linux-kernel - Re: [PATCH v3 11/11] virtio: balloon: Add freeze, restore handlers to support S4

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Date:	Thu, 17 Nov 2011 17:59:20 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Virtualization List <virtualization@...ts.linux-foundation.org>,
	levinsasha928@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 11/11] virtio: balloon: Add freeze, restore handlers
 to support S4

On (Thu) 17 Nov 2011 [14:25:08], Michael S. Tsirkin wrote:
> On Thu, Nov 17, 2011 at 05:27:42PM +0530, Amit Shah wrote:
> > Delete the vqs on the freeze callback to prepare for hibernation.
> > Re-create the vqs in the restore callback to resume normal function.
> > 
> > Signed-off-by: Amit Shah <amit.shah@...hat.com>
> > ---
> >  drivers/virtio/virtio_balloon.c |   20 ++++++++++++++++++++
> >  1 files changed, 20 insertions(+), 0 deletions(-)
> > 
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index 1ff3cf4..90149d1 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -363,6 +363,22 @@ static void __devexit virtballoon_remove(struct virtio_device *vdev)
> >  	kfree(vb);
> >  }
> >  
> > +#ifdef CONFIG_PM
> > +static int virtballoon_freeze(struct virtio_device *vdev)
> > +{
> > +	/* Now we reset the device so we can clean up the queues. */
> > +	vdev->config->reset(vdev);
> > +
> 
> This is a weird thing to do. We normally delete vqs then reset.

No, all the drivers first reset, then delete.

> For example, I don't know whether we guarantee what happens
> to MSI-X vectors assigned meanwhile if you reset.
> IIRC qemu doesn't use MSI-X for the balloon, but nothing
> prevents it.
> 
> > +	vdev->config->del_vqs(vdev);
> 
> What prevents requests being submitted at this point?
> I see all kind of handling of freezing in the balloon thread ...
> maybe that gives us some guarantees, but if yes
> it makes sense to add a comment to point this out.

Once reset, the host won't write anything to the vqs anyway..

		Amit
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