[Qemu-devel] [PATCH v2] migration: avoid filling ignore-shared ramblock when in incoming migration

[Catherine Ho]

Commit 18269069c310 ("migration: Introduce ignore-shared capability")
addes ignore-shared capability to bypass the shared ramblock (e,g,
membackend + numa node). It does good to live migration.

This commit expectes that QEMU doesn't write to guest RAM until
VM starts, but it does on aarch64 qemu:
Backtrace:
1  0x000055f4a296dd84 in address_space_write_rom_internal () at exec.c:3458
2  0x000055f4a296de3a in address_space_write_rom () at exec.c:3479
3  0x000055f4a2d519ff in rom_reset () at hw/core/loader.c:1101
4  0x000055f4a2d475ec in qemu_devices_reset () at hw/core/reset.c:69
5  0x000055f4a2c90a28 in qemu_system_reset () at vl.c:1675
6  0x000055f4a2c9851d in main () at vl.c:4552

Actually, on arm64 virt marchine, ramblock "dtb" will be filled into ram
during rom_reset. In ignore-shared incoming case, this rom filling
is not required since all the data has been stored in memory backend file.

Fixes: commit 18269069c310 ("migration: Introduce ignore-shared capability")
Signed-off-by: Catherine Ho <catherine.h...@gmail.com>
Suggested-by: Yury Kotov <yury-ko...@yandex-team.ru>
---
 hw/core/loader.c          | 15 +++++++++++++++
 include/exec/cpu-common.h |  1 +
 migration/ram.c           |  2 +-
 3 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/hw/core/loader.c b/hw/core/loader.c
index fe5cb24122..861a03335b 100644
--- a/hw/core/loader.c
+++ b/hw/core/loader.c
@@ -53,6 +53,7 @@
 #include "hw/nvram/fw_cfg.h"
 #include "exec/memory.h"
 #include "exec/address-spaces.h"
+#include "exec/cpu-common.h"
 #include "hw/boards.h"
 #include "qemu/cutils.h"
 
@@ -1086,6 +1087,9 @@ int rom_add_option(const char *file, int32_t bootindex)
 static void rom_reset(void *unused)
 {
     Rom *rom;
+    MemoryRegion *mr;
+    hwaddr hw_addr;
+    hwaddr l;
 
     QTAILQ_FOREACH(rom, &roms, next) {
         if (rom->fw_file) {
@@ -1094,6 +1098,17 @@ static void rom_reset(void *unused)
         if (rom->data == NULL) {
             continue;
         }
+
+        /* bypass the rom blob in ignore-shared migration case*/
+        if (runstate_check(RUN_STATE_INMIGRATE)) {
+            rcu_read_lock();
+            mr = address_space_translate(rom->as, rom->addr, &hw_addr, &l,
+                                         true, MEMTXATTRS_UNSPECIFIED);
+            rcu_read_unlock();
+            if (mr->ram_block != NULL && ramblock_is_ignored(mr->ram_block))
+                continue;
+        }
+
         if (rom->mr) {
             void *host = memory_region_get_ram_ptr(rom->mr);
             memcpy(host, rom->data, rom->datasize);
diff --git a/include/exec/cpu-common.h b/include/exec/cpu-common.h
index cef8b88a2a..c80b7248a6 100644
--- a/include/exec/cpu-common.h
+++ b/include/exec/cpu-common.h
@@ -76,6 +76,7 @@ void *qemu_ram_get_host_addr(RAMBlock *rb);
 ram_addr_t qemu_ram_get_offset(RAMBlock *rb);
 ram_addr_t qemu_ram_get_used_length(RAMBlock *rb);
 bool qemu_ram_is_shared(RAMBlock *rb);
+bool ramblock_is_ignored(RAMBlock *block);
 bool qemu_ram_is_uf_zeroable(RAMBlock *rb);
 void qemu_ram_set_uf_zeroable(RAMBlock *rb);
 bool qemu_ram_is_migratable(RAMBlock *rb);
diff --git a/migration/ram.c b/migration/ram.c
index 35bd6213e9..d6de9d335d 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -159,7 +159,7 @@ out:
     return ret;
 }
 
-static bool ramblock_is_ignored(RAMBlock *block)
+bool ramblock_is_ignored(RAMBlock *block)
 {
     return !qemu_ram_is_migratable(block) ||
            (migrate_ignore_shared() && qemu_ram_is_shared(block));
-- 
2.17.1