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

[Dagaen Golomb <dgolomb@xxxxxxxxxxxxxx>]

> 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

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