On Thu, Jul 31, 2014 at 01:25:20PM -0700, David Miller wrote: > From: Zoltan Kiss <zoltan.k...@citrix.com> > Date: Wed, 30 Jul 2014 14:25:30 +0100 [...] > Secondly, for something like UDP you can't just split the packet up > like this, or for any other datagram protocol for that matter. > > I know you're in a difficult situation, but I just can't see this > being an acceptable approach to solving the problem right now. > > Where does the MAX_SKB_FRAGS + 1 limit really come from, the size of > the TX queue? >
It stems from the implicit transimit protocol since inception of netfront / netback. Sigh. > If you were to have a 64-slot TX queue, you ought to be able to handle > this theoretical 51 slot SKB. > There's two problems: 1. IIRC a single page ring has 256 slots, allowing 64 slots packet yields 4 in-flight packets in worst case. 2. Older netback could not handle this large number of slots and it's likely to deem the frontend malicious. For #1, we don't actually care that much if guest screws itself by generating 64 slot packets. #2 is more concerning. Wei. > And I don't think it's so theoretical, a carefully crafted sequence of > sendfile() calls during a TCP_CORK sequence should be able to do it. > -- > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
