Re: [PATCH] vfio pci: kernel support of error recovery only for non fatal error

[Alex Williamson]

On Mon, 20 Mar 2017 20:50:39 +0800
Cao jin <caoj.fnst@xxxxxxxxxxxxxx> wrote:

> Sorry for late.
> 
> On 03/14/2017 06:06 AM, Alex Williamson wrote:
> > On Mon, 27 Feb 2017 15:28:43 +0800
> > Cao jin <caoj.fnst@xxxxxxxxxxxxxx> wrote:
> >   
> >> 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.  
> > 
> > Perhaps you're only focusing on AER, but don't the error handlers we're
> > using support both AER and EEH generically?  I don't think we can
> > completely disregard how this affects EEH behavior, if at all.
> >   
> 
> After taking a rough look at the EEH,  find that EEH always feed
> error_detected with pci_channel_io_frozen, from perspective of
> error_detected, EEH is not affected.  
> 
> I am not sure about a question: when assign devices in spapr host,
> should all functions/devices in a PE be bound to vfio? I am kind of
> confused about the relationship between a PE & a tce iommu group

AIUI, yes all devices within the PE are part of the same IOMMU group
and therefore all endpoints must be bound to vfio or pci-stub.

> >>
> >> 1. Correctable errors
> >>    There is no need to report these to guest. So let's not.  
> > 
> > What does this patch change to make this happen?  I don't see
> > anything.  Was this always the case?  No change?
> >   
> 
> yes, no change on correctable error.
> 
> >>
> >> 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.  
> > 
> > Ok, so no change here either.
> >   
> >> 2. Non-fatal errors
> >>    Here we could make progress by reporting them to guest
> >>    and have guest handle them.  
> > 
> > In practice, what actual errors do we expect userspace to see as
> > non-fatal errors? It would be useful for the commit log to describe
> > the actual benefit we're going to see by splitting out non-fatal errors
> > for the user (not always a guest) to see separately.  Justify that this
> > is actually useful.
> >   
> >>
> >>    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.  
> > 
> > This outline format was really more useful for Michael to try to
> > generate discussion, for a commit log, I'd much rather see a definitive
> > statement such as:
> > 
> >  "To maintain backwards compatibility with userspace, non-fatal errors
> >  will continue to trigger via the existing error interrupt index if a
> >  non-fatal signaling mechanism has not been registered."
> >   
> >>    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.
> >>
> >>       Note that although this is really against the documentation, 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).
> >>
> >>       Fixing this won't be easy. However, this is not a regression.
> >>
> >>       Also note this does nothing about interrupts, documentation
> >>       suggests returning IRQ_NONE until reset.
> >>       Again, not a regression.  
> > 
> > So again, no change here.  I'm not sure what this adds to the commit
> > log, perhaps we can reference this as a link to Michael's original
> > proposal.
> >    
> >>    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.  
> > 
> > This needs more description and seems a bit misleading.  This patch
> > adds a slot_reset handler, such that if the slot is reset, we notify
> > the user, essentially promoting the non-fatal error to fatal.  But what
> > condition gets us to this point?  AIUI, AER is a voting scheme and if
> > any driver affected says they need a reset, everyone gets a reset.  So
> > the PF driver we're talking about here is not vfio-pci and it's not the
> > user, the user has no way to signal that the device is completely
> > broken, this only handles the case of other collateral devices with
> > native host drivers that might signal this, right?
> >   
> 
> Yes, same understanding as you, if I don't miss something from michael.
> 
> > It seems like this is where this patch has the greatest exposure to
> > regressions.  If we take the VM use case, previously we could have a
> > non-AER aware guest and the hypervisor could stop the VM on all
> > errors.  Now the hypervisor might support the distinction between fatal
> > and non-fatal, but the guest may still not have AER support.  That
> > doesn't imply a problem with this approach, the user (hypervisor) would
> > be at fault for any difference in handling in that case.
> >   
> 
> >>  
> >> +static pci_ers_result_t vfio_pci_aer_slot_reset(struct pci_dev *pdev)
> >> +{
> >> +	struct vfio_pci_device *vdev;
> >> +	struct vfio_device *device;
> >> +	static pci_ers_result_t err = PCI_ERS_RESULT_NONE;
> >> +
> >> +	device = vfio_device_get_from_dev(&pdev->dev);
> >> +	if (!device)
> >> +		goto err_dev;
> >> +
> >> +	vdev = vfio_device_data(device);
> >> +	if (!vdev)
> >> +		goto err_data;
> >> +
> >> +	mutex_lock(&vdev->igate);
> >> +
> >> +	if (vdev->err_trigger)
> >> +		eventfd_signal(vdev->err_trigger, 1);  
> > 
> > What about the case where the user has not registered for receiving
> > non-fatal errors, now we send an error signal on both error_detected
> > and slot_reset.  Is that useful/desirable?
> >   
> 
> Not desirable, but seems not harmful, guest user will stop anyway. How
> to avoid this case gracefully seems not easy.

"Not harmful" is presuming the behavior of the user.  QEMU might not be
the only consumer of these events.  Is it possible to receive a
slot_reset without first receiving an error_detected?  If not then we
can easily track our action for one to decide on the behavior for the
other.  Thanks,

Alex