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

[mdroth]

On Thu, Mar 14, 2013 at 10:51:49AM -0400, Stefan Berger wrote:
> 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.

I addressed this earlier. Your visitor doesn't have tag each
element: if it know it's handling an array (because we told it via
start_carray()), it can buffer them internally and tag the array en
masse when end_carray() is issued.

> 
>    Stefan
>