From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:56152) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UG9sN-0006cu-5A for qemu-devel@nongnu.org; Thu, 14 Mar 2013 11:15:36 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UG9sL-0007gs-Gk for qemu-devel@nongnu.org; Thu, 14 Mar 2013 11:15:27 -0400 Received: from mail-ye0-f170.google.com ([209.85.213.170]:42697) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UG9sL-0007gh-Bd for qemu-devel@nongnu.org; Thu, 14 Mar 2013 11:15:25 -0400 Received: by mail-ye0-f170.google.com with SMTP id m15so404371yen.29 for ; Thu, 14 Mar 2013 08:15:25 -0700 (PDT) Sender: fluxion Date: Thu, 14 Mar 2013 10:11:21 -0500 From: mdroth Message-ID: <20130314151121.GB9093@vm> References: <1363200988-17865-1-git-send-email-jschopp@linux.vnet.ibm.com> <1363200988-17865-6-git-send-email-jschopp@linux.vnet.ibm.com> <20130313205200.GA6188@vm> <5140F6F8.3080101@linux.vnet.ibm.com> <20130313231836.GA9093@vm> <51412C5B.1000003@linux.vnet.ibm.com> <20130314121849.GJ4005@vm> <5141D302.3050607@linux.vnet.ibm.com> <20130314142856.GM4005@vm> <5141E405.1070701@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5141E405.1070701@linux.vnet.ibm.com> Subject: Re: [Qemu-devel] [PATCH 5/9] qapi_sized_buffer List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Berger Cc: Joel Schopp , qemu-devel@nongnu.org, Michael Tsirkin 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 >