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

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Date:   Tue, 12 Apr 2022 10:21:54 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Halil Pasic <pasic@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Marc Zyngier <maz@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH V2 4/5] virtio-pci: implement synchronize_vqs()

On Mon, Apr 11, 2022 at 4:56 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Mon, Apr 11, 2022 at 04:22:19PM +0800, Jason Wang wrote:
> > On Sun, Apr 10, 2022 at 3:51 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > >
> > > On Fri, Apr 08, 2022 at 03:03:07PM +0200, Halil Pasic wrote:
> > > > 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.
> > >
> > > Right.
> > >
> > > I am currently thinking instead of making this optional in the
> > > core we should make it mandatory, and have transports which do not
> > > need to sync have an empty stub with documentation explaining why.
> > >
> > > Also, do we want to document this sync is explicitly for irq enable/disable?
> > > synchronize_irq_enable_disable?
> >
> > I would not since the transport is not guaranteed to use an interrupt
> > for callbacks.
>
> OK but let's then document this in more detail.
> More readers will wonder about what is the callback
> trying to accomplish, and Halil requested that as well.
>
> For example, let's document why is sync required on enable.

Ok.

Thanks

>
> > >
> > >
> > > > >
> > > > > >
> > > > > >> ---
> > > > > >>  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.
> >
> > That's my feeling as well.
> >
> > Thanks
> >
> > > >
> > > > Sorry for the late response. I intend to revisit this on Monday. If
> > > > I don't please feel encouraged to ping.
> > > >
> > > > Regards,
> > > > Halil
> > >
>