Re: [Qemu-devel] [QEMU PATCH v8 2/3] migration: migrate QTAILQ

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

* Jianjun Duan (duanj@linux.vnet.ibm.com) wrote:
> 
> 
> On 10/26/2016 09:29 AM, 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        | 46 +++++++++++++++++++++++++++++++
> >>  migration/trace-events      |  4 +++
> >>  migration/vmstate.c         | 67 +++++++++++++++++++++++++++++++++++++++++++++
> >>  4 files changed, 137 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..e9378fa 100644
> >> --- a/include/qemu/queue.h
> >> +++ b/include/qemu/queue.h
> >> @@ -438,4 +438,50 @@ struct {                                                                \
> >>  #define QTAILQ_PREV(elm, headname, field) \
> >>          (*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))
> >>  
> >> +#define RAW_FIELD(base, offset)                                                \
> >> +        ((char *) (base) + offset)
> >> +
> >> +/*
> >> + * Offsets of layout of a tail queue head.
> >> + */
> >> +#define QTAILQ_FIRST_OFFSET 0
> >> +#define QTAILQ_LAST_OFFSET (sizeof(void *))
> > 
> > OK, you might want to add some asserts somewhere in a .c just to catch if
> > any of these offsets change.
> > 
> if the layout of QTAILQ changes, the version id for the vmsd should also
> be changed. It will break migration between different versions. However
> the operation doesn't depend on these values.

No, changing layout of QTAILQ doesn't need to change the version id of vmsd;
it's an internal change.  But if someone does make the change and forgets
to update your OFFSET macros it'll cause memory corruption.
You could catch that with an assert (possibly build time).

> >> + * Raw access of elements of a tail queue
> >> + */
> >> +#define QTAILQ_RAW_FIRST(head)                                                 \
> >> +        (*((void **) (RAW_FIELD(head,  QTAILQ_FIRST_OFFSET))))
> >> +#define QTAILQ_RAW_LAST(head)                                                  \
> >> +        (*((void ***) (RAW_FIELD(head,  QTAILQ_LAST_OFFSET))))
> >> +
> >> +/*
> >> + * Offsets of layout of a tail queue element.
> >> + */
> >> +#define QTAILQ_NEXT_OFFSET 0
> >> +#define QTAILQ_PREV_OFFSET (sizeof(void *))
> >> +
> >> +/*
> >> + * Raw access of elements of a tail entry
> >> + */
> >> +#define QTAILQ_RAW_NEXT(elm, entry)                                            \
> >> +        (*((void **) (RAW_FIELD(elm, entry + QTAILQ_NEXT_OFFSET))))
> >> +#define QTAILQ_RAW_PREV(elm, entry)                                            \
> >> +        (*((void ***) (RAW_FIELD(elm, entry + QTAILQ_PREV_OFFSET))))
> >> +/*
> >> + * 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..fcf808e 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;
> > 
> > You could just make that a break since you're returning ret
> > (that's otherwise all 0).  Still, not important, but could
> > be tidied if you need to regenerate the patch.
> > (and you could remove the 2nd space after elm =).

> By returning it, the trace file will miss "get_qtailq_end', that
> is useful debug information.
> You are right about the space.

But that's what you're doing now, you're missing the trace call
and returning 'ret' that will always be 0 if it gets there.

Dave

> 
> Thanks,
> Jianjun
> 
> > 
> >> +        }
> >> +        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,
> >> +};
> > 
> > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > 
> >> -- 
> >> 1.9.1
> >>
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK