Re: [Xen-devel] [PATCH net v4] xen-netback: Fix grant ref resolution in RX path

[Sander Eikelenboom <linux@xxxxxxxxxxxxxx>]

Thursday, May 15, 2014, 12:37:49 PM, you wrote:

> On Thu, 2014-05-15 at 11:08 +0100, Zoltan Kiss wrote:
>> The original series for reintroducing grant mapping for netback had a patch 
>> [1]
>> to handle receiving of packets from an another VIF. Grant copy on the 
>> receiving
>> side needs the grant ref of the page to set up the op.
>> The original patch assumed (wrongly) that the frags array haven't changed. In
>> the case reported by Sander, the sending guest sent a packet where the linear
>> buffer and the first frag were under PKT_PROT_LEN (=128) bytes.
>> xenvif_tx_submit() then pulled up the linear area to 128 bytes, and ditched 
>> the
>> first frag. The receiving side had an off-by-one problem when gathered the 
>> grant
>> refs.
>> This patch fixes that by checking whether the actual frag's page pointer is 
>> the
>> same as the page in the original frag list. It can handle any kind of 
>> changes on
>> the original frags array, like:
>> - removing granted frags from the array at any point
>> - adding local pages to the frags list anywhere
>> - reordering the frags
>> It's optimized to the most common case, when there is 1:1 relation between 
>> the
>> frags and the list, plus works optimal when frags are removed from the end or
>> the beginning.
>> 
>> [1]: 3e2234: xen-netback: Handle foreign mapped pages on the guest RX path
>> 
>> Reported-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
>> Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>

> Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

> (Remind me, do we need both this and Eric's fix, or is it either/or?)

The regression i reported goes away with Zoltan's fix alone (I tested Zoltan's 
fix v3, without Eric's patch, pulled onto 3.15-rc5).

In earlier tests Eric's patch didn't make a difference in my testcase.

So Zoltan's patch fixes the reported regression.
Eric's patch is a additional correctness fix, independent of the reported 
regression.

--
Sander




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