On Mon, Mar 06, 2017 at 05:04:41PM +0100, Cornelia Huck wrote: > On Mon, 6 Mar 2017 16:21:13 +0100 > Halil Pasic <pa...@linux.vnet.ibm.com> wrote: > > > On 03/06/2017 03:56 PM, Cornelia Huck wrote: > > > On Fri, 3 Mar 2017 14:08:37 +0100 > > > Halil Pasic <pa...@linux.vnet.ibm.com> wrote: > > > > > >> On 03/03/2017 01:50 PM, Cornelia Huck wrote: > > >>> On Fri, 3 Mar 2017 13:43:32 +0100 > > >>> Halil Pasic <pa...@linux.vnet.ibm.com> wrote: > > >>> > > >>>> On 03/03/2017 01:21 PM, Cornelia Huck wrote: > > >>>>> On Thu, 2 Mar 2017 19:59:42 +0100 > > >>>>> Halil Pasic <pa...@linux.vnet.ibm.com> wrote: > > [...] > > >> I admit, I did not investigate this thoroughly, also because the patch > > >> is flawed regarding multi-thread anyway. After a quick investigation > > >> it seems the linux guest won't auto-reset the device so the guest should > > >> end up with a not working device. I think it's pretty likely that the > > >> admin will check the logs if the device was important. > > > > > > Thinking a bit more about this, it seems setting the device broken is > > > not the right solution for exactly that reason. Setting the virtio > > > device broken is a way to signal the guest to 'you did something > > > broken; please reset the device and start anew' (and that's how current > > > callers use it). In our case, this is not the guest's fault. > > > > Do we have something to just say stuff broken without blaming the guest? > > And device reset might not be that stupid at all in the given situation, > > if we want to save what can be saved from the perspective of the guest. > > (After reset stuff should work again until we hit the race again -- and > > since turning ioeventfd on/off should not happen that often during normal > > operation it could help limit damage suffered -- e.g. controlled shutdown). > > Checking again, the spec says > > DEVICE_NEEDS_RESET (64) Indicates that the device has experienced an > error from which it can’t recover. > > Nothing about 'guest error'. > > The only problem is that legacy devices don't have that state, which > means they'll have a broken device through no fault of their own. > > > > > > > > > Maybe go back to the assert 'solution'? But I'm not sure that's enough > > > if production builds disable asserts... > > > > > > > I will wait a bit, maybe other virtio folks are going to have an > > opinion too. > > > > My concern about the assert solution is that for production it is > > either too rigorous (kill off, hopefully with a dump) or not > > enough (as you have mentioned, if NDEBUG assert does nothing). > > > > > > I think there are setups where a loss of device does not have to be > > fatal, and I would not like to be the one who makes it fatal (for the > > guest). > > Basically, it's a host bug (and not a bug specific to a certain > device). Moving the device which was impacted to a broken state may be > a useful mitigation. > > But yes, let's hear some other opinions.
We don't support NDEBUG really so I think an assert is fine for now. Handling unexpected errors more gracefully is laudable but I think we want a more systematic approach than just open-coding it in this specific place. -- MST
