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

[Dagaen Golomb <dgolomb@xxxxxxxxxxxxxx>]

> 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

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