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postcopy shared docs

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Add some notes to the migration documentation for shared memory
postcopy.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
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Dr. David Alan Gilbert 2018-03-12 17:21:24 +00:00 committed by Michael S. Tsirkin
parent 4275cd99c6
commit 1dc61e7b37
1 changed files with 41 additions and 0 deletions
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@ -577,3 +577,44 @@ Postcopy now works with hugetlbfs backed memory:
hugepages works well, however 1GB hugepages are likely to be problematic
since it takes ~1 second to transfer a 1GB hugepage across a 10Gbps link,
and until the full page is transferred the destination thread is blocked.
Postcopy with shared memory
---------------------------
Postcopy migration with shared memory needs explicit support from the other
processes that share memory and from QEMU. There are restrictions on the type of
memory that userfault can support shared.
The Linux kernel userfault support works on `/dev/shm` memory and on `hugetlbfs`
(although the kernel doesn't provide an equivalent to `madvise(MADV_DONTNEED)`
for hugetlbfs which may be a problem in some configurations).
The vhost-user code in QEMU supports clients that have Postcopy support,
and the `vhost-user-bridge` (in `tests/`) and the DPDK package have changes
to support postcopy.
The client needs to open a userfaultfd and register the areas
of memory that it maps with userfault. The client must then pass the
userfaultfd back to QEMU together with a mapping table that allows
fault addresses in the clients address space to be converted back to
RAMBlock/offsets. The client's userfaultfd is added to the postcopy
fault-thread and page requests are made on behalf of the client by QEMU.
QEMU performs 'wake' operations on the client's userfaultfd to allow it
to continue after a page has arrived.
.. note::
There are two future improvements that would be nice:
a) Some way to make QEMU ignorant of the addresses in the clients
address space
b) Avoiding the need for QEMU to perform ufd-wake calls after the
pages have arrived
Retro-fitting postcopy to existing clients is possible:
a) A mechanism is needed for the registration with userfault as above,
and the registration needs to be coordinated with the phases of
postcopy. In vhost-user extra messages are added to the existing
control channel.
b) Any thread that can block due to guest memory accesses must be
identified and the implication understood; for example if the
guest memory access is made while holding a lock then all other
threads waiting for that lock will also be blocked.