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

[Michael S. Tsirkin]

On Mon, Mar 20, 2017 at 08:50:39PM +0800, Cao jin 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
> 
> >>
> >> 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.

I actually see a clean way to do this.

Let's add yet another eventfd to trigger
when hosts resets the device itself.  vdev->host_reset ?

Users can use the same one as err_trigger if they like,
it will be up to them.

Alex?

> -- 
> Sincerely,
> Cao jin
> 
> 
>
>