netdev - Re: [PATCH] net-packet: tx timestamping on tpacket ring

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Message-ID: <CA+FuTScTDYreuynVhFLnPb12YgMPJ2oW=6F+esDTtTRp3RNB_g@mail.gmail.com>
Date:	Sat, 13 Apr 2013 20:49:56 -0400
From:	Willem de Bruijn <willemb@...gle.com>
To:	David Miller <davem@...emloft.net>
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	Paul Chavent <paul.chavent@...ra.fr>,
	Richard Cochran <richardcochran@...il.com>,
	Eric Dumazet <edumazet@...gle.com>,
	daniel.borkmann@....ee.ethz.ch, xemul@...allels.com,
	ebiederm@...ssion.com, netdev@...r.kernel.org
Subject: Re: [PATCH] net-packet: tx timestamping on tpacket ring

On Sat, Apr 13, 2013 at 8:16 PM, Willem de Bruijn <willemb@...gle.com> wrote:
> On Sat, Apr 13, 2013 at 8:04 PM, Willem de Bruijn <willemb@...gle.com> wrote:
>> On Sat, Apr 13, 2013 at 6:47 PM, David Miller <davem@...emloft.net> wrote:
>>> From: Daniel Borkmann <dborkman@...hat.com>
>>> Date: Sun, 14 Apr 2013 00:18:48 +0200
>>>
>>>>> +            flush_dcache_page(pgv_to_page(&h1->tp_sec));
>>>>> +            flush_dcache_page(pgv_to_page(&h1->tp_usec));
>>>>
>>>> Hmm, not sure, but could we also flush the dcache only once?
>>>
>>> Indeed, I truly hope that headers never straddle pages.
>>
>> I should have checked the alignment restrictions on frames. Frames
>> must be a multiple of 16 B as well as larger than the header (obviously),
>> so this can indeed never happen.
>
> Actually, 48 B is a multiple of 16, so should be accepted, and 85
> frames on a page leaves half a frame for the next. I'll check whether
> this is right. Even if so, it would still not matter for these time
> offsets, as they start at 16 or 20 B offset from the start of the
> frame.

48 B is too small (Because less than TPACKET_HDRLEN), but it can
be triggered with 80 B frames. Daniel, thanks for submitting the
selftest: both the timestamp and this alignment question were now very
easy to test by just changing a few lines in your code.
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