Re: [PATCH v2] virtio: Add polling virtio-mmio device reset completion

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Sep 26, 2024 at 11:21:25AM +0800, Jason Wang wrote:
> On Thu, Sep 26, 2024 at 10:27 AM Jianguo Sun <quic_jianguos@quicinc.com> wrote:
> >
> > We write 0 to device_status to initiate reset of a virtio-mmio device.
> > The reset operation itself may or may not be completed by the time
> > write instruction completes. Add polling device_status to return
> > 0 to ensure reset completion before reinitializing the device.
> >
> > Change-Id: If15d11d090dfd0d4972ad35f49af03e076872413
> > Signed-off-by: Jianguo Sun <quic_jianguos@quicinc.com>
> > Signed-off-by: Sreenad Menon <quic_sreemeno@quicinc.com>
> > Signed-off-by: Anant Goel <quic_anantg@quicinc.com>
> > ---
> >  drivers/virtio/virtio_mmio.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
> > index 90e784e7b721..cf1a4a3dd35d 100644
> > --- a/drivers/virtio/virtio_mmio.c
> > +++ b/drivers/virtio/virtio_mmio.c
> > @@ -70,6 +70,7 @@
> >  #include <linux/virtio_config.h>
> >  #include <uapi/linux/virtio_mmio.h>
> >  #include <linux/virtio_ring.h>
> > +#include <linux/delay.h>
> >
> >
> >
> > @@ -269,6 +270,11 @@ static void vm_reset(struct virtio_device *vdev)
> >
> >         /* 0 status means a reset. */
> >         writel(0, vm_dev->base + VIRTIO_MMIO_STATUS);
> > +       /* After writing 0 to device_status, the driver MUST wait for a read of
> > +        * device_status to return 0 before reinitializing the device.
> > +        */
> 
> Is this copied from the spec? I can only see a similar description in
> the PCI but not MMIO part.

Not in spec. We can add something like this in spec, but it
can not be a MUST, has to be a SHOULD.

> > +       while (readl(vm_dev->base + VIRTIO_MMIO_STATUS))
> > +               msleep(1);
> 
> Again, we probably need a feature bit to unbreak existing MMIO devices?
> 
> THanks

We can do this unconditionally since legacy devices return 0
immediately, feature negotiation happens after reset, won't be useful
here.


> >  }
> >
> >
> > --
> > 2.17.1
> >