On Fri, 2018-12-21 at 13:44 +0100, Paolo Bonzini wrote: > On 20/12/18 19:28, Adalbert Lazăr wrote: > > * figure out why this cannot be used with KSM enabled; > > Perhaps it would help if you explained how it fails. :) We zeroed in on KSM by trial and error (ie. turn off mm features one by one and see which one makes the problem go away). The issue we observed was that after several hours of running without problems, some Windows guests would hit a BSOD with PAGE_FAULT_IN_NONPAGED_AREA (0x50). Usually, while idle. Using qemu's guest debugging feature we determined that one of the maps was "unshared", that is our introspection tool running in its own VM saw one contents (AFAICT all zeros) and the guest another. The page was modified by our application and it was very important for the change to be undone when the guest reached a certain execution point. It was essentially a hook that had to be removed. Failure to do so would cause subtle corruptions and eventually a guest crash. We have not investigated further (after having lost several weeks chasing it) and were pretty surprised by the find, given that KSM needs userspace support (it does not appear to kick in on its own) and ksmtuned was not running. Regards, -- Mihai Donțu
