Re: [PATCH v2 1/3] virtio: update reset callback to return status

[Max Gurtovoy <mgurtovoy@xxxxxxxxxx>]

On 4/13/2021 7:42 AM, Michael S. Tsirkin wrote:
> On Tue, Apr 13, 2021 at 01:53:02AM +0300, Max Gurtovoy wrote:
> > On 4/13/2021 12:23 AM, Michael S. Tsirkin wrote:
> > > On Mon, Apr 12, 2021 at 04:03:02PM +0300, Max Gurtovoy wrote:
> > > > On 4/12/2021 3:04 PM, Michael S. Tsirkin wrote:
> > > > > On Mon, Apr 12, 2021 at 02:55:27PM +0300, Max Gurtovoy wrote:
> > > > > > On 4/8/2021 6:56 PM, Michael S. Tsirkin wrote:
> > > > > > > On Thu, Apr 08, 2021 at 12:56:52PM +0300, Max Gurtovoy wrote:
> > > > > > > > On 4/8/2021 11:58 AM, Jason Wang wrote:
> > > > > > > > > 在 2021/4/8 下午4:11, Max Gurtovoy 写道:
> > > > > > > > > > The reset device operation, usually is an operation that might fail from
> > > > > > > > > > various reasons. For example, the controller might be in a bad state and
> > > > > > > > > > can't answer to any request. Usually, the paravirt SW based virtio
> > > > > > > > > > devices always succeed in reset operation but this is not the case for
> > > > > > > > > > HW based virtio devices.
> > > > > > > > > I would like to know under what condition that the reset operation may
> > > > > > > > > fail (except for the case of a bugg guest).
> > > > > > > > The controller might not be ready or stuck. This is a real use case for many
> > > > > > > > PCI devices.
> > > > > > > >
> > > > > > > > For real devices the FW might be in a bad state and it can happen also for
> > > > > > > > paravirt device if you have a bug in the controller code or if you entered
> > > > > > > > some error flow (Out of memory).
> > > > > > > >
> > > > > > > > You don't want to be stuck because of one bad device.
> > > > > > > OK so maybe we can do more to detect the bad device.
> > > > > > > Won't we get all 1's on a read in this case?
> > > > > > No. how can we guarantee it ?
> > > > > Well this is what you tend to get if e.g. you disable device memory.
> > > > >
> > > > > Anyway, you know about hardware, I don't ... It's not returning 0 after
> > > > > reset as it should ... what does it return? Hopefully not random noise -
> > > > > I don't think it's very practical to write a driver for a device that
> > > > > starts doing that at random times ...
> > > > The device may return 0x40 (NEEDS_RESET). It doesn't have to return all 1's.
> > > >
> > > > For paravirt devices, think of a situation that you can't allocate some
> > > > internal buffers (malloc failed) and you want to wait for few seconds until
> > > > the system memory will free some pages.
> > > >
> > > > So you may return NEEDS_RESET that indicates some error state of the device.
> > > > Once the system memory freed by other application for example, your internal
> > > > virtio device malloc succeeded and you may return 0.
> > > >
> > > > In this case, you don't want to stall the other virtio devices to probe
> > > > (they might be real HW devices that driven by the same driver), right ?
> > > So the device is very busy then? Not sure it's smart to just assume
> > > it's safe to free all memory allocated for it then ...
> > >
> > > I guess the lesson is don't make device reset depend on malloc
> > > of some memory?
> > The device is not ready yet. And the malloc is just one example I gave you
> > to emphasize the case.
> >
> > Another example can be a bad FW installed.
> Surely this will wedge the host device to the point where vdpa won't
> even be probed?

what VDPA device ?


> > The host/guest driver is trying to enable the device but the device is not
> > ready. This is the real life use case and I gave many examples for reasons
> > for device not to be ready. For paravirt and HW devices.
> >
> > Endless loop and stalling next devices probe is not the way to go. PCI
> > drivers can't allow this to happen.
> >
> > Think of a situation that host it's booting from virtio-blk device but it
> > has another virtio-net device that has bad FW or other internal error (it
> > doesn't matter what it is for the example) and the virtio pci driver is
> > probing virtio-net device first.
> >
> > The host will never boot successfully. This is fatal.
> It's not nice for sure. But broken hardware is broken hardware.

what does it mean ?

broken SW is broken SW as well.

And maybe it's not something that can't be solved manually. why do you 
insist on breaking application in case of error flows ?

what will happen if, for example, virtiofsd process will be killed ? do 
you want the entire guest to suffer ?


