Re: [Qemu-devel] [PATCH 5/9] qapi_sized_buffer

[Stefan Berger <stefanb@linux.vnet.ibm.com>]

On 03/14/2013 10:28 AM, mdroth wrote:
> On Thu, Mar 14, 2013 at 09:39:14AM -0400, Stefan Berger wrote:
>> On 03/14/2013 08:18 AM, mdroth wrote:
>>> On Wed, Mar 13, 2013 at 09:48:11PM -0400, Stefan Berger wrote:
>>>> On 03/13/2013 07:18 PM, mdroth wrote:
>>>>> On Wed, Mar 13, 2013 at 06:00:24PM -0400, Stefan Berger wrote:
>>>>>> On 03/13/2013 04:52 PM, mdroth wrote:
>>>>>>
>>>>> Visitors don't have any knowledge of the data structures they're visiting
>>>>> outside of what we tell them via the visit_*() API.
>>>>>
>>>>> [...]
>>>>>
>>>>> For example, a visitor for a 16-element array of:
>>>>>
>>>>> typedef struct ComplexType {
>>>>>      int32_t foo;
>>>>>      char *bar;
>>>>> } ComplexType;
>>>>>
>>>>> would look something like:
>>>>>
>>>>> visit_start_carray(v, ...); // instruct visitor how to calculate offsets
>>>>> for (i = 0; i < 16; i++) {
>>>>>      visit_type_ComplexType(v, ...) // instruct visitor how to handle elem
>>>>>      visit_next_carray(v, ...); // instruct visitor to move to next offset
>>>>> }
>>>>> visit_end_carray(v, ...); // instruct visitor to finalize array
>>>> Given this example above, I think we will need the sized buffer. The
>>>> sized buffer targets  binary arrays and their encoding. If I was to
>>>> encode an 'unsigned char[n]' (e.g., n=200) using n, or n/2 or n/4
>>>> loops like above breaking it apart in u8, u16 or u32 respectively I
>>>> think this would 'not bed good' also considering the 2 bytes for tag
>>>> and length being added by ASN.1 for every such datatype
>>>> (u8,u16,u32). The sized buffer allows you to for example take a
>>>> memory page and write it out in one chunk adding a few bytes of
>>>> ASN.1 'decoration' around the actual data.
>>> You could do it with this interface as well actually. The Visitor will
>>> need to maintain some internal state to differentiate what it does with
>>> subsequent visit_type*/visit_next_carray/visit_end_carry. There's no
>>> reason it couldn't also track the elem size so it could tag a buffer
>>> "en masse" when visit_end_carray() gets called.
>> It depends on what you pass into visit_start_carray. In your case if
>> you pass in ComplexType you would pass in a sizeof(ComplexType) for
>> the size of each element presumably. The problem is now you have
>> char *foo, a string pointer, hanging off of this structure. How
>> would you handle that? Serializing ComplexType's foo and pointer
>> obviously won't do it.
> Why not?  visit_type_ComplexType() knows how to deal with
> the individual fields, including the string pointer. I'm not sure
> what's at issue here.
>
> In this case the handling for ComplexType would look something like:
>
> visit_type_Complex:
>      visit_start_struct
>      visit_type_uin32 //foo
>      visit_type_str //bar
>      visit_end_struct
>
> Granted, strings are easier to deal with. If char * was instead a plain
> old uint8_t*, we'd need a nested call to start_carray for each element.
> in this case it would look something like:
>
> visit_type_Complex:
>      visit_start_struct
>      visit_type_uin32 //foo field
>      visit_start_carray //bar field
>      for (i = 0; i < len_of_bar; i++):
>          visit_type_uint8
>          visit_next_carray
>      visit_end_carray

You really want to create a separate element for each element in this 
potentially large binary array? I guess it depends on the underlying 
data, but this has the potential of generating a lot of control code 
around each such byte... As said, for ASN.1 encoding, each such byte 
would be decorated with a tag and a length value, consuming 2 more bytes 
per byte.

    Stefan