On Sat, Oct 17, 2020 at 03:24:08PM +0200, Jason A. Donenfeld wrote: > 4c. The guest kernel maintains an array of physical addresses that are > MADV_WIPEONFORK. The hypervisor knows about this array and its > location through whatever protocol, and before resuming a > moved/snapshotted/duplicated VM, it takes the responsibility for > memzeroing this memory. The huge pro here would be that this > eliminates all races, and reduces complexity quite a bit, because the > hypervisor can perfectly synchronize its bringup (and SMP bringup) > with this, and it can even optimize things like on-disk memory > snapshots to simply not write out those pages to disk. > > A 4c-like approach seems like it'd be a lot of bang for the buck -- we > reuse the existing mechanism (MADV_WIPEONFORK), so there's no new > userspace API to deal with, and it'd be race free, and eliminate a lot > of kernel complexity. Clearly this has a chance to break applications, right? If there's an app that uses this as a non-system-calls way to find out whether there was a fork, it will break when wipe triggers without a fork ... For example, imagine: MADV_WIPEONFORK copy secret data to MADV_DONTFORK fork used to work, with this change it gets 0s instead of the secret data. I am also not sure it's wise to expose each guest process to the hypervisor like this. E.g. each process needs a guest physical address of its own then. This is a finite resource. The mmap interface proposed here is somewhat baroque, but it is certainly simple to implement ... -- MST