> Try not to have it is still best advice.
> Making very sure this will never regress any useful flows is high
> priority though.  Thus the focus on addressing specific issues
> not just falling over the moment something seems a bit fishy.
>
> > Error flows are critical when working with real PCI HW and I understand that
> > in paravirt devices with strong hypervisor you will be ok in 99% of the time
> > but you need to be aware also for bugs and error flows in both paravirt and
> > HW world.
> >
> > So first we need to handle this endless loop (with this patch set or with
> > async probing mechanism) and later we should update the specification.
> >
> > The virtio world now is not only guest and paravirt devices. Bare metal
> > hosts start using virtio devices and drivers more and more.
> Excellent so can we please stop talking hypotheticals and discuss how
> does actual hardware behave, and how best to recover in each case?

Why hypothetical ? if you say that SW bugs in paravirt controllers or 
some system stress is hypothetical so I dis-agree.

For HW devices, I gave you an example of broken FW.

In DPU solutions you enter a world with entire matrix of errors that can 
happen.

Error sometimes are recoverable, but virtio driver probes in sync way 
and this is wrong.

I gave you an example of booting from virtio-blk device that might be fatal.


> Recovering from weird scenarious like bad firmware is nice to have but
> not at risk of e.g. corrupting kernel memory. Which blindly removing the
> driver with an active device has a good chance to do.

weird ? This is the reality. If we will ignore it, this will not be fixed.

Error flows are not nice to have. They are the building blocks of 
developing robust solutions.


> Just to stress the importance of addressing actual issues,
> how was this patch even tested? Worth mentioning in the commit log.

With NVIDIA virtio-blk SNAP device.

I plugged virtio-blk pci function to host pci subsystem, and I entered 
the controller to error state.

Then I plugged another function in good state but it was never probed 
without this patch set.

With this patch set, after a timeout the first device failed to probed 
and cleaned all needed resources and the second device probed successfully.

later on, I moved the first controller from error state to good state 
and re-scanned the pci bus to re-probe it (There are other ways to cause 
the driver to re-probe it).

> And by the way, there are actually use-cases I am aware of where we spin
> forever and really should not, for example it would be super nice to
> support surprise removal so host can drop the driver after device is
> gone without waiting for guest. This allows allocating the device to
> another VM.  In that case too, an out of the blue timeout and blindly
> assuming all is well is not the way to go, you need to probe device to
> check whether it's dead or alive.
> Maybe your error recovery can fit in such a framework.
Spinning forever in kernel is not a good practice and we can think of 
many scenarios. But we should start somewhere and continue improving the 
stack.


