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

On 5/15/16 8:41 PM, Dagaen Golomb wrote:
>> 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.
> Ah, of course. It seems both the working and non-working domains are
> using /proc/...
> Regards,
> Dagaen Golomb

How are you starting them? Can you confirm its attempting /dev/ first?
Xen 4.7 should prefer /dev/.

Doug Goldstein

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