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

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B7237@saturn3.aculab.com>
Date:	Tue, 14 May 2013 16:20:03 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Michael Chan" <mchan@...adcom.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.

It can't be very far away, approx 1 in 65k checks would fail.
You could do the finer test afterwards.

> The new skb that we allocate to workaround the condition may be
> even closer to 4G and may hit the real bug condition.

If the 'fix' is to relocate the skb you are doomed to lose regardless
of the check - unless you are willing to reallocate a lot of times,
and without freeing the old skb.
I'd assumed the 'fix' was to disable the relevant offload.

> 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.

I was looking at the number of branches in the hot path, not whether
the values were already in registers.

	David



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