> > > > > > > > > > This commit is also a preparation for adding a timeout mechanism for
> > > > > > > > > > resetting virtio devices.
> > > > > > > > > >
> > > > > > > > > > Signed-off-by: Max Gurtovoy <mgurtovoy@xxxxxxxxxx>
> > > > > > > > > > ---
> > > > > > > > > >
> > > > > > > > > > changes from v1:
> > > > > > > > > >       - update virtio_ccw.c (Cornelia)
> > > > > > > > > >       - update virtio_uml.c
> > > > > > > > > >       - update mlxbf-tmfifo.c
> > > > > > > > > Note that virtio driver may call reset, so you probably need to convert
> > > > > > > > > them.
> > > > > > > > I'm sure I understand.
> > > > > > > >
> > > > > > > > Convert to what ?
> > > > > > > >
> > > > > > > > Thanks.
> > > > > > > >
> > > > > > > > > Thanks
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > > ---
> > > > > > > > > >       arch/um/drivers/virtio_uml.c             |  4 +++-
> > > > > > > > > >       drivers/platform/mellanox/mlxbf-tmfifo.c |  4 +++-
> > > > > > > > > >       drivers/remoteproc/remoteproc_virtio.c   |  4 +++-
> > > > > > > > > >       drivers/s390/virtio/virtio_ccw.c         |  9 ++++++---
> > > > > > > > > >       drivers/virtio/virtio.c                  | 22 +++++++++++++++-------
> > > > > > > > > >       drivers/virtio/virtio_mmio.c             |  3 ++-
> > > > > > > > > >       drivers/virtio/virtio_pci_legacy.c       |  4 +++-
> > > > > > > > > >       drivers/virtio/virtio_pci_modern.c       |  3 ++-
> > > > > > > > > >       drivers/virtio/virtio_vdpa.c             |  4 +++-
> > > > > > > > > >       include/linux/virtio_config.h            |  5 +++--
> > > > > > > > > >       10 files changed, 43 insertions(+), 19 deletions(-)
> > > > > > > > > >
> > > > > > > > > > diff --git a/arch/um/drivers/virtio_uml.c b/arch/um/drivers/virtio_uml.c
> > > > > > > > > > index 91ddf74ca888..b6e66265ed32 100644
> > > > > > > > > > --- a/arch/um/drivers/virtio_uml.c
> > > > > > > > > > +++ b/arch/um/drivers/virtio_uml.c
> > > > > > > > > > @@ -827,11 +827,13 @@ static void vu_set_status(struct virtio_device
> > > > > > > > > > *vdev, u8 status)
> > > > > > > > > >           vu_dev->status = status;
> > > > > > > > > >       }
> > > > > > > > > >       -static void vu_reset(struct virtio_device *vdev)
> > > > > > > > > > +static int vu_reset(struct virtio_device *vdev)
> > > > > > > > > >       {
> > > > > > > > > >           struct virtio_uml_device *vu_dev = to_virtio_uml_device(vdev);
> > > > > > > > > >             vu_dev->status = 0;
> > > > > > > > > > +
> > > > > > > > > > +    return 0;
> > > > > > > > > >       }
> > > > > > > > > >         static void vu_del_vq(struct virtqueue *vq)
> > > > > > > > > > diff --git a/drivers/platform/mellanox/mlxbf-tmfifo.c
> > > > > > > > > > b/drivers/platform/mellanox/mlxbf-tmfifo.c
> > > > > > > > > > index bbc4e71a16ff..c192b8ac5d9e 100644
> > > > > > > > > > --- a/drivers/platform/mellanox/mlxbf-tmfifo.c
> > > > > > > > > > +++ b/drivers/platform/mellanox/mlxbf-tmfifo.c
> > > > > > > > > > @@ -980,11 +980,13 @@ static void
> > > > > > > > > > mlxbf_tmfifo_virtio_set_status(struct virtio_device *vdev,
> > > > > > > > > >       }
> > > > > > > > > >         /* Reset the device. Not much here for now. */
> > > > > > > > > > -static void mlxbf_tmfifo_virtio_reset(struct virtio_device *vdev)
> > > > > > > > > > +static int mlxbf_tmfifo_virtio_reset(struct virtio_device *vdev)
> > > > > > > > > >       {
> > > > > > > > > >           struct mlxbf_tmfifo_vdev *tm_vdev = mlxbf_vdev_to_tmfifo(vdev);
> > > > > > > > > >             tm_vdev->status = 0;
> > > > > > > > > > +
> > > > > > > > > > +    return 0;
> > > > > > > > > >       }
> > > > > > > > > >         /* Read the value of a configuration field. */
> > > > > > > > > > diff --git a/drivers/remoteproc/remoteproc_virtio.c
> > > > > > > > > > b/drivers/remoteproc/remoteproc_virtio.c
> > > > > > > > > > index 0cc617f76068..ca9573c62c3d 100644
> > > > > > > > > > --- a/drivers/remoteproc/remoteproc_virtio.c
> > > > > > > > > > +++ b/drivers/remoteproc/remoteproc_virtio.c
