linux-kernel - Re: [PATCH V2 4/5] virtio-pci: implement synchronize

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Date:   Fri, 8 Apr 2022 15:03:07 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, peterz@...radead.org,
        maz@...nel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, tglx@...utronix.de,
        Halil Pasic <pasic@...ux.ibm.com>
Subject: Re: [PATCH V2 4/5] virtio-pci: implement synchronize_vqs()

On Wed, 06 Apr 2022 15:04:32 +0200
Cornelia Huck <cohuck@...hat.com> wrote:

> On Wed, Apr 06 2022, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
> > On Wed, Apr 06, 2022 at 04:35:37PM +0800, Jason Wang wrote:  
> >> This patch implements PCI version of synchronize_vqs().
> >> 
> >> Cc: Thomas Gleixner <tglx@...utronix.de>
> >> Cc: Peter Zijlstra <peterz@...radead.org>
> >> Cc: "Paul E. McKenney" <paulmck@...nel.org>
> >> Cc: Marc Zyngier <maz@...nel.org>
> >> Signed-off-by: Jason Wang <jasowang@...hat.com>  
> >
> > Please add implementations at least for ccw and mmio.  
> 
> I'm not sure what (if anything) can/should be done for ccw...

If nothing needs to be done I would like to have at least a comment in
the code that explains why. So that somebody who reads the code
doesn't wonder: why is virtio-ccw not implementing that callback.

> 
> >  
> >> ---
> >>  drivers/virtio/virtio_pci_common.c | 14 ++++++++++++++
> >>  drivers/virtio/virtio_pci_common.h |  2 ++
> >>  drivers/virtio/virtio_pci_legacy.c |  1 +
> >>  drivers/virtio/virtio_pci_modern.c |  2 ++
> >>  4 files changed, 19 insertions(+)
> >> 
> >> diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
> >> index d724f676608b..b78c8bc93a97 100644
> >> --- a/drivers/virtio/virtio_pci_common.c
> >> +++ b/drivers/virtio/virtio_pci_common.c
> >> @@ -37,6 +37,20 @@ void vp_synchronize_vectors(struct virtio_device *vdev)
> >>  		synchronize_irq(pci_irq_vector(vp_dev->pci_dev, i));
> >>  }
> >>  
> >> +void vp_synchronize_vqs(struct virtio_device *vdev)
> >> +{
> >> +	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
> >> +	int i;
> >> +
> >> +	if (vp_dev->intx_enabled) {
> >> +		synchronize_irq(vp_dev->pci_dev->irq);
> >> +		return;
> >> +	}
> >> +
> >> +	for (i = 0; i < vp_dev->msix_vectors; ++i)
> >> +		synchronize_irq(pci_irq_vector(vp_dev->pci_dev, i));
> >> +}
> >> +  
> 
> ...given that this seems to synchronize threaded interrupt handlers?
> Halil, do you think ccw needs to do anything? (AFAICS, we only have one
> 'irq' for channel devices anyway, and the handler just calls the
> relevant callbacks directly.)

Sorry I don't understand enough yet. A more verbose documentation on
"virtio_synchronize_vqs - synchronize with virtqueue callbacks" would
surely benefit me. It may be more than enough for a back-belt but it
ain't enough for me to tell what is the callback supposed to accomplish.

I will have to study this discussion and the code more thoroughly.
Tentatively I side with Jason and Michael in a sense, that I don't
believe virtio-ccw is safe against rough interrupts.

Sorry for the late response. I intend to revisit this on Monday. If
I don't please feel encouraged to ping.

Regards,
Halil