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

On Mon, May 16, 2016 at 3:56 PM, Dagaen Golomb <dgolomb@xxxxxxxxxxxxxx> wrote:
> 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.

I installed a package for xenstore-utils 4.6, and installed over the
4.4 in my guest. Issue persists.

Dagaen Golomb

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