netdev - Re: [Xen-devel] [PATCH] xen-netfront: correct MAX_TX

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Date:	Fri, 03 Feb 2012 13:27:56 +0100
From:	Laszlo Ersek <lersek@...hat.com>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"jeremy@...p.org" <jeremy@...p.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Ian Campbell <Ian.Campbell@...rix.com>
Subject: Re: [Xen-devel] [PATCH] xen-netfront: correct MAX_TX_TARGET	calculation.

On 01/27/12 11:36, Wei Liu wrote:
> On Thu, 2012-01-26 at 18:19 +0000, Konrad Rzeszutek Wilk wrote:
>> On Thu, Jan 26, 2012 at 05:23:23PM +0000, Wei Liu wrote:
>>
>> Can you give some more details please? What is the impact of
>> not having this? Should it be backported to stable?
>>
>
> I think it will not cause crash, only the scratch space size is
> affected, thus impacting tx batching a bit.
>
> As the tx structure is bigger than rx structure. I think scratch space
> size is likely to shrink after correction.

It also seems to affect the netfront_tx_slot_available() function, 
making it stricter (likely). Before the patch, the function may have 
reported available slots when there were none, causing spurious(?) queue 
wakeups in xennet_maybe_wake_tx(), and not stopping the queue in 
xennet_start_xmit() when it should have(?).

It seems there are no further uses of TX_MAX_TARGET, and for bounds 
checking NET_TX_RING_SIZE was used (which was always correct). So I 
guess the typo may have caused some performance degradation.

I can't either prove or disprove a DoS-like busy loop in the pre-patch form.

Laszlo
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