Re: [Qemu-devel] [very-WIP 1/7] migration: Add VMSTATE_WITH_TMP

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Jianjun Duan (duanj@linux.vnet.ibm.com) wrote:
> 
> 
> On 10/16/2016 08:31 PM, David Gibson wrote:
> > On Tue, Oct 11, 2016 at 06:18:30PM +0100, Dr. David Alan Gilbert (git) wrote:
> >> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> >>
> >> VMSTATE_WITH_TMP is for handling structures where some calculation
> >> or rearrangement of the data needs to be performed before the data
> >> hits the wire.
> >> For example,  where the value on the wire is an offset from a
> >> non-migrated base, but the data in the structure is the actual pointer.
> >>
> >> To use it, a temporary type is created and a vmsd used on that type.
> >> The first element of the type must be 'parent' a pointer back to the
> >> type of the main structure.  VMSTATE_WITH_TMP takes care of allocating
> >> and freeing the temporary before running the child vmsd.
> >>
> >> The post_load/pre_save on the child vmsd can copy things from the parent
> >> to the temporary using the parent pointer and do any other calculations
> >> needed; it can then use normal VMSD entries to do the actual data
> >> storage without having to fiddle around with qemu_get_*/qemu_put_*
> >>
> If customized put/get can do transformation and dumping/loading data
> to/from the parent structure, you don't have to go through
> pre_save/post_load, and may get rid of parent pointer.

Yes but I'd rather try and get rid of the customized put/get from
every device, because then people start using qemu_put/qemu_get in them all.

Dave

> 
> Thanks,
> Jianjun
> 
> >> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > 
> > The requirement for the parent pointer is a little clunky, but I don't
> > quickly see a better way, and it is compile-time verified.  As noted
> > elsewhere I think this is a really useful approach which could allow a
> > bunch of internal state cleanups while preserving migration.
> > 
> > Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> > 
> >> ---
> >>  include/migration/vmstate.h | 20 ++++++++++++++++++++
> >>  migration/vmstate.c         | 38 ++++++++++++++++++++++++++++++++++++++
> >>  2 files changed, 58 insertions(+)
> >>
> >> diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
> >> index 9500da1..efb0e90 100644
> >> --- a/include/migration/vmstate.h
> >> +++ b/include/migration/vmstate.h
> >> @@ -259,6 +259,7 @@ extern const VMStateInfo vmstate_info_cpudouble;
> >>  extern const VMStateInfo vmstate_info_timer;
> >>  extern const VMStateInfo vmstate_info_buffer;
> >>  extern const VMStateInfo vmstate_info_unused_buffer;
> >> +extern const VMStateInfo vmstate_info_tmp;
> >>  extern const VMStateInfo vmstate_info_bitmap;
> >>  extern const VMStateInfo vmstate_info_qtailq;
> >>  
> >> @@ -651,6 +652,25 @@ extern const VMStateInfo vmstate_info_qtailq;
> >>      .offset     = offsetof(_state, _field),                          \
> >>  }
> >>  
> >> +/* Allocate a temporary of type 'tmp_type', set tmp->parent to _state
> >> + * and execute the vmsd on the temporary.  Note that we're working with
> >> + * the whole of _state here, not a field within it.
> >> + * We compile time check that:
> >> + *    That _tmp_type contains a 'parent' member that's a pointer to the
> >> + *        '_state' type
> >> + *    That the pointer is right at the start of _tmp_type.
> >> + */
> >> +#define VMSTATE_WITH_TMP(_state, _tmp_type, _vmsd) {                 \
> >> +    .name         = "tmp",                                           \
> >> +    .size         = sizeof(_tmp_type) +                              \
> >> +                    QEMU_BUILD_BUG_EXPR(offsetof(_tmp_type, parent) != 0) + \
> >> +                    type_check_pointer(_state,                       \
> >> +                        typeof_field(_tmp_type, parent)),            \
> >> +    .vmsd         = &(_vmsd),                                        \
> >> +    .info         = &vmstate_info_tmp,                               \
> >> +    .flags        = VMS_LINKED,                                      \
> >> +}
> >> +
> >>  #define VMSTATE_UNUSED_BUFFER(_test, _version, _size) {              \
> >>      .name         = "unused",                                        \
> >>      .field_exists = (_test),                                         \
> >> diff --git a/migration/vmstate.c b/migration/vmstate.c
> >> index 2157997..f2563c5 100644
> >> --- a/migration/vmstate.c
> >> +++ b/migration/vmstate.c
> >> @@ -925,6 +925,44 @@ const VMStateInfo vmstate_info_unused_buffer = {
> >>      .put  = put_unused_buffer,
> >>  };
> >>  
> >> +/* vmstate_info_tmp, see VMSTATE_WITH_TMP, the idea is that we allocate
> >> + * a temporary buffer and the pre_load/pre_save methods in the child vmsd
> >> + * copy stuff from the parent into the child and do calculations to fill
> >> + * in fields that don't really exist in the parent but need to be in the
> >> + * stream.
> >> + */
> >> +static int get_tmp(QEMUFile *f, void *pv, size_t size, VMStateField *field)
> >> +{
> >> +    int ret;
> >> +    const VMStateDescription *vmsd = field->vmsd;
> >> +    int version_id = field->version_id;
> >> +    void *tmp = g_malloc(size);
> >> +
> >> +    /* Writes the parent field which is at the start of the tmp */
> >> +    *(void **)tmp = pv;
> >> +    ret = vmstate_load_state(f, vmsd, tmp, version_id);
> >> +    g_free(tmp);
> >> +    return ret;
> >> +}
> >> +
> >> +static void put_tmp(QEMUFile *f, void *pv, size_t size, VMStateField *field,
> >> +                    QJSON *vmdesc)
> >> +{
> >> +    const VMStateDescription *vmsd = field->vmsd;
> >> +    void *tmp = g_malloc(size);
> >> +
> >> +    /* Writes the parent field which is at the start of the tmp */
> >> +    *(void **)tmp = pv;
> >> +    vmstate_save_state(f, vmsd, tmp, vmdesc);
> >> +    g_free(tmp);
> >> +}
> >> +
> >> +const VMStateInfo vmstate_info_tmp = {
> >> +    .name = "tmp",
> >> +    .get = get_tmp,
> >> +    .put = put_tmp,
> >> +};
> >> +
> >>  /* bitmaps (as defined by bitmap.h). Note that size here is the size
> >>   * of the bitmap in bits. The on-the-wire format of a bitmap is 64
> >>   * bit words with the bits in big endian order. The in-memory format
> > 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK