From 9458a9a1df1a4c719e24512394d548c1fc7abd22 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Tue, 6 Feb 2018 18:37:39 +0100 Subject: [PATCH] memory: fix race between TCG and accesses to dirty bitmap There is a race between TCG and accesses to the dirty log: vCPU thread reader thread ----------------------- ----------------------- TLB check -> slow path notdirty_mem_write write to RAM set dirty flag clear dirty flag TLB check -> fast path read memory write to RAM Fortunately, in order to fix it, no change is required to the vCPU thread. However, the reader thread must delay the read after the vCPU thread has finished the write. This can be approximated conservatively by run_on_cpu, which waits for the end of the current translation block. A similar technique is used by KVM, which has to do a synchronous TLB flush after doing a test-and-clear of the dirty-page flags. Reported-by: Dr. David Alan Gilbert Signed-off-by: Paolo Bonzini --- exec.c | 31 +++++++++++++++++++++++++++++++ include/exec/memory.h | 12 ++++++++++++ memory.c | 10 +++++++++- migration/ram.c | 1 + 4 files changed, 53 insertions(+), 1 deletion(-) diff --git a/exec.c b/exec.c index 4aaa14b075..731b79d2e4 100644 --- a/exec.c +++ b/exec.c @@ -197,6 +197,7 @@ typedef struct subpage_t { static void io_mem_init(void); static void memory_map_init(void); +static void tcg_log_global_after_sync(MemoryListener *listener); static void tcg_commit(MemoryListener *listener); static MemoryRegion io_mem_watch; @@ -905,6 +906,7 @@ void cpu_address_space_init(CPUState *cpu, int asidx, newas->cpu = cpu; newas->as = as; if (tcg_enabled()) { + newas->tcg_as_listener.log_global_after_sync = tcg_log_global_after_sync; newas->tcg_as_listener.commit = tcg_commit; memory_listener_register(&newas->tcg_as_listener, as); } @@ -3142,6 +3144,35 @@ void address_space_dispatch_free(AddressSpaceDispatch *d) g_free(d); } +static void do_nothing(CPUState *cpu, run_on_cpu_data d) +{ +} + +static void tcg_log_global_after_sync(MemoryListener *listener) +{ + CPUAddressSpace *cpuas; + + /* Wait for the CPU to end the current TB. This avoids the following + * incorrect race: + * + * vCPU migration + * ---------------------- ------------------------- + * TLB check -> slow path + * notdirty_mem_write + * write to RAM + * mark dirty + * clear dirty flag + * TLB check -> fast path + * read memory + * write to RAM + * + * by pushing the migration thread's memory read after the vCPU thread has + * written the memory. + */ + cpuas = container_of(listener, CPUAddressSpace, tcg_as_listener); + run_on_cpu(cpuas->cpu, do_nothing, RUN_ON_CPU_NULL); +} + static void tcg_commit(MemoryListener *listener) { CPUAddressSpace *cpuas; diff --git a/include/exec/memory.h b/include/exec/memory.h index d99eb25d2e..fddc2ff48a 100644 --- a/include/exec/memory.h +++ b/include/exec/memory.h @@ -425,6 +425,7 @@ struct MemoryListener { void (*log_clear)(MemoryListener *listener, MemoryRegionSection *section); void (*log_global_start)(MemoryListener *listener); void (*log_global_stop)(MemoryListener *listener); + void (*log_global_after_sync)(MemoryListener *listener); void (*eventfd_add)(MemoryListener *listener, MemoryRegionSection *section, bool match_data, uint64_t data, EventNotifier *e); void (*eventfd_del)(MemoryListener *listener, MemoryRegionSection *section, @@ -1687,6 +1688,17 @@ MemoryRegionSection memory_region_find(MemoryRegion *mr, */ void memory_global_dirty_log_sync(void); +/** + * memory_global_dirty_log_sync: synchronize the dirty log for all memory + * + * Synchronizes the vCPUs with a thread that is reading the dirty bitmap. + * This function must be called after the dirty log bitmap is cleared, and + * before dirty guest memory pages are read. If you are using + * #DirtyBitmapSnapshot, memory_region_snapshot_and_clear_dirty() takes + * care of doing this. + */ +void memory_global_after_dirty_log_sync(void); + /** * memory_region_transaction_begin: Start a transaction. * diff --git a/memory.c b/memory.c index c90a2cf629..4aa38eb5b1 100644 --- a/memory.c +++ b/memory.c @@ -2127,9 +2127,12 @@ DirtyBitmapSnapshot *memory_region_snapshot_and_clear_dirty(MemoryRegion *mr, hwaddr size, unsigned client) { + DirtyBitmapSnapshot *snapshot; assert(mr->ram_block); memory_region_sync_dirty_bitmap(mr); - return cpu_physical_memory_snapshot_and_clear_dirty(mr, addr, size, client); + snapshot = cpu_physical_memory_snapshot_and_clear_dirty(mr, addr, size, client); + memory_global_after_dirty_log_sync(); + return snapshot; } bool memory_region_snapshot_get_dirty(MemoryRegion *mr, DirtyBitmapSnapshot *snap, @@ -2620,6 +2623,11 @@ void memory_global_dirty_log_sync(void) memory_region_sync_dirty_bitmap(NULL); } +void memory_global_after_dirty_log_sync(void) +{ + MEMORY_LISTENER_CALL_GLOBAL(log_global_after_sync, Forward); +} + static VMChangeStateEntry *vmstate_change; void memory_global_dirty_log_start(void) diff --git a/migration/ram.c b/migration/ram.c index 35552c090b..b01a37e7ca 100644 --- a/migration/ram.c +++ b/migration/ram.c @@ -1857,6 +1857,7 @@ static void migration_bitmap_sync(RAMState *rs) rcu_read_unlock(); qemu_mutex_unlock(&rs->bitmap_mutex); + memory_global_after_dirty_log_sync(); trace_migration_bitmap_sync_end(rs->num_dirty_pages_period); end_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);