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Re: [Qemu-devel] [PATCH v6] vfio error recovery: kernel support

From: Alex Williamson
Subject: Re: [Qemu-devel] [PATCH v6] vfio error recovery: kernel support
Date: Wed, 5 Apr 2017 13:38:22 -0600

On Wed, 5 Apr 2017 16:54:33 +0800
Cao jin <address@hidden> wrote:

> Sorry for late. Distracted by other problem for a while.
> On 03/31/2017 02:16 AM, Alex Williamson wrote:
> > On Thu, 30 Mar 2017 21:00:35 +0300  
> >>>>>>       
> >>>>>>>
> >>>>>>> I also asked in my previous comments to provide examples of errors 
> >>>>>>> that
> >>>>>>> might trigger correctable errors to the user, this comment seems to
> >>>>>>> have been missed.  In my experience, AERs generated during device
> >>>>>>> assignment are generally hardware faults or induced by bad guest
> >>>>>>> drivers.  These are cases where a single fatal error is an appropriate
> >>>>>>> and sufficient response.  We've scaled back this support to the point
> >>>>>>> where we're only improving the situation of correctable errors and I'm
> >>>>>>> not convinced this is worthwhile and we're not simply checking a box 
> >>>>>>> on
> >>>>>>> an ill-conceived marketing requirements document.        
> >>>>>>
> >>>>>> Sorry. I noticed that question: "what actual errors do we expect
> >>>>>> userspace to see as non-fatal errors?", but I am confused about it.
> >>>>>> Correctable, non-fatal, fatal errors are clearly defined in PCIe spec,
> >>>>>> and Uncorrectable Error Severity Register will tell which is fatal, and
> >>>>>> which is non-fatal, this register is configurable, they are device
> >>>>>> specific as I guess. AER core driver distinguish them by
> >>>>>> pci_channel_io_normal/pci_channel_io_frozen,  So I don't understand 
> >>>>>> your
> >>>>>> question. Or
> >>>>>>
> >>>>>> Or, Do you mean we could list the default non-fatal error of
> >>>>>> Uncorrectable Error Severity Register which is provided by PCIe spec?  
> >>>>>>     
> >>>>>
> >>>>> I'm trying to ask why is this patch series useful.  It's clearly
> >>>>> possible for us to signal non-fatal errors for a device to a guest, but
> >>>>> why is it necessarily a good idea to do so?  What additional RAS
> >>>>> feature is gained by this?  Can we give a single example of a real
> >>>>> world scenario where a guest has been shutdown due to a non-fatal error
> >>>>> that the guest driver would have handled?      
> >>>>
> >>>> We've been discussing AER for months if not years.
> >>>> Isn't it a bit too late to ask whether AER recovery
> >>>> by guests it useful at all?    
> >>>
> >>>
> >>> Years, but I think that is more indicative of the persistence of the
> >>> developers rather than growing acceptance on my part.  For the majority
> >>> of that we were headed down the path of full AER support with the guest
> >>> able to invoke bus resets.  It was a complicated solution, but it was
> >>> more clear that it had some value.   Of course that's been derailed
> >>> due to various issues and we're now on this partial implementation that
> >>> only covers non-fatal errors that we assume the guest can recover from
> >>> without providing it mechanisms to do bus resets.  Is there actual
> >>> value to this or are we just trying to fill an AER checkbox on
> >>> someone's marketing sheet?  I don't think it's too much to ask for a
> >>> commit log to include evidence or discussion about how a feature is
> >>> actually a benefit to implement.    
> >>
> >> Seems rather self evident but ok.  So something like
> >>
> >> With this patch, guest is able to recover from non-fatal correctable
> >> errors - as opposed to stopping the guest with no ability to
> >> recover which was the only option previously.
> >>
> >> Would this address your question?  
> > 
> > 
> > No, that's just restating the theoretical usefulness of this.  Have you
> > ever seen a non-fatal error?  Does it ever trigger?  If we can't
> > provide a real world case of this being useful, can we at least discuss
> > the types of things that might trigger a non-fatal error for which the
> > guest could recover?  In patch 3/3 Cao Jin claimed we have a 50% chance
> > of reducing VM stop conditions, but I suspect this is just a misuse of
> > statistics, just because there are two choices, fatal vs non-fatal,
> > does not make them equally likely.  Do we have any idea of the
> > incidence rate of non-fatal errors?  Is it non-zero?  Thanks,
> >   
> Apparently, I don't have experience to induce non-fatal error, device
> error is more of a chance related with the environment(temperature,
> humidity, etc) as I understand.
> After reading the discussion, can I construe that the old design with
> full AER support is preferred than this new one?  The core issue of the
> old design is that the second host link reset make the subsequent
> guest's register reading fail, and I think this can be solved by test
> the device's accessibility(read device' register, all F's means
> unaccessible. IIRC, EEH of Power also use this way to test device's
> accessiblity) and delay guest's reading if device is temporarily
> unaccessible.

What is the actual AER handling requirement you're trying to solve?  Is
it simply to check a box on a marketing spec sheet for anything related
to AER with device assignment or is it to properly handle a specific
class of errors which a user might actually see and thus provide some
tangible RAS benefit?  Without even anecdotal incidence rates of
non-fatal errors and no plan for later incorporating fatal error
handling, this feels like we're just trying to implement anything to do
with AER with no long term vision.

The previous intention of trying to handle all sorts of AER faults
clearly had more value, though even there the implementation and
configuration requirements restricted the practicality.  For instance
is AER support actually useful to a customer if it requires all ports
of a multifunction device assigned to the VM?  This seems more like a
feature targeting whole system partitioning rather than general VM
device assignment use cases.  Maybe that's ok, but it should be a clear
design decision.

I think perhaps the specific issue of the device being in reset while
the guest tries to access it is only a symptom of deeper problems.  Who
owns resetting the bus on error?  My position was that we can't make
the user solely responsible for that since we should never trust the
user.  If the host handles the reset, then does that imply we have both
host and guest triggered resets and how do we handle the device during
the host reset.  It's not clear how we can necessarily correlate a
guest reset to a specific host reset.  Would it make more sense to
allow the host AER handling to release some control to the user with
vfio doing cleanup at the end.  These are hard problems that need to be
thought through.  I don't want to continue on a path of "can I just fix
this one next thing and it will be ok?".  The entire previous design is
suspect.  If you want to move this area forward, take ownership of it
and propose a complete, end-to-end solution.  Thanks,


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