> > > > > > > > > > @@ -191,7 +191,7 @@ static void rproc_virtio_set_status(struct
> > > > > > > > > > virtio_device *vdev, u8 status)
> > > > > > > > > >           dev_dbg(&vdev->dev, "status: %d\n", status);
> > > > > > > > > >       }
> > > > > > > > > >       -static void rproc_virtio_reset(struct virtio_device *vdev)
> > > > > > > > > > +static int rproc_virtio_reset(struct virtio_device *vdev)
> > > > > > > > > >       {
> > > > > > > > > >           struct rproc_vdev *rvdev = vdev_to_rvdev(vdev);
> > > > > > > > > >           struct fw_rsc_vdev *rsc;
> > > > > > > > > > @@ -200,6 +200,8 @@ static void rproc_virtio_reset(struct
> > > > > > > > > > virtio_device *vdev)
> > > > > > > > > >             rsc->status = 0;
> > > > > > > > > >           dev_dbg(&vdev->dev, "reset !\n");
> > > > > > > > > > +
> > > > > > > > > > +    return 0;
> > > > > > > > > >       }
> > > > > > > > > >         /* provide the vdev features as retrieved from the firmware */
> > > > > > > > > > diff --git a/drivers/s390/virtio/virtio_ccw.c
> > > > > > > > > > b/drivers/s390/virtio/virtio_ccw.c
> > > > > > > > > > index 54e686dca6de..52b32555e746 100644
> > > > > > > > > > --- a/drivers/s390/virtio/virtio_ccw.c
> > > > > > > > > > +++ b/drivers/s390/virtio/virtio_ccw.c
> > > > > > > > > > @@ -732,14 +732,15 @@ static int virtio_ccw_find_vqs(struct
> > > > > > > > > > virtio_device *vdev, unsigned nvqs,
> > > > > > > > > >           return ret;
> > > > > > > > > >       }
> > > > > > > > > >       -static void virtio_ccw_reset(struct virtio_device *vdev)
> > > > > > > > > > +static int virtio_ccw_reset(struct virtio_device *vdev)
> > > > > > > > > >       {
> > > > > > > > > >           struct virtio_ccw_device *vcdev = to_vc_device(vdev);
> > > > > > > > > >           struct ccw1 *ccw;
> > > > > > > > > > +    int ret;
> > > > > > > > > >             ccw = ccw_device_dma_zalloc(vcdev->cdev, sizeof(*ccw));
> > > > > > > > > >           if (!ccw)
> > > > > > > > > > -        return;
> > > > > > > > > > +        return -ENOMEM;
> > > > > > > > > >             /* Zero status bits. */
> > > > > > > > > >           vcdev->dma_area->status = 0;
> > > > > > > > > > @@ -749,8 +750,10 @@ static void virtio_ccw_reset(struct
> > > > > > > > > > virtio_device *vdev)
> > > > > > > > > >           ccw->flags = 0;
> > > > > > > > > >           ccw->count = 0;
> > > > > > > > > >           ccw->cda = 0;
> > > > > > > > > > -    ccw_io_helper(vcdev, ccw, VIRTIO_CCW_DOING_RESET);
> > > > > > > > > > +    ret = ccw_io_helper(vcdev, ccw, VIRTIO_CCW_DOING_RESET);
> > > > > > > > > >           ccw_device_dma_free(vcdev->cdev, ccw, sizeof(*ccw));
> > > > > > > > > > +
> > > > > > > > > > +    return ret;
> > > > > > > > > >       }
> > > > > > > > > >         static u64 virtio_ccw_get_features(struct virtio_device *vdev)
> > > > > > > > > > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> > > > > > > > > > index 4b15c00c0a0a..ddbfd5b5f3bd 100644
> > > > > > > > > > --- a/drivers/virtio/virtio.c
> > > > > > > > > > +++ b/drivers/virtio/virtio.c
> > > > > > > > > > @@ -338,7 +338,7 @@ int register_virtio_device(struct virtio_device
> > > > > > > > > > *dev)
> > > > > > > > > >           /* Assign a unique device index and hence name. */
> > > > > > > > > >           err = ida_simple_get(&virtio_index_ida, 0, 0, GFP_KERNEL);
> > > > > > > > > >           if (err < 0)
> > > > > > > > > > -        goto out;
> > > > > > > > > > +        goto out_err;
> > > > > > > > > >             dev->index = err;
> > > > > > > > > >           dev_set_name(&dev->dev, "virtio%u", dev->index);
> > > > > > > > > > @@ -349,7 +349,9 @@ int register_virtio_device(struct virtio_device
> > > > > > > > > > *dev)
> > > > > > > > > >             /* We always start by resetting the device, in case a previous
> > > > > > > > > >            * driver messed it up.  This also tests that code path a
> > > > > > > > > > little. */
> > > > > > > > > > -    dev->config->reset(dev);
> > > > > > > > > > +    err = dev->config->reset(dev);
> > > > > > > > > > +    if (err)
> > > > > > > > > > +        goto out_ida;
