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

[Paolo Bonzini <pbonzini@redhat.com>]

----- 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.

> +        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.

Otherwise looks great now!

Paolo

> +    }
> +
> +    QTAILQ_RAW_FOREACH(elm, pv, entry) {
> +        vmstate_info_bool.put(f, &link, sizeof(bool), NULL, NULL);
> +        vmstate_save_state(f, vmsd, elm, vmdesc);
> +    }
> +    link = false;
> +    vmstate_info_bool.put(f, &link, sizeof(bool), NULL, NULL);
> +    if (vmdesc) {
> +        json_end_array(vmdesc);
> +    }
> +}
> +const VMStateInfo vmstate_info_qtailq = {
> +    .name = "qtailq",
> +    .get  = get_qtailq,
> +    .put  = put_qtailq,
> +};
> --
> 1.9.1
> 
>