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Date:   Mon, 13 Sep 2021 02:41:33 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     virtualization <virtualization@lists.linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "Hetzelt, Felicitas" <f.hetzelt@tu-berlin.de>,
        "kaplan, david" <david.kaplan@amd.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Subject: Re: [PATCH 7/9] virtio-pci: harden INTX interrupts
Message-ID: <20210913023757-mutt-send-email-mst@kernel.org>
References: <20210913055353.35219-1-jasowang@redhat.com>
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On Mon, Sep 13, 2021 at 02:36:54PM +0800, Jason Wang wrote:
> On Mon, Sep 13, 2021 at 2:33 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Mon, Sep 13, 2021 at 01:53:51PM +0800, Jason Wang wrote:
> > > This patch tries to make sure the virtio interrupt handler for INTX
> > > won't be called after a reset and before virtio_device_ready(). We
> > > can't use IRQF_NO_AUTOEN since we're using shared interrupt
> > > (IRQF_SHARED). So this patch tracks the INTX enabling status in a new
> > > intx_soft_enabled variable and toggle it during in
> > > vp_disable/enable_vectors(). The INTX interrupt handler will check
> > > intx_soft_enabled before processing the actual interrupt.
> > >
> > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> >
> >
> > Not all that excited about all the memory barriers for something
> > that should be an extremely rare event (for most kernels -
> > literally once per boot). Can't we do better?
> 
> I'm not sure, but do we need to care about the slow path (INTX)?

Otherwise we won't try to support this, right?

> (Or do you have a better approach?)
> 
> Thanks

Don't know really, maybe rcu or whatever?
But let's try to be much more specific - is there anything
specific we are trying to protect against here?


> >
> > > ---
> > >  drivers/virtio/virtio_pci_common.c | 18 ++++++++++++++++--
> > >  drivers/virtio/virtio_pci_common.h |  1 +
> > >  2 files changed, 17 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
> > > index 0b9523e6dd39..835197151dc1 100644
> > > --- a/drivers/virtio/virtio_pci_common.c
> > > +++ b/drivers/virtio/virtio_pci_common.c
> > > @@ -30,8 +30,12 @@ void vp_disable_vectors(struct virtio_device *vdev)
> > >       struct virtio_pci_device *vp_dev = to_vp_device(vdev);
> > >       int i;
> > >
> > > -     if (vp_dev->intx_enabled)
> > > +     if (vp_dev->intx_enabled) {
> > > +             vp_dev->intx_soft_enabled = false;
> > > +             /* ensure the vp_interrupt see this intx_soft_enabled value */
> > > +             smp_wmb();
> > >               synchronize_irq(vp_dev->pci_dev->irq);
> > > +     }
> > >
> > >       for (i = 0; i < vp_dev->msix_vectors; ++i)
> > >               disable_irq(pci_irq_vector(vp_dev->pci_dev, i));
> > > @@ -43,8 +47,12 @@ void vp_enable_vectors(struct virtio_device *vdev)
> > >       struct virtio_pci_device *vp_dev = to_vp_device(vdev);
> > >       int i;
> > >
> > > -     if (vp_dev->intx_enabled)
> > > +     if (vp_dev->intx_enabled) {
> > > +             vp_dev->intx_soft_enabled = true;
> > > +             /* ensure the vp_interrupt see this intx_soft_enabled value */
> > > +             smp_wmb();
> > >               return;
> > > +     }
> > >
> > >       for (i = 0; i < vp_dev->msix_vectors; ++i)
> > >               enable_irq(pci_irq_vector(vp_dev->pci_dev, i));
> > > @@ -97,6 +105,12 @@ static irqreturn_t vp_interrupt(int irq, void *opaque)
> > >       struct virtio_pci_device *vp_dev = opaque;
> > >       u8 isr;
> > >
> > > +     if (!vp_dev->intx_soft_enabled)
> > > +             return IRQ_NONE;
> > > +
> > > +     /* read intx_soft_enabled before read others */
> > > +     smp_rmb();
> > > +
> > >       /* reading the ISR has the effect of also clearing it so it's very
> > >        * important to save off the value. */
> > >       isr = ioread8(vp_dev->isr);
> > > diff --git a/drivers/virtio/virtio_pci_common.h b/drivers/virtio/virtio_pci_common.h
> > > index a235ce9ff6a5..3c06e0f92ee4 100644
> > > --- a/drivers/virtio/virtio_pci_common.h
> > > +++ b/drivers/virtio/virtio_pci_common.h
> > > @@ -64,6 +64,7 @@ struct virtio_pci_device {
> > >       /* MSI-X support */
> > >       int msix_enabled;
> > >       int intx_enabled;
> > > +     bool intx_soft_enabled;
> > >       cpumask_var_t *msix_affinity_masks;
> > >       /* Name strings for interrupts. This size should be enough,
> > >        * and I'm too lazy to allocate each name separately. */
> > > --
> > > 2.25.1
> >