netdev - Re: [PATCH] Rate should be u64 to avoid integer overflow at high speeds (>= ~35Gbit)

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Date:	Thu, 14 Mar 2013 00:08:13 -0400
From:	Bill Fink <billfink@...dspring.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, Thomas Graf <tgraf@...g.ch>,
	Chris Friesen <chris.friesen@...band.com>,
	Vimal <j.vimal@...il.com>, netdev@...r.kernel.org,
	shemminger <shemminger@...tta.com>
Subject: Re: [PATCH] Rate should be u64 to avoid integer overflow at high
 speeds (>= ~35Gbit)

On Wed, 13 Mar 2013, Stephen Hemminger wrote:

> On Wed, 13 Mar 2013 11:29:50 -0400
> Bill Fink <billfink@...dspring.com> wrote:
> 
> > On Wed, 13 Mar 2013, Eric Dumazet wrote:
> > 
> > > On Wed, 2013-03-13 at 02:01 -0400, Bill Fink wrote:
> > > 
> > > > The last time this was discussed appears to be (on 2011-03-28):
> > > > 
> > > > 	http://marc.info/?l=linux-netdev&m=130128741907282&w=2
> > > > 
> > > > where Maciej Żenczykowski argued that creating a new 64-bit
> > > > Netlink attribute for this would be much more complex than for
> > > > the IFLA_STATS64 support.  There was no reply.
> > > > 
> > > > Providing a new multiplier/shift parameter would be a simple
> > > > way to extend support for higher rates, and would not break
> > > > existing user space that doesn't require the higher rates.
> > > > I imagine the user would not explicitly specify the multiplier/
> > > > shift parameter, but would just normally specify the desired
> > > > rate, and a newer tc would figure out what multiplier/shift
> > > > to use if a high enough rate demanded it.  To maintain user
> > > > space compatibility, the kernel should report back the same
> > > > rate and multiplier/shift it was given, and the newer tc would
> > > > convert it back to the user's originally specified rate.  Older
> > > > user space that was fine with the ~34 Gbps rate limitation would
> > > > always have the default multiplier of 1 or shift of 0 bits, and
> > > > would see the exact same unmultiplied/unshifted rate it always
> > > > did.
> > > 
> > > We already said no to such a hack. Maybe its not clear enough ?
> > > 
> > > netlink allows us to a proper way, and Thomas Graf explained how we
> > > expect the thing to be done.
> > > 
> > > Yes, this is not a one liner patch, its a bit more of work, and its how
> > > it will be done when someone does the job.
> > 
> > I've no problem with that since it is a cleaner solution, but
> > one that requires significantly more work.  I was only arguing
> > that the multiplier/shift approach was also a workable solution
> > and should be simpler to implement.  But since there appears to
> > be developer consensus that it's not a desired method, I'm fine
> > with going along with that expert opinion.
> > 
> > 						-Bill
> 
> As others have said the multiplier shift approach is a not a workable
> solution because it is likely to cause too many compatibility surprises.
> Older kernels would ignore the multiplier and therefore not give the users
> the effective rate they wanted.

Hopefully they would get an error saying that the rate was not
supported by the running kernel, from a failure of trying to
set the multiplier/shift.  You can't get new features from an
old kernel.  But anything working today should still work since
if your rate is less than or equal to ~34 Gbps, your multiplier
would be 1 (or shift of 0 bits), and thus the effective rate is
unmodified from what the user specified (and thus no need to even
use the new interface).

Today if you specify a rate greater than ~34 Gbps, you don't get
what you expected, since from what I understand the value just
gets silently truncated so 40 Gbps results in 5.64 Gbps.  See:

	http://marc.info/?l=linux-netdev&m=130103727012841&w=2

So I don't think anything would get broken that isn't already
broken (or just currently impossible to do).

But I've already said that I'm fine with a more proper solution.
I just hope someone will implement it before another 2 years
passes, as I suspect 100G NICs will become available in that
timeframe.

					-Bill
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