Re: [Qemu-ppc] [Qemu-devel] [QEMU PATCH v5 4/6] migration: migrate QTAILQ

[Jianjun Duan]

On 10/06/2016 12:01 PM, Dr. David Alan Gilbert wrote:
> * Jianjun Duan (address@hidden) wrote:
>>
>>
>> On 10/05/2016 09:56 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. In our approach such a structure is tagged
>>>> with VMS_LINKED. We then modified vmstate_save_state and vmstate_load_state
>>>> so that when VMS_LINKED is encountered, put and get from VMStateInfo are
>>>> called respectively. 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.
>>>
>>> I think we're going to need a way to have a more flexible
>>> loops; and thus my choice here wouldn't be to use the .get/.put together
>>> with the VMSD; but I think we'll end up needing a new
>>> data structure, maybe a VMStateLoop *loop in VMStateField.
>>>
>>> So would it be easier if you added that new member, then you wouldn't have 
>>> to
>>> modify every get() and put() function that already exists in the previous 
>>> patch.
>>>
>>> Specifically, your format of QTAILQ is perfectly reasonable - a
>>> byte before each entry which is 1 to indicate there's an entry or 0
>>> to indicate termination, but there are lots of other variants, e.g.
>>>
>>>    a) put_scsi_requests uses that byte to hold a flag, so it's 0,1,2
>>>       0 still means terminate but 1 or 2 set a flag in the structure.
>>
>> I quickly take a look of put_scsi_requests. It is transferring a QTAILQ of
>> SCSIRequest. However it goes into the structure inside to dump the
>> elements out.
>> If using my approach, I would have a VMSD for SCSIRequest. The
>> additional byte used to indicate the end of the queue would lie outside
>> the SCSCIRequest data block, so there would be no confusion.
> 
> Hmm OK; I don't think it's that easy but we'll see.
> 
It is more complicated if we want to use the exact stream as is now.
IMO VMStateInfo provides enough flexibility and is used for migrating
scsi_requests. I would stick with it if the same stream layout is to be
used.
> However, can I make one much simpler request; please split this patch
> so that the VMSTATE_LINKED and 
> vmstate_save_state/vmstate_load_state/vmfield_get_type_name
> are in one patch, while the QTAILQ patches are in a separate patch.
> (I'd be OK if you moved the VMSTATE_LINKED into the previous patch).
>

OK.

> I've just been thinking about a different use for the same mechanism;
> I want to do a:
>   VMSTATE_WITH_TMP(t1*, type1, type2, vmsd)
> 
> which also sets the LINKED, where the .get/.put allocate a temporary
> structure (of type/size type2), set up *tmp = t1 and then do the 
> vmstate_load/save
> using the vmsd on the temporary; something like (untested):
> 
> static int get_tmp(QEMUFile *f, void *pv, size_t unused_size, VMStateField 
> *field)
> {
>     const VMStateDescription *vmsd = field->vmsd;
>     size_t size = field->size;
>     int version_id = field->version_id;
>     void *tmp = gmalloc(size);
>     int ret;
>     
>     *(void **)tmp = pv;
>     ret = vmstate_load_state(f, vmsd, tmp, version_id);
>     gfree(tmp);
>     return ret;
> }
> 
> This can be in a generic macro; and we would impose that type2 must be a 
> struct
> with the first element is 'type1* parent' (compile checked).
> This would work nicely for where we have to do some maths to generate some
> temporary results prior to migration; the .pre_save of the vmsd can read the 
> data
> from pv->parent and write it to the other fields but not have to use
> qemu_get_*/qemu_put_* at all.
>

This could be a special instance of VMStateInfo.

> Dave
> 
>>
>>>    b) slirp_state_load also uses a null byte termination but not off a 
>>> QTAILQ
>>>       (although I think it could be flipped for one) (it uses '42' for the
>>>       non-0 value, but looks like it could become 1)
>>
>>>    c) virtio_blk also rolls it's own linked list but again with the 0/1 byte
>>>
>>>   Now how would I modify your QTAILQ load/store to do (a) without copying 
>>> the whole
>>> thing?
>>>
>>> Dave
>>>
>>>>
>>>> Signed-off-by: Jianjun Duan <address@hidden>
>>>> ---
>>>>  include/migration/vmstate.h | 26 ++++++++++++++++++
>>>>  include/qemu/queue.h        | 32 ++++++++++++++++++++++
>>>>  migration/trace-events      |  4 +++
>>>>  migration/vmstate.c         | 66 
>>>> +++++++++++++++++++++++++++++++++++++++++++++
>>>>  4 files changed, 128 insertions(+)
>>>>
>>>> diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
>>>> index 459dd4a..e60c994 100644
>>>> --- a/include/migration/vmstate.h
>>>> +++ b/include/migration/vmstate.h
>>>> @@ -186,6 +186,12 @@ 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.
>>>> +     * When this flag is set in VMStateField, info->get/put will
>>>> +     * be used in vmstate_load/save_state instead of recursive call.
>>>> +     * User should implement set info to handle the concerned data 
>>>> structure.
>>>> +     */
>>>> +    VMS_LINKED            = 0x8000,
>>>>  };
>>>>  
>>>>  struct VMStateField {
>>>> @@ -246,6 +252,7 @@ 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)
>>>> @@ -657,6 +664,25 @@ 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,                                          \
>>>> +    .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..12c3f80 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)
>>>> +
>>>>  #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 66802cb..192db8a 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);
>>>> @@ -121,6 +123,8 @@ 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_LINKED) {
>>>> +                    ret = field->info->get(f, addr, size, field);
>>>>                  } else {
>>>>                      ret = field->info->get(f, addr, size, NULL);
>>>>  
>>>> @@ -193,6 +197,8 @@ static const char *vmfield_get_type_name(VMStateField 
>>>> *field)
>>>>  
>>>>      if (field->flags & VMS_STRUCT) {
>>>>          type = "struct";
>>>> +    } else if (field->flags & VMS_LINKED) {
>>>> +        type = "linked";
>>>>      } else if (field->info->name) {
>>>>          type = field->info->name;
>>>>      }
>>>> @@ -327,6 +333,8 @@ 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_LINKED) {
>>>> +                    field->info->put(f, addr, size, field, vmdesc_loop);
>>>>                  } else {
>>>>                      field->info->put(f, addr, size, NULL, NULL);
>>>>                  }
>>>> @@ -939,3 +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;
>>>> +    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);
>>>
>>> Can you make those error_report's please - if it fails we want to
>>> see why in the log.
>>>
>>> Dave
>>>
>>>> +        return -EINVAL;
>>>> +    }
>>>> +    if (version_id < vmsd->minimum_version_id) {
>>>> +        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);
>>>> +    }
>>>> +
>>>> +    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
>>>>
>>> --
>>> Dr. David Alan Gilbert / address@hidden / Manchester, UK
>>>
>> Thanks,
>> Jianjun
>>
>>
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK
> 
Thanks,
Jianjun