Re: [Qemu-devel] [PATCH v3 3/4] savevm: define new unambiguous migration format

[Anthony Liguori <anthony@codemonkey.ws>]

On 08/12/2011 10:11 AM, Anthony Liguori wrote:
> On 08/12/2011 04:59 AM, Paolo Bonzini wrote:
>> With the current migration format, VMS_STRUCTs with subsections
>> are ambiguous. The protocol cannot tell whether a 0x5 byte after
>> the VMS_STRUCT is a subsection or part of the parent data stream.
>> In the past QEMU assumed it was always a part of a subsection; after
>> commit eb60260 (savevm: fix corruption in vmstate_subsection_load().,
>> 2011-02-03) the choice depends on whether the VMS_STRUCT has subsections
>> defined.
>>
>> Unfortunately, this means that if a destination has no subsections
>> defined for the struct, it will happily read subsection data into
>> its own fields. And if you are "lucky" enough to stumble on a
>> zero byte at the right time, it will be interpreted as QEMU_VM_EOF
>> and migration will be interrupted.
>>
>> There is no way out of this except defining an incompatible
>> migration protocol with a sentinel at the end of embedded structs.
>> Of course, this is restricted to new machine models.
>>
>> Signed-off-by: Paolo Bonzini<pbonzini@redhat.com>
>
> Because of the change we made for 0.15, this is no longer strictly
> needed. It only matters if we add a subsection to a structure, right?

I'm definitely not saying we shouldn't do this BTW.  But trying to 
understand whether we can position this as a strictly optional feature 
(iow, a capability) or whether this requires a hard break in the 
migration protocol.

Regards,

Anthony Liguori

>
> Regards,
>
> Anthony Liguori
>
>> ---
>> hw/boards.h | 3 +++
>> hw/pc_piix.c | 5 +++++
>> savevm.c | 24 ++++++++++++++++--------
>> 3 files changed, 24 insertions(+), 8 deletions(-)
>>
>> diff --git a/hw/boards.h b/hw/boards.h
>> index 560dbaf..f20d58e 100644
>> --- a/hw/boards.h
>> +++ b/hw/boards.h
>> @@ -5,6 +5,9 @@
>>
>> #include "qdev.h"
>>
>> +#define QEMU_VM_FILE_VERSION_0_15 0x00000003
>> +#define QEMU_VM_FILE_VERSION 0x00000004
>> +
>> typedef void QEMUMachineInitFunc(ram_addr_t ram_size,
>> const char *boot_device,
>> const char *kernel_filename,
>> diff --git a/hw/pc_piix.c b/hw/pc_piix.c
>> index 88a5f3b..8af1f6f 100644
>> --- a/hw/pc_piix.c
>> +++ b/hw/pc_piix.c
>> @@ -284,6 +284,7 @@ static QEMUMachine pc_machine_v0_15 = {
>> .desc = "Standard PC",
>> .init = pc_init_pci,
>> .max_cpus = 255,
>> + .migration_format = QEMU_VM_FILE_VERSION_0_15,
>> };
>>
>> static QEMUMachine pc_machine_v0_13 = {
>> @@ -291,6 +292,7 @@ static QEMUMachine pc_machine_v0_13 = {
>> .desc = "Standard PC",
>> .init = pc_init_pci_no_kvmclock,
>> .max_cpus = 255,
>> + .migration_format = QEMU_VM_FILE_VERSION_0_15,
>> .compat_props = (GlobalProperty[]) {
>> {
>> .driver = "virtio-9p-pci",
>> @@ -330,6 +332,7 @@ static QEMUMachine pc_machine_v0_12 = {
>> .desc = "Standard PC",
>> .init = pc_init_pci_no_kvmclock,
>> .max_cpus = 255,
>> + .migration_format = QEMU_VM_FILE_VERSION_0_15,
>> .compat_props = (GlobalProperty[]) {
>> {
>> .driver = "virtio-serial-pci",
>> @@ -373,6 +376,7 @@ static QEMUMachine pc_machine_v0_11 = {
>> .desc = "Standard PC, qemu 0.11",
>> .init = pc_init_pci_no_kvmclock,
>> .max_cpus = 255,
>> + .migration_format = QEMU_VM_FILE_VERSION_0_15,
>> .compat_props = (GlobalProperty[]) {
>> {
>> .driver = "virtio-blk-pci",
>> @@ -424,6 +428,7 @@ static QEMUMachine pc_machine_v0_10 = {
>> .desc = "Standard PC, qemu 0.10",
>> .init = pc_init_pci_no_kvmclock,
>> .max_cpus = 255,
>> + .migration_format = QEMU_VM_FILE_VERSION_0_15,
>> .compat_props = (GlobalProperty[]) {
>> {
>> .driver = "virtio-blk-pci",
>> diff --git a/savevm.c b/savevm.c
>> index 87f2b71..a362ad7 100644
>> --- a/savevm.c
>> +++ b/savevm.c
>> @@ -158,6 +158,14 @@ void qemu_announce_self(void)
>>
>> #define IO_BUF_SIZE 32768
>>
>> +#define QEMU_VM_EOF 0x00
>> +#define QEMU_VM_SECTION_START 0x01
>> +#define QEMU_VM_SECTION_PART 0x02
>> +#define QEMU_VM_SECTION_END 0x03
>> +#define QEMU_VM_SECTION_FULL 0x04
>> +#define QEMU_VM_SUBSECTION 0x05
>> +#define QEMU_VM_SUBSECTIONS_END 0x06
>> +
>> struct QEMUFile {
>> QEMUFilePutBufferFunc *put_buffer;
>> QEMUFileGetBufferFunc *get_buffer;
>> @@ -1358,6 +1366,11 @@ int vmstate_load_state(QEMUFile *f, const
>> VMStateDescription *vmsd,
>> }
>> if (field->flags& VMS_STRUCT) {
>> ret = vmstate_load_state(f, field->vmsd, addr, field->vmsd->version_id);
>> + if (qemu_current_migration_format()>= 4) {
>> + if (qemu_get_byte(f) != QEMU_VM_SUBSECTIONS_END) {
>> + return -EINVAL;
>> + }
>> + }
>> } else {
>> ret = field->info->get(f, addr, size);
>>
>> @@ -1429,6 +1442,9 @@ void vmstate_save_state(QEMUFile *f, const
>> VMStateDescription *vmsd,
>> }
>> if (field->flags& VMS_STRUCT) {
>> vmstate_save_state(f, field->vmsd, addr);
>> + if (qemu_current_migration_format()>= 4) {
>> + qemu_put_byte(f, QEMU_VM_SUBSECTIONS_END);
>> + }
>> } else {
>> field->info->put(f, addr, size);
>> }
>> @@ -1458,14 +1474,6 @@ static void vmstate_save(QEMUFile *f,
>> SaveStateEntry *se)
>>
>> #define QEMU_VM_FILE_MAGIC 0x5145564d
>> #define QEMU_VM_FILE_VERSION_COMPAT 0x00000002
>> -#define QEMU_VM_FILE_VERSION 0x00000003
>> -
>> -#define QEMU_VM_EOF 0x00
>> -#define QEMU_VM_SECTION_START 0x01
>> -#define QEMU_VM_SECTION_PART 0x02
>> -#define QEMU_VM_SECTION_END 0x03
>> -#define QEMU_VM_SECTION_FULL 0x04
>> -#define QEMU_VM_SUBSECTION 0x05
>>
>> bool qemu_savevm_state_blocked(Monitor *mon)
>> {
>
>