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

[Anthony Liguori <anthony@codemonkey.ws>]

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?

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)
>   {