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

[Dagaen Golomb <dgolomb@xxxxxxxxxxxxxx>]

> >>>>> 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/...
>
> Then according to Doug, "Anything after 3.14 will give you
> > deadlocks if you try to use /proc/xen/xenbus.". Maybe the non-working domU uses kernel version after 3.14.

It does, being the custom kernel on version 4.1.0. But Dom0 uses this same exact kernel and reads/writes just fine! The only solution if this is indeed the problem appears to be changing the kernel source we build on or some hacky method such as symlinking /proc/.. to /dev/.., there has to be an elegant real solution to this...

Regards,
Dagaen Golomb

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