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

On Mon, May 16, 2016 at 12:11 PM, Dagaen Golomb <dgolomb@xxxxxxxxxxxxxx> wrote:
> On Mon, May 16, 2016 at 12:03 PM, Dagaen Golomb <dgolomb@xxxxxxxxxxxxxx> 
> wrote:
>> On Mon, May 16, 2016 at 11:55 AM, Doug Goldstein <cardoe@xxxxxxxxxx> wrote:
>>> 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/.

For all kernels in my domU, without setting any environment variables
they use /proc/.
For 4.1.0 this did not work, but works with /dev/ when using
environment variable.
Is this supposed to try /dev/ before /proc/? Because this doesn't
appear the case according to strace.
I think this is a bug.

Dagaen Golomb

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