> > > > > > > > > >             /* Acknowledge that we've seen the device. */
> > > > > > > > > >           virtio_add_status(dev, VIRTIO_CONFIG_S_ACKNOWLEDGE);
> > > > > > > > > > @@ -362,10 +364,14 @@ int register_virtio_device(struct
> > > > > > > > > > virtio_device *dev)
> > > > > > > > > >            */
> > > > > > > > > >           err = device_add(&dev->dev);
> > > > > > > > > >           if (err)
> > > > > > > > > > -        ida_simple_remove(&virtio_index_ida, dev->index);
> > > > > > > > > > -out:
> > > > > > > > > > -    if (err)
> > > > > > > > > > -        virtio_add_status(dev, VIRTIO_CONFIG_S_FAILED);
> > > > > > > > > > +        goto out_ida;
> > > > > > > > > > +
> > > > > > > > > > +    return 0;
> > > > > > > > > > +
> > > > > > > > > > +out_ida:
> > > > > > > > > > +    ida_simple_remove(&virtio_index_ida, dev->index);
> > > > > > > > > > +out_err:
> > > > > > > > > > +    virtio_add_status(dev, VIRTIO_CONFIG_S_FAILED);
> > > > > > > > > >           return err;
> > > > > > > > > >       }
> > > > > > > > > >       EXPORT_SYMBOL_GPL(register_virtio_device);
> > > > > > > > > > @@ -408,7 +414,9 @@ int virtio_device_restore(struct virtio_device *dev)
> > > > > > > > > >             /* We always start by resetting the device, in case a previous
> > > > > > > > > >            * driver messed it up. */
> > > > > > > > > > -    dev->config->reset(dev);
> > > > > > > > > > +    ret = dev->config->reset(dev);
> > > > > > > > > > +    if (ret)
> > > > > > > > > > +        goto err;
> > > > > > > > > >             /* Acknowledge that we've seen the device. */
> > > > > > > > > >           virtio_add_status(dev, VIRTIO_CONFIG_S_ACKNOWLEDGE);
> > > > > > > > > > diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
> > > > > > > > > > index 56128b9c46eb..12b8f048c48d 100644
> > > > > > > > > > --- a/drivers/virtio/virtio_mmio.c
> > > > > > > > > > +++ b/drivers/virtio/virtio_mmio.c
> > > > > > > > > > @@ -256,12 +256,13 @@ static void vm_set_status(struct virtio_device
> > > > > > > > > > *vdev, u8 status)
> > > > > > > > > >           writel(status, vm_dev->base + VIRTIO_MMIO_STATUS);
> > > > > > > > > >       }
> > > > > > > > > >       -static void vm_reset(struct virtio_device *vdev)
> > > > > > > > > > +static int vm_reset(struct virtio_device *vdev)
> > > > > > > > > >       {
> > > > > > > > > >           struct virtio_mmio_device *vm_dev = to_virtio_mmio_device(vdev);
> > > > > > > > > >             /* 0 status means a reset. */
> > > > > > > > > >           writel(0, vm_dev->base + VIRTIO_MMIO_STATUS);
> > > > > > > > > > +    return 0;
> > > > > > > > > >       }
> > > > > > > > > >         diff --git a/drivers/virtio/virtio_pci_legacy.c
> > > > > > > > > > b/drivers/virtio/virtio_pci_legacy.c
> > > > > > > > > > index d62e9835aeec..0b5d95e3efa1 100644
> > > > > > > > > > --- a/drivers/virtio/virtio_pci_legacy.c
> > > > > > > > > > +++ b/drivers/virtio/virtio_pci_legacy.c
> > > > > > > > > > @@ -89,7 +89,7 @@ static void vp_set_status(struct virtio_device
> > > > > > > > > > *vdev, u8 status)
> > > > > > > > > >           iowrite8(status, vp_dev->ioaddr + VIRTIO_PCI_STATUS);
> > > > > > > > > >       }
> > > > > > > > > >       -static void vp_reset(struct virtio_device *vdev)
> > > > > > > > > > +static int vp_reset(struct virtio_device *vdev)
> > > > > > > > > >       {
> > > > > > > > > >           struct virtio_pci_device *vp_dev = to_vp_device(vdev);
> > > > > > > > > >           /* 0 status means a reset. */
> > > > > > > > > > @@ -99,6 +99,8 @@ static void vp_reset(struct virtio_device *vdev)
> > > > > > > > > >           ioread8(vp_dev->ioaddr + VIRTIO_PCI_STATUS);
> > > > > > > > > >           /* Flush pending VQ/configuration callbacks. */
> > > > > > > > > >           vp_synchronize_vectors(vdev);
> > > > > > > > > > +
> > > > > > > > > > +    return 0;
> > > > > > > > > >       }
> > > > > > > > > >         static u16 vp_config_vector(struct virtio_pci_device *vp_dev,
> > > > > > > > > > u16 vector)
> > > > > > > > > > diff --git a/drivers/virtio/virtio_pci_modern.c
