netdev - RE: [PATCH v2 net 2/2] tg3: Fix data corruption on 5725 with TSO

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Date:	Tue, 14 May 2013 08:04:29 -0700
From:	"Michael Chan" <mchan@...adcom.com>
To:	"David Laight" <David.Laight@...LAB.COM>
cc:	"Nithin Nayak Sujir" <nsujir@...adcom.com>,
	"Eric Dumazet" <eric.dumazet@...il.com>, davem@...emloft.net,
	netdev@...r.kernel.org, stable@...r.kernel.org
Subject: RE: [PATCH v2 net 2/2] tg3: Fix data corruption on 5725 with
 TSO

On Tue, 2013-05-14 at 09:40 +0100, David Laight wrote:
> > >>>> +        if (tg3_asic_rev(tp) == ASIC_REV_5762 && mss) {
> > >>>> +                u32 base = (u32) mapping & 0xffffffff;
> > >>>> +
> > >>>> +                return ((base + len + (mss & 0x3fff)) < base);
> ... 
> > For the bug to occur, the fragment does not have to span a 4G boundary. If it is
> > within MSS bytes (9.6k) of a 4G boundary, it triggers the failure.
> 
> Would it be worth simplifying the test to assume that 'len'
> is 64k and 'mss' 9.6k?
> (commenting on the actual condition.)
> The number of false positives would be small, but the test
> a lot quicker.
> The '(u32)mapping + (0x10000 + 9600) < (u32)mapping' test might
> even be faster than the ' tg3_asic_rev(tp) == ASIC_REV_5762' one. 

I think that if we do this and detect a false positive, it may be very
far from the 4G boundary.  The new skb that we allocate to workaround
the condition may be even closer to 4G and may hit the real bug
condition.

The mss and len values are accessed many times in this immediate code
path just before setting the TX BD, gcc should be able to optimize this
quite nicely.


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