Re: Guest migration between different Ryzen CPU generations

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 31 May 2022 13:00:07 -0400
mike tancsa <mike@sentex.net> wrote:

> Hello,
> 
>      I have been using kvm since the Ubuntu 18 and 20.x LTS series of 
> kernels and distributions without any issues on a whole range of Guests 
> up until now. Recently, we spun up an Ubuntu LTS 22 hypervisor to add to 
> the mix and eventually upgrade to. Hardware is a series of Ryzen 7 CPUs 
> (3700x).  Migrations back and forth without issue for Ubuntu 20.x 
> kernels.  The first Ubuntu 22 machine was on identical hardware and all 
> was good with that too. The second Ubuntu 22 based machine was spun up 
> with a newer gen Ryzen, a 5800x.  On the initial kernel version that 
> came with that release back in April, migrations worked as expected 
> between hardware as well as different kernel versions and qemu / KVM 
> versions that come default with the distribution. Not sure if migrations 
> between kernel and KVM versions "accidentally" worked all these years, 
> but they did.  However, we ran into an issue with the kernel 
> 5.15.0-33-generic (possibly with 5.15.0-30 as well) thats part of 
> Ubuntu.  Migrations no longer worked to older generation CPUs.  I could 
> send a guest TO the box and all was fine, but upon sending the guest to 
> another hypervisor, the sender would see it as successfully migrated, 
> but the VM would typically just hang, with 100% CPU utilization, or 
> sometimes crash.  I tried a 5.18 kernel from May 22nd and again the 
> behavior is different. If I specify the CPU as EPYC or EPYC-IBPB, I can 
> migrate back and forth.

perhaps you are hitting issue fixed by:
https://lore.kernel.org/lkml/CAJ6HWG66HZ7raAa+YK0UOGLF+4O3JnzbZ+a-0j8GNixOhLk9dA@mail.gmail.com/T/


> Quick summary
> 
> On Ubuntu 20.04 LTS with latest Ubuntu updates, I can migrate VMs back 
> and forth between a 3700x and a 5800x without issue. Guests are a mix of 
> Ubuntu, Fedora and FreeBSD
> On Ubuntu 22 LTS, with the original kernel from release day, I can 
> migrate VMs back and forth between a 3700x and a 5800x without issue
> On Ubuntu 22 LTS with everything up to date as of mid May 2022, I can 
> migrate from the 3700X to the 5800x without issue. But going from the 
> 5800x to the 3700x results in a migrated VM that either crashes inside 
> the VM or has the CPU pegged at 100% spinning its wheels with the guest 
> frozen and needing a hard reset. This is with --live or without and with 
> --unsafe or without. The crash / hang happens once the VM is fully 
> migrated with the sender thinking it was successfully sent and the 
> receiver thinking it successfully arrived in.
> On stock Ubuntu 22 (5.15.0-33-generic) I can migrate back and forth to 
> Ubuntu 20 as long as the hardware / cpu is identical (in this case, 3700x)
> On Ubuntu 22 LTS with everything up to date as of mid May 2022 with 
> 5.18.0-051800-generic #202205222030 SMP PREEMPT_DYNAMIC Sun May 22. I 
> can migrate VMs back and forth that have as its CPU def EPYC or 
> EPYC-IBPB. If the def (in my one test case anyways) is Nehalem then I 
> get a frozen VM on migration back to the 3700X.
> 
> Some more details at
> 
> https://ubuntuforums.org/showthread.php?t=2475399
> 
> Is this a bug ? Expected behavior ?  Is there a better place to ask 
> these questions ?
> 
> Thanks in advance!
> 
>      ---Mike
>