Re: [Qemu-devel] [PATCH] qapi: pad GenericList value fields to 64 bits

[Luiz Capitulino]

On Sun, 26 May 2013 22:20:58 -0500
Michael Roth <address@hidden> wrote:

> With the introduction of native list types, we now have types such as
> int64List where the 'value' field is not a pointer, but the actual
> 64-bit value.
> 
> On 32-bit architectures, this can lead to situations where 'next' field
> offset in GenericList does not correspond to the 'next' field in the
> types that we cast to GenericList when using the visit_next_list()
> interface, causing issues when we attempt to traverse linked list
> structures of these types.
> 
> To fix this, pad the 'value' field of GenericList and other
> schema-defined/native *List types out to 64-bits.
> 
> This is less memory-efficient for 32-bit architectures, but allows us to
> continue to rely on list-handling interfaces that target GenericList to
> simply visitor implementations.
> 
> In the future we can improve efficiency by defaulting to using native C
> array backends to handle list of non-pointer types, which would be more
> memory efficient in itself and allow us to roll back this change.

I'm also concerned with the small complexity this change is adding.
How hard would it be to do the proper solution with arrays instead?

> 
> Signed-off-by: Michael Roth <address@hidden>
> ---
>  include/qapi/visitor.h          |    5 ++++-
>  scripts/qapi-types.py           |   10 ++++++++--
>  tests/test-qmp-output-visitor.c |    5 ++++-
>  3 files changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
> index 1fef18c..28c21d8 100644
> --- a/include/qapi/visitor.h
> +++ b/include/qapi/visitor.h
> @@ -18,7 +18,10 @@
>  
>  typedef struct GenericList
>  {
> -    void *value;
> +    union {
> +        void *value;
> +        uint64_t padding;
> +    };
>      struct GenericList *next;
>  } GenericList;
>  
> diff --git a/scripts/qapi-types.py b/scripts/qapi-types.py
> index fd42d71..ddcfed9 100644
> --- a/scripts/qapi-types.py
> +++ b/scripts/qapi-types.py
> @@ -22,7 +22,10 @@ def generate_fwd_struct(name, members, builtin_type=False):
>  
>  typedef struct %(name)sList
>  {
> -    %(type)s value;
> +    union {
> +        %(type)s value;
> +        uint64_t padding;
> +    };
>      struct %(name)sList *next;
>  } %(name)sList;
>  ''',
> @@ -35,7 +38,10 @@ typedef struct %(name)s %(name)s;
>  
>  typedef struct %(name)sList
>  {
> -    %(name)s *value;
> +    union {
> +        %(name)s *value;
> +        uint64_t padding;
> +    };
>      struct %(name)sList *next;
>  } %(name)sList;
>  ''',
> diff --git a/tests/test-qmp-output-visitor.c b/tests/test-qmp-output-visitor.c
> index 0942a41..b2fa9a7 100644
> --- a/tests/test-qmp-output-visitor.c
> +++ b/tests/test-qmp-output-visitor.c
> @@ -295,7 +295,10 @@ static void 
> test_visitor_out_struct_errors(TestOutputVisitorData *data,
>  
>  typedef struct TestStructList
>  {
> -    TestStruct *value;
> +    union {
> +        TestStruct *value;
> +        uint64_t padding;
> +    };
>      struct TestStructList *next;
>  } TestStructList;
>