Re: [Qemu-devel] [QEMU RFC PATCH v3 4/6] Migration: migrate QTAILQ

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

On 06/01/2016 08:29 AM, Paolo Bonzini wrote:
> 
> 
> On 31/05/2016 23:53, Jianjun Duan wrote:
>>
>>
>> On 05/31/2016 12:54 PM, Paolo Bonzini wrote:
>>>
>>>
>>> ----- Original Message -----
>>>> From: "Jianjun Duan" <duanj@linux.vnet.ibm.com>
>>>> To: qemu-devel@nongnu.org
>>>> Cc: qemu-ppc@nongnu.org, duanj@linux.vnet.ibm.com, dmitry@daynix.com, "peter maydell" <peter.maydell@linaro.org>,
>>>> kraxel@redhat.com, mst@redhat.com, david@gibson.dropbear.id.au, pbonzini@redhat.com, veroniabahaa@gmail.com,
>>>> quintela@redhat.com, "amit shah" <amit.shah@redhat.com>, mreitz@redhat.com, kwolf@redhat.com, rth@twiddle.net,
>>>> aurelien@aurel32.net, "leon alrae" <leon.alrae@imgtec.com>, blauwirbel@gmail.com, "mark cave-ayland"
>>>> <mark.cave-ayland@ilande.co.uk>, mdroth@linux.vnet.ibm.com
>>>> Sent: Tuesday, May 31, 2016 8:02:42 PM
>>>> Subject: [QEMU RFC PATCH v3 4/6] Migration: migrate QTAILQ
>>>>
>>>> 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. In our approach such a structure is tagged
>>>> with VMS_CSTM. We then modified vmstate_save_state and vmstate_load_state
>>>> so that when VMS_CSTM is encountered, put and get from VMStateInfo are
>>>> called respectively. 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 | 22 +++++++++++++
>>>>  include/qemu/queue.h        | 32 ++++++++++++++++++
>>>>  migration/vmstate.c         | 79
>>>>  +++++++++++++++++++++++++++++++++++++++++++++
>>>>  3 files changed, 133 insertions(+)
>>>>
>>>> diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
>>>> index 56a4171..da4ef7f 100644
>>>> --- a/include/migration/vmstate.h
>>>> +++ b/include/migration/vmstate.h
>>>> @@ -185,6 +185,8 @@ enum VMStateFlags {
>>>>       * to determine the number of entries in the array. Only valid in
>>>>       * combination with one of VMS_VARRAY*. */
>>>>      VMS_MULTIPLY_ELEMENTS = 0x4000,
>>>> +    /* For fields which need customized handling, such as QTAILQ in
>>>> queue.h*/
>>>> +    VMS_CSTM            = 0x8000,
>>>
>>> Please call this VMS_LINKED.  It can be adapted to other data
>>> structures in qemu/queue.h if there is a need later on.
>>>
>>>>  };
>>>>  
>>>>  struct VMStateField {
>>>> @@ -245,6 +247,7 @@ 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)
>>>> @@ -656,6 +659,25 @@ 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_CSTM,                                            \
>>>> +    .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 f781aa2..003e368 100644
>>>> --- a/include/qemu/queue.h
>>>> +++ b/include/qemu/queue.h
>>>> @@ -437,3 +437,35 @@ struct {
>>>> \
>>>>          (*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))
>>>>  
>>>>  #endif  /* !QEMU_SYS_QUEUE_H_ */
>>>> +
>>>> +/*
>>>> + * 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)
>>>> diff --git a/migration/vmstate.c b/migration/vmstate.c
>>>> index 644ba1f..ff56650 100644
>>>> --- a/migration/vmstate.c
>>>> +++ b/migration/vmstate.c
>>>> @@ -5,7 +5,9 @@
>>>>  #include "migration/vmstate.h"
>>>>  #include "qemu/bitops.h"
>>>>  #include "qemu/error-report.h"
>>>> +#include "qemu/queue.h"
>>>>  #include "trace.h"
>>>> +#include "migration/qjson.h"
>>>>  
>>>>  static void vmstate_subsection_save(QEMUFile *f, const VMStateDescription
>>>>  *vmsd,
>>>>                                      void *opaque, QJSON *vmdesc);
>>>> @@ -120,6 +122,8 @@ int vmstate_load_state(QEMUFile *f, const
>>>> VMStateDescription *vmsd,
>>>>                  if (field->flags & VMS_STRUCT) {
>>>>                      ret = vmstate_load_state(f, field->vmsd, addr,
>>>>                                               field->vmsd->version_id);
>>>> +                } else if (field->flags & VMS_CSTM) {
>>>> +                    ret = field->info->get(f, addr, size, field);
>>>>                  } else {
