Re: [PATCH RFC] vfio error recovery: kernel support

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

On Thu, Jan 19, 2017 at 03:10:56PM -0700, Alex Williamson wrote:
> On Thu, 19 Jan 2017 22:16:03 +0200
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > This is a design and an initial patch for kernel side for AER
> > support in VFIO.
> > 
> > 0. What happens now (PCIE AER only)
> >    Fatal errors cause a link reset.
> >    Non fatal errors don't.
> >    All errors stop the VM eventually, but not immediately
> >    because it's detected and reported asynchronously.
> >    Interrupts are forwarded as usual.
> >    Correctable errors are not reported to guest at all.
> >    Note: PPC EEH is different. This focuses on AER.
> > 
> > 1. Correctable errors
> >    I don't see a need to report these to guest. So let's not.
> > 
> > 2. Fatal errors
> >    It's not easy to handle them gracefully since link reset
> >    is needed. As a first step, let's use the existing mechanism
> >    in that case.
> >    
> > 2. Non-fatal errors
> >    Here we could make progress by reporting them to guest
> >    and have guest handle them.
> >    Issues:
> >     a. this behaviour should only be enabled with new userspace
> >        old userspace should work without changes
> >     Suggestion: One way to address this would be to add a new eventfd
> >     non_fatal_err_trigger. If not set, invoke err_trigger.
> > 
> >     b. drivers are supposed to stop MMIO when error is reported
> >     if vm keeps going, we will keep doing MMIO/config
> >     Suggestion 1: ignore this. vm stop happens much later when userspace runs anyway,
> >     so we are not making things much worse
> >     Suggestion 2: try to stop MMIO/config, resume on resume call
> > 
> >     Patch below implements Suggestion 1.
> 
> Although this is really against the documentation,

documentation is out of sync with code unfortunately.
I have a todo to rewrite it to match reality, for now
you will have to read the recovery function code.
Fortunately it is rather short.

> which states
> error_detected() is the point at which the driver should quiesce the
> device and not touch it further (until diagnostic poking at
> mmio_enabled or full access at resume callback).

Right. But note it's not a regression.

> >     c. PF driver might detect that function is completely broken,
> >     if vm keeps going, we will keep doing MMIO/config
> >     Suggestion 1: ignore this. vm stop happens much later when userspace runs anyway,
> >     so we are not making things much worse
> >     Suggestion 2: detect this and invoke err_trigger to stop VM
> > 
> >     Patch below implements Suggestion 2.
> > 
> > Aside: we currently return PCI_ERS_RESULT_DISCONNECT when device
> > is not attached. This seems bogus, likely based on the confusing name.
> > We probably should return PCI_ERS_RESULT_CAN_RECOVER.
> 
> Not sure I agree here, if we get called for the error_detected callback
> and we can't find a handle for the device, we certainly don't want to
> see any of the other callbacks for this device and we can't do anything
> about recovering it.

But we aren't actually driving it from any VMs so it's in the same
state it was and not doing any DMA or MMIO.

>  What's wrong with putting the device into a
> failed state in that case?

That you will wedge the PF too for no good reason.

> I actually question whether CAN_RECOVER is really the right return for
> the existing path.  If we consider this to be a fatal error, should we
> be voting NEED_RESET?  We're certainly not doing anything to return the
> device to a working state.


Yes we do - we stop VM and reset device on VM shutdown.
At least for VFs this is likely enough as by design they
must not wedge each other on driver bugs.

>  Should we be more harsh if err_trigger is
> not registered, putting the device into DISCONNECT?  Should only the new
> path you've added below for non-fatal errors return CAN_RECOVER?

So anyone assigning PFs deserves the resulting pain. I don't
want to speculate about the best strategy there.
But for VFs I think CAN_RECOVER is reasonable because
they should be independent of each other.

Also pls note any status except CAN_RECOVER mostly just wedges hardware
ATM. Maybe AER should do link resets more aggressively but it does not.

