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

From: Cao jin
Subject: Re: [Qemu-devel] [PATCH v6] vfio error recovery: kernel support
Date: Wed, 5 Apr 2017 16:54:33 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0

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

Cao jin

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