Subject: Re: [PATCHv1 net] xen-netback: support frontends without feature-rx-notify again

From: Ian Campbell <[email-masked]>
Date: Mon, 5 Jan 2015 11:51:43 +0000

On Thu, 2014-12-18 at 11:13 +0000, David Vrabel wrote:
> Commit bc96f648df1bbc2729abbb84513cf4f64273a1f1 (xen-netback: make
> feature-rx-notify mandatory) incorrectly assumed that there were no
> frontends in use that did not support this feature.  But the frontend
> driver in MiniOS does not and since this is used by (qemu) stubdoms,
> these stopped working.
> 
> Netback sort of works as-is in this mode except:
> 
> - If there are no Rx requests and the internal Rx queue fills, only
>   the drain timeout will wake the thread.  The default drain timeout
>   of 10 s would give unacceptable pauses.
> 
> - If an Rx stall was detected and the internal Rx queue is drained,
>   then the Rx thread would never wake.
> 
> Handle these two cases (when feature-rx-notify is disabled) by:
> 
> - Reducing the drain timeout to 30 ms.
> 
> - Disabling Rx stall detection.
> 
> Reported-by: John <[email-masked]>
> Tested-by: John <[email-masked]>
> Signed-off-by: David Vrabel <[email-masked]>

FYI I've seen a report[0] that "Windows 2012 R2 domu with GPLPV drivers"
also suffered without feature-rx-notify support in the backend.

Ian.

[0] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=767261#103