> > The following patch does not change that.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > 
> > ---
> > 
> > The patch is completely untested. Let's discuss the design first.
> > Cao jin, if this is deemed acceptable please take it from here.
> > 
> > diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> > index dce511f..fdca683 100644
> > --- a/drivers/vfio/pci/vfio_pci.c
> > +++ b/drivers/vfio/pci/vfio_pci.c
> > @@ -1292,7 +1292,9 @@ static pci_ers_result_t vfio_pci_aer_err_detected(struct pci_dev *pdev,
> >  
> >  	mutex_lock(&vdev->igate);
> >  
> > -	if (vdev->err_trigger)
> > +	if (state == pci_channel_io_normal && vdev->non_fatal_err_trigger)
> > +		eventfd_signal(vdev->err_trigger, 1);
> 
> s/err_trigger/non_fatal_err_trigger/
> 
> > +	else if (vdev->err_trigger)
> >  		eventfd_signal(vdev->err_trigger, 1);
> >  
> >  	mutex_unlock(&vdev->igate);
> > @@ -1302,8 +1304,38 @@ static pci_ers_result_t vfio_pci_aer_err_detected(struct pci_dev *pdev,
> >  	return PCI_ERS_RESULT_CAN_RECOVER;
> >  }
> >  
> > +static pci_ers_result_t vfio_pci_aer_slot_reset(struct pci_dev *pdev,
> > +						pci_channel_state_t state)
> > +{
> > +	struct vfio_pci_device *vdev;
> > +	struct vfio_device *device;
> > +
> > +	device = vfio_device_get_from_dev(&pdev->dev);
> > +	if (!device)
> > +		goto err_dev;
> > +
> > +	vdev = vfio_device_data(device);
> > +	if (!vdev)
> > +		goto err_dev;
> 
> s/err_dev/err_data/
> 
> > +
> > +	mutex_lock(&vdev->igate);
> > +
> > +	if (vdev->err_trigger)
> > +		eventfd_signal(vdev->err_trigger, 1);
> > +
> > +	mutex_unlock(&vdev->igate);
> > +
> > +	vfio_device_put(device);
> > +
> > +err_data:
> > +	vfio_device_put(device);
> > +err_dev:
> > +	return PCI_ERS_RESULT_RECOVERED;
> > +}
> > +
> >  static const struct pci_error_handlers vfio_err_handlers = {
> >  	.error_detected = vfio_pci_aer_err_detected,
> > +	.slot_reset = vfio_pci_aer_slot_reset,
> >  };
> >  
> >  static struct pci_driver vfio_pci_driver = {
> > diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
> > index 1c46045..e883db5 100644
> > --- a/drivers/vfio/pci/vfio_pci_intrs.c
> > +++ b/drivers/vfio/pci/vfio_pci_intrs.c
> > @@ -611,6 +611,17 @@ static int vfio_pci_set_err_trigger(struct vfio_pci_device *vdev,
> >  					       count, flags, data);
> >  }
> >  
> > +static int vfio_pci_set_non_fatal_err_trigger(struct vfio_pci_device *vdev,
> > +				    unsigned index, unsigned start,
> > +				    unsigned count, uint32_t flags, void *data)
> > +{
> > +	if (index != VFIO_PCI_NON_FATAL_ERR_IRQ_INDEX || start != 0 || count > 1)
> > +		return -EINVAL;
> > +
> > +	return vfio_pci_set_ctx_trigger_single(&vdev->non_fatal_err_trigger,
> > +					       count, flags, data);
> > +}
> > +
> >  static int vfio_pci_set_req_trigger(struct vfio_pci_device *vdev,
> >  				    unsigned index, unsigned start,
> >  				    unsigned count, uint32_t flags, void *data)
> > @@ -664,6 +675,14 @@ int vfio_pci_set_irqs_ioctl(struct vfio_pci_device *vdev, uint32_t flags,
> >  			break;
> >  		}
> >  		break;
> > +	case VFIO_PCI_NON_FATAL_ERR_IRQ_INDEX:
> > +		switch (flags & VFIO_IRQ_SET_ACTION_TYPE_MASK) {
> > +		case VFIO_IRQ_SET_ACTION_TRIGGER:
> > +			if (pci_is_pcie(vdev->pdev))
> > +				func = vfio_pci_set_err_trigger;
> > +			break;
> > +		}
> > +		break;
> >  	case VFIO_PCI_REQ_IRQ_INDEX:
> >  		switch (flags & VFIO_IRQ_SET_ACTION_TYPE_MASK) {
> >  		case VFIO_IRQ_SET_ACTION_TRIGGER:
> > diff --git a/drivers/vfio/pci/vfio_pci_private.h b/drivers/vfio/pci/vfio_pci_private.h
> > index f37c73b..c27a507 100644
> > --- a/drivers/vfio/pci/vfio_pci_private.h
> > +++ b/drivers/vfio/pci/vfio_pci_private.h
> > @@ -93,6 +93,7 @@ struct vfio_pci_device {
> >  	struct pci_saved_state	*pci_saved_state;
> >  	int			refcnt;
> >  	struct eventfd_ctx	*err_trigger;
> > +	struct eventfd_ctx	*non_fatal_err_trigger;
> >  	struct eventfd_ctx	*req_trigger;
> >  	struct list_head	dummy_resources_list;
> >  };
> > 
> 
> VFIO_PCI_NON_FATAL_ERR_IRQ_INDEX never got defined.
> 
> So if we think the link is ok, we notify a non-fatal event to the user,
> but we don't do anything about preventing access to the device between
> error_detected and resume as the documentation indicates we should.  If
> the system does a slot reset anyway, perhaps as a response to another
> driver on the same bus, we promote to fatal error signaling.  If we
> have no user signaling mechanism, shouldn't that also mark the device
> failed via returning DISCONNECT?  On the QEMU side, we'd still need to
> try to guess whether the VM is attempting a link reset is in response to
> the AER event and QEMU would need to vm_stop() in that case, right?
> Thanks,
> 
> Alex