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

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Message-Id: <20130415.133154.1066916854069237592.davem@davemloft.net>
Date:	Mon, 15 Apr 2013 13:31:54 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	David.Laight@...LAB.COM
Cc:	dborkman@...hat.com, willemb@...gle.com, paul.chavent@...ra.fr,
	richardcochran@...il.com, 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

From: "David Laight" <David.Laight@...LAB.COM>
Date: Mon, 15 Apr 2013 10:45:55 +0100

>> > +	case TPACKET_V1:
>> > +		h1 = frame;
>> > +		h1->tp_sec = ts.tv_sec;
>> > +		h1->tp_usec = ts.tv_nsec / NSEC_PER_USEC;
>> > +
>> > +		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?
> 
> If it isn't a silly question, why is the dcache being flushed
> here at all?

Anything that gets written into userspace mapped pages via the
kernel linear mapping of the pages must be D-cache flushed into
order to achieve coherency on cpus that have virtually indexed
caches.
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