netdev - Re: [PATCH net-next v5 4/9] xen-netback: Change RX path for mapped SKB fragments

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Message-ID: <20140227124327.GD16241@zion.uk.xensource.com>
Date:	Thu, 27 Feb 2014 12:43:27 +0000
From:	Wei Liu <wei.liu2@...rix.com>
To:	Zoltan Kiss <zoltan.kiss@...rix.com>
CC:	Zoltan Kiss <zoltan.kiss@...aman.hu>,
	Ian Campbell <Ian.Campbell@...rix.com>, <wei.liu2@...rix.com>,
	<xen-devel@...ts.xenproject.org>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <jonathan.davies@...rix.com>
Subject: Re: [PATCH net-next v5 4/9] xen-netback: Change RX path for mapped
 SKB fragments

On Mon, Feb 24, 2014 at 03:08:31PM +0000, Zoltan Kiss wrote:
> On 24/02/14 13:49, Zoltan Kiss wrote:
> >On 22/02/14 23:18, Zoltan Kiss wrote:
> >>On 18/02/14 17:45, Ian Campbell wrote:
> >>>On Mon, 2014-01-20 at 21:24 +0000, Zoltan Kiss wrote:
> >>>
> >>>Re the Subject: change how? Perhaps "handle foreign mapped pages on the
> >>>guest RX path" would be clearer.
> >>Ok, I'll do that.
> >>
> >>>
> >>>>RX path need to know if the SKB fragments are stored on
> >>>>pages from another
> >>>>domain.
> >>>Does this not need to be done either before the mapping change
> >>>or at the
> >>>same time? -- otherwise you have a window of a couple of commits where
> >>>things are broken, breaking bisectability.
> >>I can move this to the beginning, to keep bisectability. I've
> >>put it here originally because none of these makes sense without
> >>the previous patches.
> >Well, I gave it a close look: to move this to the beginning as a
> >separate patch I would need to put move a lot of definitions from
> >the first patch to here (ubuf_to_vif helper,
> >xenvif_zerocopy_callback etc.). That would be the best from bisect
> >point of view, but from patch review point of view even worse than
> >now. So the only option I see is to merge this with the first 2
> >patches, so it will be even bigger.
> Actually I was stupid, we can move this patch earlier and introduce
> stubs for those 2 functions. But for the another two patches (#6 and
> #8) it's still true that we can't move them before, only merge them
> into the main, as they heavily rely on the main patch. #6 is
> necessary for Windows frontends, as they are keen to send too many
> slots. #8 is quite a rare case, happens only if a guest wedge or
> malicious, and sits on the packet.
> So my question is still up: do you prefer perfect bisectability or
> more segmented patches which are not that pain to review?
> 

What's the diff stat if you merge those patches?

> >And based on that principle, patch #6 and #8 should be merged
> >there as well, as they solve corner cases introduced by the grant
> >mapping.
> >I don't know how much the bisecting requirements are written in
> >stone. At this moment, all the separate patches compile, but after
> >#2 there are new problems solved in #4, #6 and #8. If someone
> >bisect in the middle of this range and run into these problems,
> >they could quite easily figure out what went wrong looking at the
> >adjacent patches. So I would recommend to keep this current order.
> >What's your opinion?
> >
> >Zoli
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