Re: [PATCH] Add an ioctl interface for simple xenstore access

[Anthony Liguori <aliguori@us.ibm.com>]

Keir Fraser wrote:
>> Hmm, wouldn't a virtual file system be a much better fit for xenstore
>> access? the kernel community considers ioctls Evil with a capital E.
>
> Also, we already support xenstore access via read/write on that device 
> file. And libxenstore knows how to target that read/write interface. 
> Why would we add an ioctl to do the same thing?
Hi Keir,

We had discussed this in a previous thread and I thought you were okay 
with this approach.  The /proc/xen/xenbus interface requires full 
parsing of the XenBus protocol.  This requires libxenstore in domU.  
Today, libxenstore is shipped with the rest of the tools package on most 
distros.  If one wants to write a Xen-aware application for a domU, that 
brings in a pretty large number of unnecessary dependencies.  Also, for 
32 bit management apps in a 64 bit environment, it makes things very 
nasty (should we build a 32 bit and 64 bit version of libxenstore?  Is 
the protocol 32/64 bit safe?).

Ideally, an app could just use a simple interface to /proc/xen/xenbus to 
access XenStore.  That solves all of the above problems elegantly.  An 
ioctl() interface seemed like the most obvious approach that wouldn't 
break existing apps.  Of course, any suggestion for a better interface 
would be appreciated.

The general problem of xenstore access in domU is a big issue for us 
(and I assume it will be for most people building Xen management 
infrastructure).  It would really help to have a good solution for 3.0.2.

Regards,

Anthony Liguori

>  -- Keir
>
>
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