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Re: [Qemu-devel] [RFC 00/13] Live memory snapshot based on userfaultfd

From: Hailiang Zhang
Subject: Re: [Qemu-devel] [RFC 00/13] Live memory snapshot based on userfaultfd
Date: Tue, 28 Feb 2017 09:48:26 +0800
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1


On 2017/2/27 23:37, Christian Pinto wrote:
Hello Hailiang,

are there any updates on this patch series? Are you planning to release
a new version?

No, userfaultfd still does not support write-protect for KVM.
You can see the newest discussion about it here:

You say there are some issues with the current snapshot-v2 version,
which issues were you referring to? On my side the only problem I have
seen was that the live snapshot was not working on ARMv8, but I have
fixed that and managed to successfully snapshot and restore a QEMU ARMv8
tcg machine on an ARMv8 host. I will gladly contribute with these fixes
once you will release a new version of the patches.

Yes, for current implementing of live snapshot, it supports tcg,
but does not support kvm mode, the reason i have mentioned above,
if you try to implement it, i think you need to start from userfaultfd
supporting KVM. There is scenario for it, But I'm blocked by other things
these days. I'm glad to discuss with you if you planed to do it.


Thanks a lot,


On 20/08/2016 08:31, Hailiang Zhang wrote:

I updated this series, but didn't post it, because there are some
problems while i tested the snapshot function.
I didn't know if it is the userfaultfd issue or not.
I don't have time to investigate it this month. I have put them in github

Anyone who want to test and modify it are welcomed!

Besides, will you join the linuxcon or KVM forum in Canada ?
I wish to see you there if you join the conference ;)


On 2016/8/18 23:56, Andrea Arcangeli wrote:
Hello everyone,

I've an aa.git tree uptodate on the master & userfault branch (master
includes other pending VM stuff, userfault branch only contains
userfault enhancements):


I didn't have time to test KVM live memory snapshot on it yet as I'm
still working to improve it. Did anybody test it? However I'd be happy
to take any bugreports and quickly solve anything that isn't working
right with the shadow MMU.

I got positive report already for another usage of the uffd WP support:


The last few things I'm working on to finish the WP support are:

1) pte_swp_mkuffd_wp equivalent of pte_swp_mksoft_dirty to mark in a
     vma->vm_flags with VM_UFFD_WP set, which swap entries were
     generated while the pte was wrprotected.

2) to avoid all false positives the equivalent of pte_mksoft_dirty is
     needed too... and that requires spare software bits on the pte
     which are available on x86. I considered also taking over the
     soft_dirty bit but then you couldn't do checkpoint restore of a
     JIT/to-native compiler that uses uffd WP support so it wasn't
     ideal. Perhaps it would be ok as an incremental patch to make the
     two options mutually exclusive to defer the arch changes that
     pte_mkuffd_wp would require for later.

3) prevent UFFDIO_ZEROPAGE if registering WP|MISSING or trigger a
     cow in userfaultfd_writeprotect.

4) WP selftest

In theory things should work ok already if the userland code is
tolerant against false positives through swap and after fork() and
KSM. For an usage like snapshotting false positives shouldn't be an
issue (it'll just run slower if you swap in the worst case), and point
3) above also isn't an issue because it's going to register into uffd
with WP only.

The current status includes:

1) WP support for anon (with false positives.. work in progress)

2) MISSING support for tmpfs and hugetlbfs

3) non cooperative support




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