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

[Jianjun Duan]

On 10/14/2016 03:44 AM, Dr. David Alan Gilbert wrote:
> * Jianjun Duan (address@hidden) 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 <address@hidden>
>> ---
>>  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.
>> +    .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.
>>  #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.

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 / address@hidden / Manchester, UK
>