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

[David Gibson]

On Tue, Oct 11, 2016 at 06:18:30PM +0100, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
> 
> 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 <address@hidden>

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 <address@hidden>

> ---
>  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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