Massive performance regression caused by recent spectre/meltdown patches

[Phillip Susi]

I recently started up a lubuntu livecd under a Xen vm on my 16.04 Xen server to do some testing. You never even see the LILO menu, and after 4 minutes 30 seconds, you finally see the desktop. The guest also reports CPU soft lockup errors because the lag is so bad.

I first tried reverting to an older kernel ( 4.10 ) that did not have the recent spectre/meltdown patches to no avail. I noticed that during this time on the dom0, qemu-system-x86 is using 100% of a cpu. I noticed qemu recently had a set of spectre/meltdown patches so I downgraded it back to 1:2.5+dfsg-5ubuntu10 and suddenly I was able see the LILO menu within one or two seconds, select from it, and get a desktop maybe 15 seconds after that.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Christian Ehrhardt  (paelzer)]

Hi Phillip,
there was no recent change to Xen and the ones in qemu in regard to spectre/meltdown are all opt-in. You have to change the guest cpu to pick the things up - well that is it in kvm, but you mentioned xen - I'm not sure at all how it works there.

Could you outline exactly:
1. which are the two versions of qemu-system-x86 that are good and bad (there were a lot of security fixes recently)?
2. the command line you use to invoke the live cd under xen?
3. if you could check, does on your system the same apply if you run it under KVM ?

@Marc - any idea which change could be involved?

[Christian Ehrhardt  (paelzer)]

Note: Probably related to bug 1752761

[Christian Ehrhardt  (paelzer)]

The suggested bug above was resolved, could you please test if this was the issue that affected you as well?

[Phillip Susi (psusi)]

Yes, it seems to be working now.