand the question. it says "avoids setting DRIVER_OK twice" -
why is that advantageous and worth calling out in the commit log?


> > 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