Bug 605697

This request was evaluated by Red Hat Product Management for inclusion in a Red
Hat Enterprise Linux maintenance release.  Product Management has requested
further review of this request by Red Hat Engineering, for potential
inclusion in a Red Hat Enterprise Linux Update release for currently deployed
products.  This request is not yet committed for inclusion in an Update
release.

*** Bug 620733 has been marked as a duplicate of this bug. ***

Getting ballooning to work for 32on64 HVM guests looks like it will require more changes than we want to make, at least for 5.6, especially considering there haven't been any customer requests for this feature. However, when a sysadmin types 'xm mem-set' on their HVM guest it's not very nice to watch it crash.  I'll attach a simple patch for the kernel that simply propagates the error (rather than BUGs).

Ugh... I spoke too soon. Looking at the balloon driver's design, it's not trivial to just propagate the error. I think we'll have to be satisfied with just guarding against this in the tools until we can get ballooning working 100% (if we decide to do so).

I'll clone this bug to userspace to work up a patch there for 5.6. And flip the flag in this kernel side bug to 5.7.

Created attachment 447743 [details]
Prevent crashing of x86 guest on x86_64 host by checking for -ENOSYS first.

I've explored the hypervisor (backport) and tools (guard) variants and adapting the patch from oracle still seems like a best solution, as the other would require to pull lots of possibly buggy code from upstream, while this patch does similar job and with ifdefs it changes only the vulnerable kernels.

Note to QE: please make sure that both echo to /proc/xen/balloon, and "xm mem-set" are tested.

We have a patch posted for 5.6 right now, so flipping flag back to 5.6...

This request was evaluated by Red Hat Product Management for inclusion in a Red
Hat Enterprise Linux maintenance release.  Product Management has requested
further review of this request by Red Hat Engineering, for potential
inclusion in a Red Hat Enterprise Linux Update release for currently deployed
products.  This request is not yet committed for inclusion in an Update
release.

in kernel-2.6.18-223.el5
You can download this test kernel (or newer) from http://people.redhat.com/jwilson/el5

Detailed testing feedback is always welcomed.

QA verified this bug with -237 kernel-xen package:

1. Start a 32bit RHEL5.5 HVM guest.
2. Make sure balloon module is loaded.
3. In Domain0, try to balloon down the pvHVM guest via xm mem-set

At step 3, the guest memory size changes from 512M to 400M. And the guest didn't crash. 

BTW, it seems that no /proc/xen/balloon interface is provided in RHEL5.5 HVM guest, so should "echo to /proc/xen/balloon" be tested too?

No, I'll check why /proc/xen is not found.  Maybe Andrew knows already though.

(In reply to comment #17)
> No, I'll check why /proc/xen is not found.  Maybe Andrew knows already though.

It's only created for xen kernels (not bare-metal/HVM kernels), because all calls to create_xen_proc_entry are wrapped with an #ifdef like this

#if defined(CONFIG_PROC_FS) && defined(CONFIG_XEN_PRIVILEGED_GUEST)
        if ((balloon_pde = create_xen_proc_entry("balloon", 0644)) == NULL) {

An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHSA-2011-0017.html