On Wed, May 11, 2022 at 4:44 PM Cornelia Huck <cohuck@xxxxxxxxxx> wrote:
>
> On Wed, May 11 2022, Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
> > On Tue, May 10, 2022 at 7:32 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> >>
> >> On Sat, May 07, 2022 at 03:19:53PM +0800, Jason Wang wrote:
> >> > diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> >> > index d8a2340f928e..23f1694cdbd5 100644
> >> > --- a/include/linux/virtio_config.h
> >> > +++ b/include/linux/virtio_config.h
> >> > @@ -256,6 +256,18 @@ void virtio_device_ready(struct virtio_device *dev)
> >> > unsigned status = dev->config->get_status(dev);
> >> >
> >> > BUG_ON(status & VIRTIO_CONFIG_S_DRIVER_OK);
> >> > +
> >> > + /*
> >> > + * The virtio_synchronize_cbs() makes sure vring_interrupt()
> >> > + * will see the driver specific setup if it sees vq->broken
> >> > + * as false.
> >> > + */
> >> > + virtio_synchronize_cbs(dev);
> >>
> >> since you mention vq->broken above, maybe add
> >> "set vq->broken to false"
> >
> > Ok.
> >
> >>
> >> > + __virtio_unbreak_device(dev);
> >> > + /*
> >> > + * The transport is expected ensure the visibility of
> >>
> >> to ensure
> >
> > Will fix.
> >
> >>
> >> > + * vq->broken
> >>
> >> let's add: "visibility by vq callbacks"
> >
> > Sure.
> >
> >>
> >> > before setting VIRTIO_CONFIG_S_DRIVER_OK.
> >> > + */
> >>
> >>
> >> Can I see some analysis of existing transports showing
> >> this is actually the case for them?
> >
> > Yes.
> >
> >> And maybe add a comment near set_status to document the
> >> requirement.
> >
> > For PCI and MMIO, we can quote the memory-barriers.txt or explain that
> > wmb() is not needed before the MMIO writel().
> > For CCW, it looks not obvious, it looks to me the IO was submitted via
> > __ssch() which has an inline assembly. Cornelia and Hali, could you
> > help me to understand if and how did virtio_ccw_set_status() can
> > ensure the visibility of the previous driver setup and vq->broken
> > here?
>
> I'm not sure I completely understand the question here, but let me try:
It's something like the following case:
CPU 0: vq->broken = false
CPU 0: set_status(DRIVER_OK)
CPU 1: vring_interrupt() { if (vq->broken) return IRQ_NONE; }
We need to make sure the CPU 1 sees the vq->broken if the interrupt is
raised after DRVER_OK.
For PCI, we use MMIO of writel() for set_status(), a wmb() is not
needed in this case according to memory-barriers.txt.
"
Note that, when using writel(), a prior
wmb() is not needed to guarantee that the cache coherent memory writes
have completed before writing to the MMIO region.
"
So CPU 1 will see the broken as false.
>
> virtio_ccw_set_status() uses a channel command to set the status, with
> the interesting stuff done inside ccw_io_helper(). That function
> - takes the subchannel lock, disabling interrupts
Then it is, for x86 the operation to disable interrupt is a full
barrier. I guess this should apply to other architecture like s390. I
see a stnsm is used in this case but a quick google doesn't tell me if
it's a barrier.
If this is true. The vring_interrupt will see broken as false.
> - does the ssch; this instruction will fail if there's already another
> I/O in progress, or an interrupt is pending for the subchannel; on
> success, it is guaranteed that we'll get an interrupt eventually
I guess ssch might imply a barrier as well, otherwise we may need a
lot of barriers before this.
Thanks
> - unlock the subchannel, and wait for the interupt handler to eventually
> process the interrupt, so I guess it should see the vq->broken value?
>
> If the I/O fails, virtio_ccw_set_status() will revert its internal
> status to the old value.
>
>
> >
> > Thanks
> >
> >>
> >> > dev->config->set_status(dev, status | VIRTIO_CONFIG_S_DRIVER_OK);
> >> > }
>
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