Re: vfio/pci: guest error recovery proposal

[Cao jin]

On 12/16/2016 07:02 AM, Michael S. Tsirkin wrote:
> 
>>  1) We need to do the right thing for the guest, I don't think we
>>     should be presuming that different reset types are equivalent,
>>     leaving gaps where we expect the guest/host to do a reset and don't
>>     follow through on other reset requests, and we need to notify the
>>     guest immediately for the error.
> c>  2) We need to do the right thing for the host, that means we should
>>     not give the user the opportunity to leave a device in a state
>>     where we haven't at least performed a bus reset on link error (this
>>     may be our current state and if so we should fix it).
> 
> Ok so here is a concrete proposal for improving guest device error
> recovery (1).  This is not trying to fix current bugs for 2, but
> also does not lock us into not fixing them.
> 
> I'll write up proposal for (2) but I feel we can't properly
> fix host without fixing (1) first and without breaking compatibility.
> 
> Background:
> 
> non-fatal errors:
> 
> - These errors are due to data link problems.
>   The problem is that a transaction was lost, so driver and device are
>   out of sync. Device reset is in theory enough to recover from these,
>   in practice some drivers might try to do link level reset instead.
> 
> 
> fatal errors:
> 
> - These errors are due to physical problems.
>   The problem is that a transaction was lost, so driver and device are
>   out of sync. Link reset might be necessary to recover from these,
>   sometimes device reset might be enough for very simple devices.
>   If a link above the device reports errors, device might have went away,
>   link reset is the only thing that might being it back.
> 
> current behaviour:
> 
> - vfio will always report that it recovered function from an error.
> - whether link reset will trigger depends on whether any other
>   function on the same link has a host driver that reports an error.
> - also, if there's a host driver that can't handle errors,
>   link reset will never trigger
> 
> 
> proposed enhancement:
> 
> 1- allow userspace to request reporting non fatal/fatal errors separately
> 2- report errors on monitor as events as well
> 3- forward correct error type to guest
> 4- set link error flag in userspace (this is optional, used for 5 below)
> 5- if guest requests link reset, and error flag is set,
>   stop vm (I hope we can distinguish this
>   from resets that happen on reboot here.
>   if yes we might not need error flag in 4 above)
> 

Hi,

I have a question about vm stop on fatal error.
Recently, When test my patches, I often saw fatal error(Malformed TLP
Status) happens, which disturbed my test. So I am wondering: why vm stop
is a better choice than qdev_unplug? Although we told user "Please
collect any data possible and then kill the guest", I still don't know
how to save any possible data. For example, if user is editing document,
vm_stop caused by a device fatal error will destroy user's effort.

-- 
Sincerely,
Cao jin
> 
> Results:
> The advantage of this is that we don't need to manage any state at all.
> Most drivers will handle non fatal errors by FLR and will recover fine.
> Drivers that attempt link reset will get vmstop which is not
> worse than what we have now.
> 
> I don't see how this can break any reasonable configuration
> that is not already broken, but we might want a flag
> to suppress aer reports to guest and just do vmstop
> unconditionally.
> Alternatively, management can pause vm itself when it sees the error.
> 
> 
> Pls remember to Cc qemu list on discussion, not just kvm.
>