netdev - Re: [PATCH] Packet mmap : allow the user to choose the offset of the tx payload.

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Date:	Sat, 06 Oct 2012 09:43:24 +0200
From:	pchavent <Paul.Chavent@...ra.fr>
To:	Daniel Borkmann <danborkmann@...earbox.net>
Cc:	<davem@...emloft.net>, <edumazet@...gle.com>,
	<xemul@...allels.com>, <herbert@...dor.hengli.com.au>,
	<netdev@...r.kernel.org>, <johann.baudy@...-log.net>,
	<uaca@...mni.uv.es>
Subject: Re: [PATCH] Packet mmap : allow the user to choose the offset of
 the tx payload.

On Fri, 5 Oct 2012 21:37:58 +0200, Daniel Borkmann wrote:
> On Fri, Oct 5, 2012 at 9:21 PM, pchavent <Paul.Chavent@...ra.fr> 
> wrote:
>> On Fri, 5 Oct 2012 16:17:12 +0200, Daniel Borkmann wrote:
>>> On Fri, Oct 5, 2012 at 3:10 PM, Paul Chavent 
>>> <Paul.Chavent@...ra.fr>
>>> wrote:
>>>>
>>>> The tx offset of packet mmap tx ring used to be :
>>>> (TPACKET2_HDRLEN - sizeof(struct sockaddr_ll))
>>>>
>>>> The problem is that depending on the usage of SOCK_DGRAM or
>>>> SOCK_RAW, the payload could be aligned or not.
>>>>
>>>> This patch allow to let the user give an offset for it's tx
>>>> payload if he desires.
>>>>
>>>> Signed-off-by: Paul Chavent <paul.chavent@...ra.fr>
>>>
>>> Can you provide an example when it doesn't hit TPACKET_ALIGNMENT?
>>
>> When we use tx ring, the user have to write at (TPACKET_HDRLEN -
>> sizeof(struct sockaddr_ll))
>>
>> This adress is aligned on TPACKET_ALIGNMENT since
>> TPACKET_HDRLEN = (TPACKET_ALIGN(sizeof(struct tpacket_hdr)) + 
>> sizeof(struct
>> sockaddr_ll))
>>
>> When we use the tx ring with SOCK_RAW option, the mac header is 
>> aligned on
>> TPACKET_ALIGNMENT, but not the payload (14 bytes away).
>
> Okay, I'm confused about your intentions, maybe I'm missing 
> something.
> The man-page of packet(7) clearly says:
>
> The socket_type is either SOCK_RAW for raw packets *including* the
> link level header or SOCK_DGRAM for cooked packets with  the link
> level header *removed*.
>
> So this is perfectly intended behavior of PF_PACKET.
>
> Cheers,
>
> Daniel

Yes, i also expect to be able to include the link level header when i 
use SOCK_RAW.

My intention is to send a frame with this payload (for example) :
typedef struct
{
   double   ts;
   uint64_t foo;
} test_t;

So i get a pointer to the raw packet :
void * raw_packet = frame_base + (TPACKET_HDRLEN - sizeof(struct 
sockaddr_ll));

I cook the header :
memcpy(raw_packet +  0, dst_addr, sizeof(dst_addr));
memcpy(raw_packet +  6, src_addr, sizeof(src_addr));
memcpy(raw_packet + 12, type    , sizeof(type));

Then i get a pointer to the beginning of payload :
test_t * payload = raw_packet + 14;

Here payload is at 58 bytes from the beginning of the frame.

Then i fill the payload :
payload->ts = 1.0;
payload->foo = 2;
...

These are misaligned accesses.

I don't care to fill the cooked header if it's misaligned, but i would 
like to be able to fill the frame directly in the ring buffer being on 
aligned boundaries.
It would allow to avoid to memcpy an instance of the payload 
structure...

Regards.

Paul.

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