Re: [PATCH] drivers/virt: vmgenid: add vm generation id driver

[Michael S. Tsirkin]

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