netdev - RE: [Xen-devel] [PATCH 4/4] xen-netback: coalesce slots before copying

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Date:	Mon, 18 Mar 2013 13:09:32 +0000
From:	James Harper <james.harper@...digoit.com.au>
To:	Wei Liu <wei.liu2@...rix.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>
CC:	"annie.li@...cle.com" <annie.li@...cle.com>,
	"ian.campbell@...rix.com" <ian.campbell@...rix.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>
Subject: RE: [Xen-devel] [PATCH 4/4] xen-netback: coalesce slots before
 copying

> 
> This patch tries to coalesce tx requests when constructing grant copy
> structures. It enables netback to deal with situation when frontend's
> MAX_SKB_FRAGS is larger than backend's MAX_SKB_FRAGS.
> 
> It defines max_skb_slots, which is a estimation of the maximum number of
> slots
> a guest can send, anything bigger than that is considered malicious. Now it is
> set to 20, which should be enough to accommodate Linux (16 to 19) and
> possibly
> Windows (19?).
> 
> +/*
> + * This is an estimation of the maximum possible frags a SKB might
> + * have, anything larger than this is considered malicious. Typically
> + * Linux has 16 to 19, Windows has 19(?).
> + */

Could you remove the "Windows has 19(?)" comment? I don't think it's helpful, even with the "(?)"... I just checked and windows 2008R2 gives GPLPV a maximum of 20 buffers in all the testing I've done, and that's after the header is coalesced so it's probably more than that. I'm pretty sure I tested windows 2003 quite a while back and I could coax it into giving ridiculous numbers of buffers when using iperf with tiny buffers.

Maybe "Windows has >19" if you need to put a number on it?

James

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