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

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/.

The previous versions used /proc/ as well according to strace, but
worked. According to others, this may be an issue with newer kernels
(3.14+) using /proc/.

I experimented with our 4.1.0 kernel. At first it still wasn't working
because I was using sudo as the program needs superuser access... each
time this cleared out any environment.
Using sudo su, then setting the environment to use /dev/, and then
running the program successfully reads! I'll consider this the
solution. Thanks everyone.

Now to make this environment setting apply to all users/shells,
directly from boot. I guess /etc/bashrc will do the trick?

Dagaen Golomb

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