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

[Thomas Lamprecht]

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

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-initiated-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