The dirty_memory[] bitmap is used for live migration, TCG self-modifying code detection, and VGA emulation. Up until now the bitmap was always accessed under the QEMU global mutex. This series makes all dirty_memory[] accesses atomic to prepare the way for threads writing to guest memory without holding the global mutex.
In particular, this series converts non-atomic dirty_memory[] accesses to atomic_or, atomic_xchg, and atomic_fetch_and so that race conditions are avoided when two threads manipulate the bitmap at the same time. There are two pieces remaining before the dirty_memory[] bitmap is truly thread-safe: 1. Convert all cpu_physical_memory_*_dirty() callers to use the API atomically. There are TCG callers who things along the lines of: if (!cpu_physical_memory_get_dirty(addr)) { cpu_physical_memory_set_dirty(addr); /* not atomic! */ } At first I considered ignoring TCG completely since it is currently not multi-threaded, but we might as well tackle this so that virtio-blk/virtio-scsi dataplane can be used with TCG eventually (they still need ioeventfd/irqfd emulation before they can support TCG). 2. Use array RCU to safely resize dirty_memory[] for memory hotplug. Currently ram_block_add() grows the bitmap in a way that is not thread-safe. Paolo has a QEMU userspace RCU implementation which I'd like to bring in for this. I am posting this RFC to get feedback because this is my first foray into the memory bitmap code. Comments appreciated! Stefan Hajnoczi (6): bitmap: add atomic set functions bitmap: add atomic test and clear memory: use atomic ops for setting dirty memory bits migration: move dirty bitmap sync to ram_addr.h memory: replace cpu_physical_memory_reset_dirty() with test-and-clear memory: make cpu_physical_memory_sync_dirty_bitmap() fully atomic arch_init.c | 46 ++---------------------------- cputlb.c | 4 +-- exec.c | 23 +++++++++++---- include/exec/ram_addr.h | 74 ++++++++++++++++++++++++++++++++++++------------- include/qemu/bitmap.h | 4 +++ include/qemu/bitops.h | 14 ++++++++++ memory.c | 11 +++----- util/bitmap.c | 47 +++++++++++++++++++++++++++++++ 8 files changed, 144 insertions(+), 79 deletions(-) -- 2.1.0