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

[Jianjun Duan <duanj@linux.vnet.ibm.com>]

On 10/28/2016 12:06 PM, Dr. David Alan Gilbert wrote:
> * 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.
> 
OK.
>> +/*
>> + * 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) ?
Yes. But I don't want to depend on the inside of the type if it is
possible. QTAILQ_FIRST is a defined access macro.

> 
>> +#define QTAILQ_LAST_OFFSET  (offsetof(typeof(dumb_q->head), tqh_last))
> 
> Similarly isnt't that just  offsetof(DUMB_Q_HEAD, tqh_last) ?
> 
Similarly, DUMB_Q_HEAD may not be a type name in the future.

Thanks,
Jianjun
>> +/*
>> + * 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
>