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

[Paolo Bonzini]

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.

Paolo