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| -rw-r--r-- | docs/devel/migration.rst | 41 |
1 files changed, 41 insertions, 0 deletions
diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst index 9d1b7657f0..e32b087f6e 100644 --- a/docs/devel/migration.rst +++ b/docs/devel/migration.rst @@ -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. |