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

[mdroth]

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
    visit_end_struct

The key is knowing the length. In open coded visitor routines we know
this, or where to get it, for routines generated from QAPI schemas
we'd a way to tell the code generators how to field the size, or state
the size in the schema directly. I had some patches to do this, but we
don't have a QAPI user that needs this yet. When we do,
visit_*_carray() should be able to handle it, so we should consolidate
around that interface since there are a lot of things to consider in
the scope of what a visitor implementation may be used for.

> would you handle that? Serializing ComplexType's foo and pointer
> obviously won't do it. You need to follow the string pointer and
> serialize that as well. So we have different use cases here when
> wanting to serialize ComplexType versus a plain array with the
> carray calls somehow having to figure it out themselves -- how ??

for a plain array we'd just replace visit_type_ComplexType() with
visit_type_uint{8,16,32,64} and change loop/elem_size params
accordingly.

> 
>    Stefan
>