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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: Tue, 26 Apr 2016 08:48:15 -0600

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.


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