> > > > > > > > > > b/drivers/virtio/virtio_pci_modern.c
> > > > > > > > > > index fbd4ebc00eb6..cc3412a96a17 100644
> > > > > > > > > > --- a/drivers/virtio/virtio_pci_modern.c
> > > > > > > > > > +++ b/drivers/virtio/virtio_pci_modern.c
> > > > > > > > > > @@ -158,7 +158,7 @@ static void vp_set_status(struct virtio_device
> > > > > > > > > > *vdev, u8 status)
> > > > > > > > > >           vp_modern_set_status(&vp_dev->mdev, status);
> > > > > > > > > >       }
> > > > > > > > > >       -static void vp_reset(struct virtio_device *vdev)
> > > > > > > > > > +static int vp_reset(struct virtio_device *vdev)
> > > > > > > > > >       {
> > > > > > > > > >           struct virtio_pci_device *vp_dev = to_vp_device(vdev);
> > > > > > > > > >           struct virtio_pci_modern_device *mdev = &vp_dev->mdev;
> > > > > > > > > > @@ -174,6 +174,7 @@ static void vp_reset(struct virtio_device *vdev)
> > > > > > > > > >               msleep(1);
> > > > > > > > > >           /* Flush pending VQ/configuration callbacks. */
> > > > > > > > > >           vp_synchronize_vectors(vdev);
> > > > > > > > > > +    return 0;
> > > > > > > > > >       }
> > > > > > > > > >         static u16 vp_config_vector(struct virtio_pci_device *vp_dev,
> > > > > > > > > > u16 vector)
> > > > > > > > > > diff --git a/drivers/virtio/virtio_vdpa.c b/drivers/virtio/virtio_vdpa.c
> > > > > > > > > > index e28acf482e0c..5fd4e627a9b0 100644
> > > > > > > > > > --- a/drivers/virtio/virtio_vdpa.c
> > > > > > > > > > +++ b/drivers/virtio/virtio_vdpa.c
> > > > > > > > > > @@ -97,11 +97,13 @@ static void virtio_vdpa_set_status(struct
> > > > > > > > > > virtio_device *vdev, u8 status)
> > > > > > > > > >           return ops->set_status(vdpa, status);
> > > > > > > > > >       }
> > > > > > > > > >       -static void virtio_vdpa_reset(struct virtio_device *vdev)
> > > > > > > > > > +static int virtio_vdpa_reset(struct virtio_device *vdev)
> > > > > > > > > >       {
> > > > > > > > > >           struct vdpa_device *vdpa = vd_get_vdpa(vdev);
> > > > > > > > > >             vdpa_reset(vdpa);
> > > > > > > > > > +
> > > > > > > > > > +    return 0;
> > > > > > > > > >       }
> > > > > > > > > >         static bool virtio_vdpa_notify(struct virtqueue *vq)
> > > > > > > > > > diff --git a/include/linux/virtio_config.h
> > > > > > > > > > b/include/linux/virtio_config.h
> > > > > > > > > > index 8519b3ae5d52..d2b0f1699a75 100644
> > > > > > > > > > --- a/include/linux/virtio_config.h
> > > > > > > > > > +++ b/include/linux/virtio_config.h
> > > > > > > > > > @@ -44,9 +44,10 @@ struct virtio_shm_region {
> > > > > > > > > >        *    status: the new status byte
> > > > > > > > > >        * @reset: reset the device
> > > > > > > > > >        *    vdev: the virtio device
> > > > > > > > > > - *    After this, status and feature negotiation must be done again
> > > > > > > > > > + *    Upon success, status and feature negotiation must be done again
> > > > > > > > > >        *    Device must not be reset from its vq/config callbacks, or in
> > > > > > > > > >        *    parallel with being added/removed.
> > > > > > > > > > + *    Returns 0 on success or error status.
> > > > > > > > > >        * @find_vqs: find virtqueues and instantiate them.
> > > > > > > > > >        *    vdev: the virtio_device
> > > > > > > > > >        *    nvqs: the number of virtqueues to find
> > > > > > > > > > @@ -82,7 +83,7 @@ struct virtio_config_ops {
> > > > > > > > > >           u32 (*generation)(struct virtio_device *vdev);
> > > > > > > > > >           u8 (*get_status)(struct virtio_device *vdev);
> > > > > > > > > >           void (*set_status)(struct virtio_device *vdev, u8 status);
> > > > > > > > > > -    void (*reset)(struct virtio_device *vdev);
> > > > > > > > > > +    int (*reset)(struct virtio_device *vdev);
> > > > > > > > > >           int (*find_vqs)(struct virtio_device *, unsigned nvqs,
> > > > > > > > > >                   struct virtqueue *vqs[], vq_callback_t *callbacks[],
> > > > > > > > > >                   const char * const names[], const bool *ctx,