Re: [Qemu-devel] [PATCH v0 0/7] Background snapshots

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Paolo Bonzini (pbonzini@redhat.com) wrote:
> On 25/07/2018 22:04, Andrea Arcangeli wrote:
> > 
> > It may look like the uffd-wp model is wish-feature similar to an
> > optimization, but without the uffd-wp model when the WP fault is
> > triggered by kernel code, the sigsegv model falls apart and requires
> > all kind of ad-hoc changes just for this single feature. Plus uffd-wp
> > has other benefits: it makes it all reliable in terms of not
> > increasing the number of vmas in use during the snapshot. Finally it
> > makes it faster too with no mmap_sem for reading and no sigsegv
> > signals.
> > 
> > The non cooperative features got merged first because there was much
> > activity on the kernel side on that front, but this is just an ideal
> > time to nail down the remaining issues in uffd-wp I think. That I
> > believe is time better spent than trying to emulate it with sigsegv
> > and changing all drivers to send new events down to qemu specific to
> > the sigsegv handling. We considered this before doing uffd for
> > postcopy too but overall it's unreliable and more work (no single
> > change was then needed to KVM code with uffd to handle postcopy and
> > here it should be the same).
> 
> I totally agree.  The hard part in userfaultfd was the changes to the
> kernel get_user_pages API, but the payback was huge because _all_ kernel
> uses (KVM, vhost-net, syscalls, etc.) just work with userfaultfd.  Going
> back to mprotect would be a huge mistake.

TBF I think Denis just wanted to get things moving, knowing that we've
not got userfault-wp yet;  what I'm curious about though is how Denis
has anything stable given that the faults can land in syscalls and
vhost etc.

Dave

> Paolo
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK