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

[Dagaen Golomb <dgolomb@xxxxxxxxxxxxxx>]

On Mon, May 16, 2016 at 3:52 PM, Dagaen Golomb <dgolomb@xxxxxxxxxxxxxx> wrote:
> On Mon, May 16, 2016 at 3:12 PM, Doug Goldstein <cardoe@xxxxxxxxxx> wrote:
>> On 5/16/16 11:20 AM, Dagaen Golomb wrote:
>>> 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.
>>>
>>> Regards,
>>> Dagaen Golomb
>>>
>>
>> Is your domU using Xen 4.7 compiled utilities? or are you using distro
>> provided xenstore libraries?
>
> I am using the newest binaries available from the distro repo (Ubuntu
> 12.04), so version 4.4.
> I'm going to guess you might say this is part of the issue. 4.4 and
> 4.7 aren't really *that* far off, though.
> Legacy interfaces should not be breaking, so I still think this is a
> bug that needs to be addressed somewhere, maybe by determining what
> exactly the issue is with new kernels and /proc/ tree XenStore access,
> if thisis what older tools will be using.

Should I try to use newer utilities? What would be the easiest method
of acquiring and installing them?
Should building and installing the *-tools directives in the xen
source do the trick? I'd like to see if this fixes the issue.

Regards,
Dagaen Golomb

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