>>>>                      ret = field->info->get(f, addr, size, NULL);
>>>>  
>>>> @@ -192,6 +196,8 @@ static const char *vmfield_get_type_name(VMStateField
>>>> *field)
>>>>  
>>>>      if (field->flags & VMS_STRUCT) {
>>>>          type = "struct";
>>>> +    } else if (field->flags & VMS_CSTM) {
>>>> +        type = "customized";
>>>>      } else if (field->info->name) {
>>>>          type = field->info->name;
>>>>      }
>>>> @@ -326,6 +332,8 @@ void vmstate_save_state(QEMUFile *f, const
>>>> VMStateDescription *vmsd,
>>>>                  }
>>>>                  if (field->flags & VMS_STRUCT) {
>>>>                      vmstate_save_state(f, field->vmsd, addr, vmdesc_loop);
>>>> +                } else if  (field->flags & VMS_CSTM) {
>>>> +                    field->info->put(f, addr, size, field, vmdesc_loop);
>>>>                  } else {
>>>>                      field->info->put(f, addr, size, NULL, NULL);
>>>>                  }
>>>> @@ -938,3 +946,74 @@ 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)
>>>> +{
>>>> +    bool link;
>>>> +    int ret = 0;
>>>> +    const VMStateDescription *vmsd = field->vmsd;
>>>> +    size_t size = field->size;
>>>> +    size_t entry = field->start;
>>>> +    int version_id = field->version_id;
>>>> +    void *elm;
>>>> +
>>>> +    trace_vmstate_load_state(vmsd->name, version_id);
>>>> +    if (version_id > vmsd->version_id) {
>>>> +        trace_vmstate_load_state_end(vmsd->name, "too new", -EINVAL);
>>>> +        return -EINVAL;
>>>> +    }
>>>> +    if (version_id < vmsd->minimum_version_id) {
>>>> +        trace_vmstate_load_state_end(vmsd->name, "too old", -EINVAL);
>>>> +        return -EINVAL;
>>>> +    }
>>>> +
>>>> +    while (true) {
>>>> +        vmstate_info_bool.get(f, &link, sizeof(bool), NULL);
>>>
>>> You can use qemu_get_byte here, and likewise qemu_put_byte below in
>>> put_qtailq.
>>
>> qemu_get/put is indeed better choice.
>>>
>>>> +        if (!link) {
>>>> +            break;
>>>> +        }
>>>> +
>>>> +        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_vmstate_load_state_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)
>>>> +{
>>>> +    bool link = true;
>>>> +    const VMStateDescription *vmsd = field->vmsd;
>>>> +    size_t entry = field->start;
>>>> +    void *elm;
>>>> +
>>>> +    if (vmdesc) {
>>>> +        json_prop_str(vmdesc, "vmsd_name", vmsd->name);
>>>> +        json_prop_int(vmdesc, "version", vmsd->version_id);
>>>> +        json_start_array(vmdesc, "fields");
>>>
>>> You need to store the fields exactly once here, even if there are
>>> 0 or >1 elements.
>>>
>> Do you mean the json entries? I think it is already doing that.
> 
> In the case of 0 entries we don't go through the loop, so the JSON
> entries are definitely missing in that case.  I'm not sure if QJSON
> handles duplicates in the case of 2+ entries.
The vmsd here is the vmsd for the queue elements. Not for the queue.
Maybe the stuff written here should be information about the qtailq
instead, but we don't have a vmsd for the queue as a whole.

As it stands, it will record the name, version of the element type. If
the queue is empty, it records nothing in the fields. otherwise it will
record each element and repeat.

In vmstate_save_state, the vmsd of the array element for a uncompressed
array is recorded every time an array element is saved. The number of
written bytes is also recorded. If the array has 0 elements, it is not
recorded.

If we want to make the behavior consistent, we should not do any json
stuff in put_qtatilq function.

If we want to record the vmsd of the queue element exactly once, I can
set the vmdesc to null after the first iteration. But we may need a
recursive function just to dump out the vmsd when the queue is empty,
and record that 0 bytes are written for each field.

I would say that let's remove the json stuff here to be consistent with
array behavior.

> 
> Thanks,
> 
> Paolo
> 
Thanks,
Jianjun