Re: [Qemu-devel] [Qemu-ppc] [QEMU PATCH v6 2/2] migration: migrate QTAILQ

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

* Jianjun Duan (duanj@linux.vnet.ibm.com) wrote:
> 
> 
> On 10/14/2016 03:44 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        | 32 ++++++++++++++++++++++++
> >>  migration/trace-events      |  4 +++
> >>  migration/vmstate.c         | 59 +++++++++++++++++++++++++++++++++++++++++++++
> >>  4 files changed, 115 insertions(+)
> >>
> >> diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
> >> index d0e37b5..4dd0aed 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
> >> + * _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,                                \
> >> +    .flags        = VMS_LINKED,                                          \
> > 
> > Hang on - you killed VMS_LINKED off in the previous patch; so this
> > doesn't work.
> > (Adding a test to tests/test-vmstate.c would have found that)
> > 
> 
> Thanks for pointing it out. Since the actual application of this patch
> is split off, my test didn't go through this path.

Yes, but you could add a new entry into test-vmstate.c for this.

> >> +    .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..d672ae0 100644
> >> --- a/include/qemu/queue.h
> >> +++ b/include/qemu/queue.h
> >> @@ -438,4 +438,36 @@ struct {                                                                \
> >>  #define QTAILQ_PREV(elm, headname, field) \
> >>          (*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))
> >>  
> >> +/*
> >> + * Offsets of layout of a tail queue head.
> >> + */
> >> +#define QTAILQ_FIRST_OFFSET 0
> >> +#define QTAILQ_LAST_OFFSET (sizeof(void *))
> >> +
> >> +/*
> >> + * Offsets of layout of a tail queue element.
> >> + */
> >> +#define QTAILQ_NEXT_OFFSET 0
> >> +#define QTAILQ_PREV_OFFSET (sizeof(void *))
> >> +
> >> +/*
> >> + * Tail queue tranversal using pointer arithmetic.
> >> + */
> >> +#define QTAILQ_RAW_FOREACH(elm, head, entry)                                   \
> >> +        for ((elm) = *((void **) ((char *) (head) + QTAILQ_FIRST_OFFSET));     \
> >> +             (elm);                                                            \
> >> +             (elm) =                                                           \
> >> +                 *((void **) ((char *) (elm) + (entry) + QTAILQ_NEXT_OFFSET)))
> >> +/*
> >> + * Tail queue insertion using pointer arithmetic.
> >> + */
> >> +#define QTAILQ_RAW_INSERT_TAIL(head, elm, entry) do {                          \
> >> +        *((void **) ((char *) (elm) + (entry) + QTAILQ_NEXT_OFFSET)) = NULL;   \
> >> +        *((void **) ((char *) (elm) + (entry) + QTAILQ_PREV_OFFSET)) =         \
> >> +            *((void **) ((char *) (head) + QTAILQ_LAST_OFFSET));               \
> >> +        **((void ***)((char *) (head) + QTAILQ_LAST_OFFSET)) = (elm);          \
> >> +        *((void **) ((char *) (head) + QTAILQ_LAST_OFFSET)) =                  \
> >> +            (void *) ((char *) (elm) + (entry) + QTAILQ_NEXT_OFFSET);          \
> >> +} while (/*CONSTCOND*/0)
> > 
> > I wonder if there's a simpler way to do this; I'm not sure this works, but something like:
> > 
> > struct QTAILQDummy {
> >     char dummy;
> > };
> > 
> > QTAILQ_HEAD(QTAILQRawHead, struct QTAILQDummy)
> > typedef QTAILQ_ENTRY(struct QTAILQDummy) QTAILQRawEntry;
> > 
> > #define QTAILQ_RAW_FOREACH(elm, head, entry)                                   \
> >         for ((elm) = ((struct QTAILQRawHead *)head)->tqh_first)                \
> >              (elm);                                                            \
> >              (elm) =                                                           \
> >              (elm) = ((QTAILQRawEntry *)((char *) (elm) + (entry)))->tqh_next
> > 
> > and then I think elm gets declared as a struct QTAILQDummy.
> > But it does avoid those FIRST_OFFSET/LAST_OFFSET/NEXT_OFFSET/PREV_OFFSET calculations.
> > 
> > Would that work?
> > 
> It is intended for QTAILQ of any type. So type is not available.

I think it might be possible to do it generally.

> >>  #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 e8eff3f..eb68fc6 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"
> >>  
> >> @@ -946,3 +947,61 @@ const VMStateInfo vmstate_info_bitmap = {
> >>      .get = get_bitmap,
> >>      .put = put_bitmap,
> >>  };
> >> +
> >> +/*get for QTAILQ */
> >> +static int get_qtailq(QEMUFile *f, void *pv, size_t unused_size,
> >> +                      VMStateField *field)
> >> +{
> >> +    int ret = 0;
> >> +    const VMStateDescription *vmsd = field->vmsd;
> >> +    size_t size = field->size;
> >> +    size_t entry = field->start;
> > 
> > Please comment these two here; I'd prefer entry_offset or something
> > as a name to make clear what it is.
> > 
> OK.
> >> +    int version_id = field->version_id;
> >> +    void *elm;
> >> +
> >> +    trace_get_qtailq(vmsd->name, version_id);
> >> +    if (version_id > vmsd->version_id) {
> >> +        trace_get_qtailq_end(vmsd->name, "too new", -EINVAL);
> >> +        return -EINVAL;
> >> +    }
> >> +    if (version_id < vmsd->minimum_version_id) {
> >> +        trace_get_qtailq_end(vmsd->name, "too old", -EINVAL);
> >> +        return -EINVAL;
> >> +    }
> > 
> > Please make those error_report's - if anything fails migration
> > I like to see it in the stderr log.
> > 
> I know previously you wanted it to be error report. But in the migration
> code the same error is always just put into trace (vmastate_load_state).
> I just want to be consistent.

I should fix those as well to add error_reports !
It's very difficult when we get an error back from a customer with just
a migration failure when the only thing we know is an -EINVAL, so I keep
adding error_report calls.

Dave

> Thanks,
> Jianjun
> >> +    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);
> >> +    }
> >> +
> >> +    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;
> >> +    size_t entry = field->start;
> >> +    void *elm;
> >> +
> >> +    trace_put_qtailq(vmsd->name, vmsd->version_id);
> >> +
> >> +    QTAILQ_RAW_FOREACH(elm, pv, entry) {
> >> +        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
> > 
> > Dave
> > 
> >>
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK