Re: [Qemu-devel] [QEMU PATCH v9 2/3] migration: migrate QTAILQ

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

* Jianjun Duan (duanj@linux.vnet.ibm.com) wrote:
> Currently we cannot directly transfer a QTAILQ instance because of the
> limitation in the migration code. Here we introduce an approach to
> transfer such structures. We created VMStateInfo vmstate_info_qtailq
> for QTAILQ. Similar VMStateInfo can be created for other data structures
> such as list.
> 
> This approach will be used to transfer pending_events and ccs_list in spapr
> state.
> 
> We also create some macros in qemu/queue.h to access a QTAILQ using pointer
> arithmetic. This ensures that we do not depend on the implementation
> details about QTAILQ in the migration code.
> 
> Signed-off-by: Jianjun Duan <duanj@linux.vnet.ibm.com>
> ---
>  include/migration/vmstate.h | 20 ++++++++++++++
>  include/qemu/queue.h        | 61 +++++++++++++++++++++++++++++++++++++++++
>  migration/trace-events      |  4 +++
>  migration/vmstate.c         | 67 +++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 152 insertions(+)
> 
> diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
> index d0e37b5..318a6f1 100644
> --- a/include/migration/vmstate.h
> +++ b/include/migration/vmstate.h
> @@ -251,6 +251,7 @@ extern const VMStateInfo vmstate_info_timer;
>  extern const VMStateInfo vmstate_info_buffer;
>  extern const VMStateInfo vmstate_info_unused_buffer;
>  extern const VMStateInfo vmstate_info_bitmap;
> +extern const VMStateInfo vmstate_info_qtailq;
>  
>  #define type_check_2darray(t1,t2,n,m) ((t1(*)[n][m])0 - (t2*)0)
>  #define type_check_array(t1,t2,n) ((t1(*)[n])0 - (t2*)0)
> @@ -662,6 +663,25 @@ extern const VMStateInfo vmstate_info_bitmap;
>      .offset       = offsetof(_state, _field),                        \
>  }
>  
> +/* For QTAILQ that need customized handling.
> + * Target QTAILQ needs be properly initialized.
> + * _type: type of QTAILQ element
> + * _next: name of QTAILQ entry field in QTAILQ element
> + * _vmsd: VMSD for QTAILQ element
> + * size: size of QTAILQ element
> + * start: offset of QTAILQ entry in QTAILQ element
> + */
> +#define VMSTATE_QTAILQ_V(_field, _state, _version, _vmsd, _type, _next)  \
> +{                                                                        \
> +    .name         = (stringify(_field)),                                 \
> +    .version_id   = (_version),                                          \
> +    .vmsd         = &(_vmsd),                                            \
> +    .size         = sizeof(_type),                                       \
> +    .info         = &vmstate_info_qtailq,                                \
> +    .offset       = offsetof(_state, _field),                            \
> +    .start        = offsetof(_type, _next),                              \
> +}
> +
>  /* _f : field name
>     _f_n : num of elements field_name
>     _n : num of elements
> diff --git a/include/qemu/queue.h b/include/qemu/queue.h
> index 342073f..16af127 100644
> --- a/include/qemu/queue.h
> +++ b/include/qemu/queue.h
> @@ -438,4 +438,65 @@ struct {                                                                \
>  #define QTAILQ_PREV(elm, headname, field) \
>          (*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))
>  
> +#define field_at_offset(base, offset, type)                                    \
> +        ((type) (((char *) (base)) + (offset)))
> +
> +typedef struct DUMB_Q_ENTRY DUMB_Q_ENTRY;
> +typedef struct DUMB_Q DUMB_Q;
> +
> +struct DUMB_Q_ENTRY {
> +        QTAILQ_ENTRY(DUMB_Q_ENTRY) next;
> +};
> +
> +struct DUMB_Q {
> +        QTAILQ_HEAD(DUMB_Q_HEAD, DUMB_Q_ENTRY) head;
> +};

OK, good we've got rid of the hard coded offsets; thanks!

> +#define dumb_q ((DUMB_Q *) 0)
> +#define dumb_qh ((typeof(dumb_q->head) *) 0)
> +#define dumb_qe ((DUMB_Q_ENTRY *) 0)

Note that 'dumb' and 'dummy' are completely different words;
this is a dummy not a dumb.

> +/*
> + * Offsets of layout of a tail queue head.
> + */
> +#define QTAILQ_FIRST_OFFSET ((size_t) &(QTAILQ_FIRST(dumb_qh)))

Isn't that just  offsetof(dumb_qh, tqh_first) ?

> +#define QTAILQ_LAST_OFFSET  (offsetof(typeof(dumb_q->head), tqh_last))

Similarly isnt't that just  offsetof(DUMB_Q_HEAD, tqh_last) ?

> +/*
> + * Raw access of elements of a tail queue
> + */
> +#define QTAILQ_RAW_FIRST(head)                                                 \
> +        (*field_at_offset(head, QTAILQ_FIRST_OFFSET, void **))
> +#define QTAILQ_RAW_LAST(head)                                                  \
> +        (*field_at_offset(head, QTAILQ_LAST_OFFSET, void ***))
> +
> +/*
> + * Offsets of layout of a tail queue element.
> + */
> +#define QTAILQ_NEXT_OFFSET ((size_t) (&QTAILQ_NEXT(dumb_qe, next)))
> +#define QTAILQ_PREV_OFFSET (offsetof(typeof(dumb_qe->next), tqe_prev))

Similar comments to the pair above.

Dave
P.S. I'm out next week, so please someone else jump in.

> +/*
> + * Raw access of elements of a tail entry
> + */
> +#define QTAILQ_RAW_NEXT(elm, entry)                                            \
> +        (*field_at_offset(elm, entry + QTAILQ_NEXT_OFFSET, void **))
> +#define QTAILQ_RAW_PREV(elm, entry)                                            \
> +        (*field_at_offset(elm, entry + QTAILQ_PREV_OFFSET, void ***))
> +/*
> + * Tail queue tranversal using pointer arithmetic.
> + */
> +#define QTAILQ_RAW_FOREACH(elm, head, entry)                                   \
> +        for ((elm) = QTAILQ_RAW_FIRST(head);                                   \
> +             (elm);                                                            \
> +             (elm) = QTAILQ_RAW_NEXT(elm, entry))
> +/*
> + * Tail queue insertion using pointer arithmetic.
> + */
> +#define QTAILQ_RAW_INSERT_TAIL(head, elm, entry) do {                          \
> +        QTAILQ_RAW_NEXT(elm, entry) = NULL;                                    \
> +        QTAILQ_RAW_PREV(elm, entry) = QTAILQ_RAW_LAST(head);                   \
> +        *QTAILQ_RAW_LAST(head) = (elm);                                        \
> +        QTAILQ_RAW_LAST(head) = &QTAILQ_RAW_NEXT(elm, entry);                  \
> +} while (/*CONSTCOND*/0)
> +
>  #endif /* QEMU_SYS_QUEUE_H */
> diff --git a/migration/trace-events b/migration/trace-events
> index dfee75a..9a6ec59 100644
> --- a/migration/trace-events
> +++ b/migration/trace-events
> @@ -52,6 +52,10 @@ vmstate_n_elems(const char *name, int n_elems) "%s: %d"
>  vmstate_subsection_load(const char *parent) "%s"
>  vmstate_subsection_load_bad(const char *parent,  const char *sub, const char *sub2) "%s: %s/%s"
>  vmstate_subsection_load_good(const char *parent) "%s"
> +get_qtailq(const char *name, int version_id) "%s v%d"
> +get_qtailq_end(const char *name, const char *reason, int val) "%s %s/%d"
> +put_qtailq(const char *name, int version_id) "%s v%d"
> +put_qtailq_end(const char *name, const char *reason) "%s %s"
>  
>  # migration/qemu-file.c
>  qemu_file_fclose(void) ""
> diff --git a/migration/vmstate.c b/migration/vmstate.c
> index d188afa..70e0472 100644
> --- a/migration/vmstate.c
> +++ b/migration/vmstate.c
> @@ -5,6 +5,7 @@
>  #include "migration/vmstate.h"
>  #include "qemu/bitops.h"
>  #include "qemu/error-report.h"
> +#include "qemu/queue.h"
>  #include "trace.h"
>  #include "migration/qjson.h"
>  
> @@ -948,3 +949,69 @@ const VMStateInfo vmstate_info_bitmap = {
>      .get = get_bitmap,
>      .put = put_bitmap,
>  };
> +
> +/* get for QTAILQ
> + * meta data about the QTAILQ is encoded in a VMStateField structure
> + */
> +static int get_qtailq(QEMUFile *f, void *pv, size_t unused_size,
> +                      VMStateField *field)
> +{
> +    int ret = 0;
> +    const VMStateDescription *vmsd = field->vmsd;
> +    /* size of a QTAILQ element */
> +    size_t size = field->size;
> +    /* offset of the QTAILQ entry in a QTAILQ element */
> +    size_t entry_offset = field->start;
> +    int version_id = field->version_id;
> +    void *elm;
> +
> +    trace_get_qtailq(vmsd->name, version_id);
> +    if (version_id > vmsd->version_id) {
> +        error_report("%s %s",  vmsd->name, "too new");
> +        trace_get_qtailq_end(vmsd->name, "too new", -EINVAL);
> +
> +        return -EINVAL;
> +    }
> +    if (version_id < vmsd->minimum_version_id) {
> +        error_report("%s %s",  vmsd->name, "too old");
> +        trace_get_qtailq_end(vmsd->name, "too old", -EINVAL);
> +        return -EINVAL;
> +    }
> +
> +    while (qemu_get_byte(f)) {
> +        elm = g_malloc(size);
> +        ret = vmstate_load_state(f, vmsd, elm, version_id);
> +        if (ret) {
> +            return ret;
> +        }
> +        QTAILQ_RAW_INSERT_TAIL(pv, elm, entry_offset);
> +    }
> +
> +    trace_get_qtailq_end(vmsd->name, "end", ret);
> +    return ret;
> +}
> +
> +/* put for QTAILQ */
> +static void put_qtailq(QEMUFile *f, void *pv, size_t unused_size,
> +                       VMStateField *field, QJSON *vmdesc)
> +{
> +    const VMStateDescription *vmsd = field->vmsd;
> +    /* offset of the QTAILQ entry in a QTAILQ element*/
> +    size_t entry_offset = field->start;
> +    void *elm;
> +
> +    trace_put_qtailq(vmsd->name, vmsd->version_id);
> +
> +    QTAILQ_RAW_FOREACH(elm, pv, entry_offset) {
> +        qemu_put_byte(f, true);
> +        vmstate_save_state(f, vmsd, elm, vmdesc);
> +    }
> +    qemu_put_byte(f, false);
> +
> +    trace_put_qtailq_end(vmsd->name, "end");
> +}
> +const VMStateInfo vmstate_info_qtailq = {
> +    .name = "qtailq",
> +    .get  = get_qtailq,
> +    .put  = put_qtailq,
> +};
> -- 
> 1.9.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK