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Re: [Qemu-devel] [PATCH v9 10/37] qapi: Make all visitors supply uint64


From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH v9 10/37] qapi: Make all visitors supply uint64 callbacks
Date: Wed, 20 Jan 2016 18:29:00 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Eric Blake <address@hidden> writes:

> Our qapi visitor contract supports multiple integer visitors,
> but left the type_uint64 visitor as optional (falling back on
> type_int64); it also marks the type_size visitor as optional
> (falling back on type_uint64 or even type_int64).
>
> Note that the default of falling back on type_int for unsigned
> visitors can cause confusing results for values larger than
> INT64_MAX (such as having to pass in a negative 2s complement
> value on input, and getting a negative result on output).
>
> This patch does not fully address the disparity in handling
> large values as negatives, but does move towards a cleaner
> situation where EVERY visitor provides both type_int64 and
> type_uint64 variants as entry points; then each client can
> either implement sane differences between the two, or document
> in place with a FIXME that there is munging going on.

Before: nobody implements type_uint64(), and the core falls back to
type_int64(), casting negative values to large positive ones.  With an
implementation of type_int64() that parses large positive values as
negative, the two casts cancel out.

After: everybody implements type_uint64() incorrectly, by reusing
type_int64() code.  The problem moves from visitor core to visitor
implementations, where we can actually fix it if we care.

Correct?

> The dealloc visitor no longer needs a type_size callback,
> since that now safely falls back to the type_uint64 callback.

Did it need the callback before this patch?

> Then, in qapi-visit-core.c, we can now use the guaranteed
> type_uint64 callback as the fallback for all smaller unsigned
> int visits.

The type_int64() callback works just fine for smaller unsigned ints, but
I agree avoiding mixed signedness by using type_uint64() make sense once
type_uint64() is mandatory.

> Signed-off-by: Eric Blake <address@hidden>
>
> ---
> v9: hoist in part of 11/35, drop Marc-Andre's R-b
> v8: no change
> v7: split off int64 callbacks and retitle, add more FIXMEs in the
> code, hoist use of type_uint64 here from 3/23, improved commit
> message
> v6: new patch, but stems from v5 23/46
> ---
>  include/qapi/visitor-impl.h  |  9 ++++++---
>  qapi/qapi-dealloc-visitor.c  | 12 ++++++------
>  qapi/qapi-visit-core.c       | 42 ++++++++++++++----------------------------
>  qapi/qmp-input-visitor.c     | 17 +++++++++++++++++
>  qapi/qmp-output-visitor.c    |  9 +++++++++
>  qapi/string-input-visitor.c  | 15 +++++++++++++++
>  qapi/string-output-visitor.c |  9 +++++++++
>  7 files changed, 76 insertions(+), 37 deletions(-)
>
> diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h
> index c263a26..45c1d3e 100644
> --- a/include/qapi/visitor-impl.h
> +++ b/include/qapi/visitor-impl.h
> @@ -40,6 +40,12 @@ struct Visitor
>      void (*type_int64)(Visitor *v, int64_t *obj, const char *name,
>                         Error **errp);
>      /* Must be set. */
> +    void (*type_uint64)(Visitor *v, uint64_t *obj, const char *name,
> +                        Error **errp);
> +    /* Optional; fallback is type_uint64().  */
> +    void (*type_size)(Visitor *v, uint64_t *obj, const char *name,
> +                      Error **errp);
> +    /* Must be set. */
>      void (*type_bool)(Visitor *v, bool *obj, const char *name, Error **errp);
>      void (*type_str)(Visitor *v, char **obj, const char *name, Error **errp);
>      void (*type_number)(Visitor *v, double *obj, const char *name,
> @@ -53,12 +59,9 @@ struct Visitor
>      void (*type_uint8)(Visitor *v, uint8_t *obj, const char *name, Error 
> **errp);
>      void (*type_uint16)(Visitor *v, uint16_t *obj, const char *name, Error 
> **errp);
>      void (*type_uint32)(Visitor *v, uint32_t *obj, const char *name, Error 
> **errp);
> -    void (*type_uint64)(Visitor *v, uint64_t *obj, const char *name, Error 
> **errp);
>      void (*type_int8)(Visitor *v, int8_t *obj, const char *name, Error 
> **errp);
>      void (*type_int16)(Visitor *v, int16_t *obj, const char *name, Error 
> **errp);
>      void (*type_int32)(Visitor *v, int32_t *obj, const char *name, Error 
> **errp);
> -    /* visit_type_size() falls back to (*type_uint64)() if type_size is 
> unset */
> -    void (*type_size)(Visitor *v, uint64_t *obj, const char *name, Error 
> **errp);
>      bool (*start_union)(Visitor *v, bool data_present, Error **errp);
>      void (*end_union)(Visitor *v, bool data_present, Error **errp);
>  };
> diff --git a/qapi/qapi-dealloc-visitor.c b/qapi/qapi-dealloc-visitor.c
> index e9b9f3f..11eb828 100644
> --- a/qapi/qapi-dealloc-visitor.c
> +++ b/qapi/qapi-dealloc-visitor.c
> @@ -140,6 +140,11 @@ static void qapi_dealloc_type_int64(Visitor *v, int64_t 
> *obj, const char *name,
>  {
>  }
>
> +static void qapi_dealloc_type_uint64(Visitor *v, uint64_t *obj,
> +                                     const char *name, Error **errp)
> +{
> +}
> +
>  static void qapi_dealloc_type_bool(Visitor *v, bool *obj, const char *name,
>                                     Error **errp)
>  {
> @@ -158,11 +163,6 @@ static void qapi_dealloc_type_anything(Visitor *v, 
> QObject **obj,
>      }
>  }
>
> -static void qapi_dealloc_type_size(Visitor *v, uint64_t *obj, const char 
> *name,
> -                                   Error **errp)
> -{
> -}
> -
>  static void qapi_dealloc_type_enum(Visitor *v, int *obj,
>                                     const char * const strings[],
>                                     const char *kind, const char *name,
> @@ -220,11 +220,11 @@ QapiDeallocVisitor *qapi_dealloc_visitor_new(void)
>      v->visitor.end_list = qapi_dealloc_end_list;
>      v->visitor.type_enum = qapi_dealloc_type_enum;
>      v->visitor.type_int64 = qapi_dealloc_type_int64;
> +    v->visitor.type_uint64 = qapi_dealloc_type_uint64;
>      v->visitor.type_bool = qapi_dealloc_type_bool;
>      v->visitor.type_str = qapi_dealloc_type_str;
>      v->visitor.type_number = qapi_dealloc_type_number;
>      v->visitor.type_any = qapi_dealloc_type_anything;
> -    v->visitor.type_size = qapi_dealloc_type_size;
>      v->visitor.start_union = qapi_dealloc_start_union;
>
>      QTAILQ_INIT(&v->stack);
> diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
> index 6295fa8..4a8ad43 100644
> --- a/qapi/qapi-visit-core.c
> +++ b/qapi/qapi-visit-core.c
> @@ -102,15 +102,15 @@ void visit_type_int(Visitor *v, int64_t *obj, const 
> char *name, Error **errp)
>
>  void visit_type_uint8(Visitor *v, uint8_t *obj, const char *name, Error 
> **errp)
>  {
> -    int64_t value;
> +    uint64_t value;
>
>      if (v->type_uint8) {
>          v->type_uint8(v, obj, name, errp);
>      } else {
>          value = *obj;
> -        v->type_int64(v, &value, name, errp);
> -        if (value < 0 || value > UINT8_MAX) {
> -            /* FIXME questionable reuse of errp if type_int64() changes
> +        v->type_uint64(v, &value, name, errp);
> +        if (value > UINT8_MAX) {
> +            /* FIXME questionable reuse of errp if type_uint64() changes
>                 value on error */
>              error_setg(errp, QERR_INVALID_PARAMETER_VALUE,
>                         name ? name : "null", "uint8_t");

You could delay adding these FIXMEs until this patch, and reduce churn.
Probably not worth the bother now.

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