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Re: [Qemu-devel] [patch v6 11/12] vfio: register aer resume notification

From: Alex Williamson
Subject: Re: [Qemu-devel] [patch v6 11/12] vfio: register aer resume notification handler for aer resume
Date: Fri, 6 May 2016 10:39:32 -0600

On Fri, 6 May 2016 09:38:41 +0800
Chen Fan <address@hidden> wrote:

> On 04/26/2016 10:48 PM, Alex Williamson wrote:
> > On Tue, 26 Apr 2016 11:39:02 +0800
> > Chen Fan<address@hidden>  wrote:
> >  
> >> On 04/14/2016 09:02 AM, Chen Fan wrote:  
> >>> On 04/12/2016 05:38 AM, Alex Williamson wrote:  
> >>>> On Tue, 5 Apr 2016 19:42:02 +0800
> >>>> Cao jin<address@hidden>  wrote:
> >>>>     
> >>>>> From: Chen Fan<address@hidden>
> >>>>>
> >>>>> for supporting aer recovery, host and guest would run the same aer
> >>>>> recovery code, that would do the secondary bus reset if the error
> >>>>> is fatal, the aer recovery process:
> >>>>>     1. error_detected
> >>>>>     2. reset_link (if fatal)
> >>>>>     3. slot_reset/mmio_enabled
> >>>>>     4. resume
> >>>>>
> >>>>> it indicates that host will do secondary bus reset to reset
> >>>>> the physical devices under bus in step 2, that would cause
> >>>>> devices in D3 status in a short time. but in qemu, we register
> >>>>> an error detected handler, that would be invoked as host broadcasts
> >>>>> the error-detected event in step 1, in order to avoid guest do
> >>>>> reset_link when host do reset_link simultaneously. it may cause
> >>>>> fatal error. we introduce a resmue notifier to assure host reset
> >>>>> completely. then do guest aer injection.  
> >>>> Why is it safe to continue running the VM between the error detected
> >>>> notification and the resume notification?  We're just pushing back the
> >>>> point at which we inject the AER into the guest, potentially negating
> >>>> any benefit by allowing the VM to consume bad data.  Shouldn't we
> >>>> instead be immediately notifying the VM on error detected, but stalling
> >>>> any access to the device until resume is signaled?  How do we know that
> >>>> resume will ever be signaled?  We have both the problem that we may be
> >>>> running on an older kernel that won't support a resume notification and
> >>>> the problem that seeing a resume notification depends on the host being
> >>>> able to successfully complete a link reset after fatal error. We can
> >>>> detect support for resume notification, but we still need a strategy
> >>>> for never receiving it.  Thanks,  
> >>> That's make sense, but I haven't came up with a good idea. do you have
> >>> any idea, Alex?  
> > I don't know that there are any good solutions here.  We need to
> > respond to the current error notifier interrupt and not regress from
> > our support there.  I think that means that if we want to switch from a
> > simple halt-on-error to a mechanism for the guest to handle recovery,
> > we need to disable access to the device between being notified that the
> > error occurred and being notified to resume.  We can do that by
> > disabling mmaps to the device and preventing access via the slow path
> > handlers.  I don't know what the best solution is for preventing access,
> > do we block and pause the VM or do we drop writes and return -1 for
> > reads, that's something that needs to be determined.  We also need to
> > inject the AER into the VM at the point we're notified of an error
> > because the VM needs to know as soon as possible to stop using the
> > device or trusting any data from it.  The next coordination point would
> > be something like the resume notifier that you've added and there are
> > numerous questions around the interaction of that with the guest
> > handling.  Clearly we can't do a guest directed bus reset until we get
> > the resume notifier, so do we block that execution path in QEMU until
> > the resume notification is received?  What happens if we don't get that
> > notification?  Is there any way that we can rely on the host having
> > done a bus reset to the point where we don't need to act on the guest
> > directed reset?  These are all things that need to be figured out.
> > Thanks,  
> Maybe we can simply pause the vcpu running and avoid the VM to
> access the device. and add two flags in VFIO_DEVICE_GET_INFO to query
> whether the vfio pci driver has a resume notifier,
> if it does not have resume notifier flags, we can directly fail to boot 
> up VM
> as with aer enabled.

We can already tell if a resume interrupt is supported between the IRQ
count in vfio_device_info and a probe with vfio_irq_info, what would
additional flags in vfio_device_info tell us beyond a resume interrupt
being supported?  Is pausing the VM acceptable from a service guarantee
perspective to users?  A bus reset can take a full second and I imagine
deeper PCI hierarchies can push that out depending on what level the
error occurs.  A second of downtime may be enough to trigger failovers
to other systems.  If we were to disable mmaps when a fault occurs, we
could trap any further device access, drop writes, return -1 for
reads.  This seems reasonable since we've already notified the VM that
the device had a fault.  The synchronization point seems like when the
guest tries to do a bus reset, we need to block that until we get the
resume notification from the host.  Perhaps if that doesn't occur after
some timeout, we would abort the guest directed bus reset altogether
and make the device disappear, perhaps even initiating an unplug of the
device to prevent it from further interacting with the VM.

> otherwise, we should wait for resume notifier coming to
> restart the cpu. about the problem of the reduplicated bus reset by host 
> and guest,
> I think qemu can according to the error is fatal or non-fatal to decide 
> whether need
> to do a bus reset on guest, I think it's not critical and could be 
> resolved later.

The vfio error interrupt doesn't signal non-fatal errors afaik.  I'm
also not sure we have an guarantee that the host has performed a bus
reset, we shouldn't necessarily design the API that strictly around the
current behavior of the Linux AER handler.  So I don't know that
there's any practical way to avoid duplicate bus resets between host
and guest recovery.  Thanks,


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