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SubjectRe: [PATCH] vfio pci: kernel support of error recovery only for non fatal error
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@cn.fujitsu.com> 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
>
>
>
>

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