On Tue, Oct 11, 2016 at 06:18:30PM +0100, Dr. David Alan Gilbert (git) wrote: > From: "Dr. David Alan Gilbert" <dgilb...@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_* > > Signed-off-by: Dr. David Alan Gilbert <dgilb...@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 <da...@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
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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