Re: [Xen-devel] Problem Reading from XenStore in DomU

[Doug Goldstein <cardoe@xxxxxxxxxx>]

On 5/15/16 8:28 PM, Dagaen Golomb wrote:
>> On 5/15/16 11:40 AM, Dagaen Golomb wrote:
>>> Hi All,
>>>
>>> I'm having an interesting issue. I am working on a project that
>>> requires me to share memory between dom0 and domUs. I have this
>>> successfully working using the grant table and the XenStore to
>>> communicate grefs.
>>>
>>> My issue is this. I have one domU running Ubuntu 12.04 with a default
>>> 3.8.x kernel that has no issue reading or writing from the XenStore.
>>> My work also requires some kernel modifications, and we have made
>>> these changes in the 4.1.0 kernel. In particular, we've only added a
>>> simple hypercall. This modified kernel is what dom0 is running, on top
>>> of Xen 4.7 rc1.
>>
>> Without reading the rest of the thread but seeing the kernel versions.
>> Can you check how you're communicating to xenstore? Is it via
>> /dev/xen/xenbus or /proc/xen/xenbus? Anything after 3.14 will give you
>> deadlocks if you try to use /proc/xen/xenbus. Xen 4.6 and newer should
>> prefer /dev/xen/xenbus. Same thing can happen with privcmd but making
>> that default didn't land until Xen 4.7. Since you're on the right
>> versions I expect you're using /dev/xen/xenbus but you never know.
> 
> How do I know which is being used? /dev/xen/xenbus is there and so is
> process/xen/xenbus. Could this be a problem with header version
> mismatches or something similar? I'm using the xen/xenstore.h header
> file for all of my xenstore interactions. I'm running Xen 4.7 so it
> should be in /dev/, and the old kernel is before 3.14 but the new one
> is after, but I would presume the standard headers are updated to
> account for this. Is there an easy way to check for this? Also, would
> the same issue cause writes to fails? Because writes from the same
> domain work fine, and appear to other domains using xenstore-ls.
> 
> Regards,
> Dagaen Golomb
> 

Use strace on the process and see what gets opened.

-- 
Doug Goldstein

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