Re: [Qemu-devel] Async savevm using userfaultfd(2)

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

* Stefan Hajnoczi (stefanha@gmail.com) wrote:
> John and I recently discussed asynchronous savevm and I wanted to post
> the ideas so they aren't forgotten.  (We're not actively working on this
> feature.)
> 
> Asynchronous savevm has the same effect as the 'savevm' monitor command:
> it saves RAM, device state, and a snapshot of all disks at the point in
> time the command was issued.
> 
> The current 'savevm' monitor command is synchronous so the guest and
> QEMU monitor are blocked while the operation runs (it can take a
> while!).  Asynchronous savevm has the advantage of allowing the guest
> and QEMU monitor to continue while the operation is running.
> 
> This sounds similar to live migration to file but remember that live
> migration's consistency point is when the guest is paused at the end of
> the iteration phase.  The user has no control over *when* live migration
> captures the guest state.  Therefore it's not a useful command for
> taking snapshots of guest state at a specific point in time - we need
> asynchronous savevm for that.
> 
> Async savevm must copy-on-write guest RAM so the guest can continue
> writing to memory while the snapshot is being saved.  Rik van Riel
> suggested using userfaultfd(2) to do this on Linux.
> 
> Unlike post-copy live migration, we want to track memory writes (instead
> of missing page faults).  The userfaultfd(2) flag
> UFFDIO_REGISTER_MODE_WP provides these semantics.  Unfortunately I think
> UFFDIO_REGISTER_MODE_WP is not yet implemented?

A prototype of this has already been written by Hailiang Zhang;
see https://lists.gnu.org/archive/html/qemu-devel/2016-08/msg03441.html

> Once UFFDIO_REGISTER_MODE_WP is available QEMU can catch writes to guest
> RAM and copy the original pages to a buffer.  If memory is dirtied too
> quickly then it's necessary to throttle the guest or fail the savevm
> operation.

The only limit there is the size of the buffer, waiting for space will
do the throttling.

Dave

> 
> Perhaps this approach can be prototyped with mprotect and a SIGSEGV
> handler if anyone wants to get async savevm going.  I don't know if
> there are any disadvantages to mprotecting guest RAM that the kvm kernel
> module is using.  Hopefully in-kernel devices and vhost will continue to
> work.
> 
> Stefan


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