KVM/QEMU live migration fails with Ubuntu wily kernel 4.2.0-30.35

[Kamal Mostafa]

On Tue, 2016-03-01 at 08:49 +0100, Thomas Lamprecht wrote:
> Hi all!
> 
> First of all, most of the information I'm posting here is available
> at:
> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1540532
> But as it seems this "bug report" won't get much attention anymore
> I'm
> posting it here again, in the hope that I do not step on any feet and
> that it gets resolved

Hi Thomas-

Thanks for your work on this -- your detailed analysis is very much
appreciated!  I'll start the process of getting your backport applied
and deployed.

We'll need a *new* Launchpad bug report to track the issue.  Can I ask
you to file one please?  (Attach your backport to the new bug, and send
me the bug number).

 -Kamal


> The Ubuntu-4.2.0-30.35 introduced a regression regarding live
> migration.
> When migrating live the VM will start on the target host but is
> completely frozen.
> This affects all types of VMs, I after receiving a bug report with a
> WIndows VM I tested it with various distros, like: Alpine Linux,
> Linux
> Mint, Ubuntu Desktop 15.10, Windows 7.
> I always get the same result, the VM is completely frozen and in no
> way
> responsive or reachable. In some cases the VM was not frozen but
> rather
> reset on the target.
> 
> A git bisect and a bit of guessing gave me the culprit, its
> 3f11933efc9ef55ecb2ac7e6d626e8d05a99a4b1 (KVM: x86: expose
> MSR_TSC_AUX
> to userspace) from the ubuntu repo, or the respective kernel upstream
> patch9dbe6cf941a6fe82933aef565e4095fb10f65023 .
> 
> Reverting this fixes the issue (with no introduced regression, as far
> as
> I've seen).
> 
> But as I tested the master and master-next branches from the Ubuntu
> Xenial kernel repos and I knew that this patch was there and live
> migration was working I reverse bisect (i.e. switch good and bad) the
> 4.4 to find the fix to this issue and backported it back to the 4.2
> Wily
> kernel and tested it successfully.
> 
> The fix is commit 81b1b9ca6d5ca5f3ce91c0095402def657cf5db3 (KVM: VMX:
> Fix host initiated access to guest MSR_TSC_AUX) upstream and the
> backport patch is attached to the Ubuntu launchpad bug link i posted
> at
> the beginning.
> https://launchpadlibrarian.net/243097677/0001-KVM-VMX-Fix-host-initia
> ted-access-to-guest-MSR_TSC_A.patch
> 
> The backport was rather trivial, I only had to replace the new
> guest_cpuid_has_rdtscp(vcpu) with the old check on the vmx struct.
> 
> If any action by my side should/must be taken to land this in ubuntu
> kernel repo, e.g. sending the patch to this list, feel free to notify
> me.
> 
> best regards,
> Thomas
> 
>