Re: [Qemu-devel] [RFC]VM live snapshot proposal

[Andrea Arcangeli <aarcange@redhat.com>]

On Wed, Mar 05, 2014 at 01:52:14AM +0000, Huangpeng (Peter) wrote:
> Hi, Andrea
> 
> Where can I get the dev-git-branch?
> I can use it to try the snapshot prototype coding.

You can find the current status in the origin/master branch here
http://git.kernel.org/cgit/linux/kernel/git/andrea/aa.git

however userlandfd is still missing so it's not yet good for
transparent userfault when it's O_DIRECT or other gup users triggering
the access (those would currently return an error to userland if they
hit on a userfault vma, and we don't want to change userland to ever
get an error or the modifications to userland are too big).

userlandfd will let the kernel wait on an event from the migration
thread and it will talk with the migration thread directly. So
userland won't be able to notice the userfault happening inside a
write() or kvm ioctl() syscall (you could notice only if you strace
the migration thread). That's more efficient too so the host scheduler
can directly switch to the migration thread without having to return
to userland first. And after remap_anon_pages completes and the host
scheduler runs the vcpu or I/O thread again, gup_fast can continue
from kernel mode where it stopped again without unnecessary exits to
userland. Making the kernel speak directly to the migration thread is
somewhat more tricky at the kernel level that what you find in aa.git
right now, but it is worth it to be transparent to all syscalls that
would trip on userfaults with gup_fast.