[Qemu-devel] [PATCH v3 2/2] loader: put FW CFG ROM files into RAM

["Michael S. Tsirkin" <mst@redhat.com>]

ROM files that are put in FW CFG are copied to guest ram, by BIOS, but
they are not backed by RAM so they don't get migrated.

Each time we change two bytes in such a ROM this breaks cross-version
migration: since we can migrate after BIOS has read the first byte but
before it has read the second one, getting an inconsistent state.

Future-proof this by creating, for each such ROM,
an MR serving as the backing store.
This MR is never mapped into guest memory, but it's registered
as RAM so it's migrated with the guest.

Naturally, this only helps for -M 1.7 and up, older machine types
will still have the cross-version migration bug.
Luckily the race window for the problem to trigger is very small,
which is also likely why we didn't notice the cross-version
migration bug in testing yet.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 hw/core/loader.c    | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++---
 hw/i386/pc_piix.c   |  2 ++
 hw/i386/pc_q35.c    |  2 ++
 include/hw/loader.h |  1 +
 4 files changed, 56 insertions(+), 3 deletions(-)

diff --git a/hw/core/loader.c b/hw/core/loader.c
index 6875b7e..32d807a 100644
--- a/hw/core/loader.c
+++ b/hw/core/loader.c
@@ -54,6 +54,8 @@
 
 #include <zlib.h>
 
+bool rom_file_in_ram = true;
+
 static int roms_loaded;
 
 /* return the size or -1 if error */
@@ -576,6 +578,7 @@ struct Rom {
     size_t datasize;
 
     uint8_t *data;
+    MemoryRegion *mr;
     int isrom;
     char *fw_dir;
     char *fw_file;
@@ -605,6 +608,26 @@ static void rom_insert(Rom *rom)
     QTAILQ_INSERT_TAIL(&roms, rom, next);
 }
 
+static void *rom_set_mr(Rom *rom, Object *owner, const char *name)
+{
+    /*
+     * Migration code expects that all RAM blocks are full pages.
+     * Round MR size up to satisfy this condition.
+     */
+    unsigned size = ROUND_UP(rom->datasize, qemu_migration_page_size());
+    void *data;
+
+    rom->mr = g_malloc(sizeof(*rom->mr));
+    memory_region_init_ram(rom->mr, owner, name, size);
+    memory_region_set_readonly(rom->mr, true);
+    vmstate_register_ram_global(rom->mr);
+
+    data = memory_region_get_ram_ptr(rom->mr);
+    memcpy(data, rom->data, rom->datasize);
+
+    return data;
+}
+
 int rom_add_file(const char *file, const char *fw_dir,
                  hwaddr addr, int32_t bootindex)
 {
@@ -646,6 +669,7 @@ int rom_add_file(const char *file, const char *fw_dir,
     if (rom->fw_file && fw_cfg) {
         const char *basename;
         char fw_file_name[56];
+        void *data;
 
         basename = strrchr(rom->fw_file, '/');
         if (basename) {
@@ -655,8 +679,15 @@ int rom_add_file(const char *file, const char *fw_dir,
         }
         snprintf(fw_file_name, sizeof(fw_file_name), "%s/%s", rom->fw_dir,
                  basename);
-        fw_cfg_add_file(fw_cfg, fw_file_name, rom->data, rom->romsize);
         snprintf(devpath, sizeof(devpath), "/rom@%s", fw_file_name);
+
+        if (rom_file_in_ram) {
+            data = rom_set_mr(rom, OBJECT(fw_cfg), devpath);
+        } else {
+            data = rom->data;
+        }
+
+        fw_cfg_add_file(fw_cfg, fw_file_name, data, rom->romsize);
     } else {
         snprintf(devpath, sizeof(devpath), "/rom@" TARGET_FMT_plx, addr);
     }
@@ -731,7 +762,12 @@ static void rom_reset(void *unused)
         if (rom->data == NULL) {
             continue;
         }
-        cpu_physical_memory_write_rom(rom->addr, rom->data, rom->datasize);
+        if (rom->mr) {
+            void *host = memory_region_get_ram_ptr(rom->mr);
+            memcpy(host, rom->data, rom->datasize);
+        } else {
+            cpu_physical_memory_write_rom(rom->addr, rom->data, rom->datasize);
+        }
         if (rom->isrom) {
             /* rom needs to be written only once */
             g_free(rom->data);
@@ -781,6 +817,9 @@ static Rom *find_rom(hwaddr addr)
         if (rom->fw_file) {
             continue;
         }
+        if (rom->mr) {
+            continue;
+        }
         if (rom->addr > addr) {
             continue;
         }
@@ -808,6 +847,9 @@ int rom_copy(uint8_t *dest, hwaddr addr, size_t size)
         if (rom->fw_file) {
             continue;
         }
+        if (rom->mr) {
+            continue;
+        }
         if (rom->addr + rom->romsize < addr) {
             continue;
         }
@@ -866,7 +908,13 @@ void do_info_roms(Monitor *mon, const QDict *qdict)
     Rom *rom;
 
     QTAILQ_FOREACH(rom, &roms, next) {
-        if (!rom->fw_file) {
+        if (rom->mr) {
+            monitor_printf(mon, "%s"
+                           " size=0x%06zx name=\"%s\"\n",
+                           rom->mr->name,
+                           rom->romsize,
+                           rom->name);
+        } else if (!rom->fw_file) {
             monitor_printf(mon, "addr=" TARGET_FMT_plx
                            " size=0x%06zx mem=%s name=\"%s\"\n",
                            rom->addr, rom->romsize,
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 95c45b8..2a5348e 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -25,6 +25,7 @@
 #include <glib.h>
 
 #include "hw/hw.h"
+#include "hw/loader.h"
 #include "hw/i386/pc.h"
 #include "hw/i386/apic.h"
 #include "hw/pci/pci.h"
@@ -252,6 +253,7 @@ static void pc_init_pci(QEMUMachineInitArgs *args)
 static void pc_init_pci_1_6(QEMUMachineInitArgs *args)
 {
     has_pci_info = false;
+    rom_file_in_ram = false;
     pc_init_pci(args);
 }
 
diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index 6bfc2ca..66e7e1b 100644
--- a/hw/i386/pc_q35.c
+++ b/hw/i386/pc_q35.c
@@ -28,6 +28,7 @@
  * THE SOFTWARE.
  */
 #include "hw/hw.h"
+#include "hw/loader.h"
 #include "sysemu/arch_init.h"
 #include "hw/i2c/smbus.h"
 #include "hw/boards.h"
@@ -220,6 +221,7 @@ static void pc_q35_init(QEMUMachineInitArgs *args)
 static void pc_q35_init_1_6(QEMUMachineInitArgs *args)
 {
     has_pci_info = false;
+    rom_file_in_ram = false;
     pc_q35_init(args);
 }
 
diff --git a/include/hw/loader.h b/include/hw/loader.h
index eb9c9a3..6145736 100644
--- a/include/hw/loader.h
+++ b/include/hw/loader.h
@@ -36,6 +36,7 @@ void pstrcpy_targphys(const char *name,
                       hwaddr dest, int buf_size,
                       const char *source);
 
+extern bool rom_file_in_ram;
 
 int rom_add_file(const char *file, const char *fw_dir,
                  hwaddr addr, int32_t bootindex);
-- 
MST