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

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Message-ID: <530B606F.2070902@citrix.com>
Date:	Mon, 24 Feb 2014 15:08:31 +0000
From:	Zoltan Kiss <zoltan.kiss@...rix.com>
To:	Zoltan Kiss <zoltan.kiss@...aman.hu>,
	Ian Campbell <Ian.Campbell@...rix.com>
CC:	<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 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?

> 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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