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Date: Wed, 22 Apr 2009 11:45:29 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] [RfC / Patch] xenner: event channel implementation.
References: <49EF1EE6.5080900@redhat.com>
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To: Gerd Hoffmann <kraxel@redhat.com>
Cc: Xen Development Mailing List <xen-devel@lists.xensource.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

Gerd Hoffmann wrote:
>   Hi,
>
> Merging the xen bits seems to be on a good way.  Time to look at 
> un-bitrotting the xenner bits ...
>
> Here is a first patch for comments.  Not useful on its own.  Right now 
> I'm looking more for comments on the way the integration is done.
>
> Event channels on Xen are managed by calling the xc_evtchn_* functions 
> provided by libxenctrl.  The library in turn does does hypercalls into 
> the xen kernel.  xenner obviously has to provide an alternative 
> implementation for these functions.  Also for others.  This patch 
> starts with just the event channels though.

With xenner, do you still use xenstore/xenconsoled?  If so, don't you 
need some LD_PRELOAD magic to make that work?

Regards,

Anthony Liguori

> It works this way:  There is a struct with function pointers to the 
> event channel functions.  The struct can be switched at runtime to the 
> xen or xenner version of the functions depending on the qemu operation 
> mode.
>
> The struct is named "xc_evtchn", the function pointer are named like 
> the xc_evtchn_* functions, but without the xc_evtchn_ prefix, i.e. 
> "xc_evtchn_open(...)" becomes xc_evtchn.open(...).
>
> The function calls in the source code (xen backend drivers) are not 
> changed directly, but using a include file with a bunch of #defines. 
> That way I don't have to change s/xc_evtchn_/xc_evtchn./ all over the 
> place.  Also xenner can easily be disabled at compile time and the 
> indirect function pointer calls simply go away then.
>
> Comments?
>
> cheers,
>   Gerd