Re: Trying cpr

["Dr. David Alan Gilbert" <dave@treblig.org>]

* Dongli Zhang (dongli.zhang@oracle.com) wrote:
> 
> 
> On 4/21/25 6:38 AM, Dr. David Alan Gilbert wrote:
> > Hi Steve,
> >   I've just had a go with cpr-transfer, it's quite interesting.
> > I was just trying it on my (AMD) desktop.
> > 
> > * I was running with qemu displaying graphics, and after migration
> > the source display got updated every time I moved my mouse into the
> > source window; the VM was still stopped, but I guess that means
> > the source GUI is still parsing the guest VRAM and displaying it.
> > I'm not sure if there's any other interactions - e.g. is there any
> > situation where the source GUI will try and write into the shared
> > guest ram?
> > 
> > * Given that you pass fd's over the CPR socket, had you considered
> > passing main migration fd's over it as well, that way you'd
> > only need one incoming.
> > 
> > * The guest noticed the time skew:
> >   timekeeping watchdog on CPU1: Marking clocksource 'tsc' as unstable because the skew is too large:
> >      'kvm-clock' wd_nsec: 556248511 wd_new: 4a93129e69 wd_alst: 4a71eaf0aa mask: (all f's)
> >      'tsc' cs_nsec: 514023131 cs_now: 1047f1d8489 cs_last: 10414538c1 mask: (all f's)
> >      Clocksource 'tsc' skewed -42225380 ns (-42 ms) over watchdog 'kvm-clock' interval of 556248511 ns (556 ms)
> >      'kvm-clock' (not 'tsc') is current clocksource
> 
> Here the guest kernel uses kvm-clock to measure the accuracy of tsc.
> 
> While there is a chance that the accuracy of tsc is broken, it is more likely
> the kvm-clock's accuracy is broken.

Well, remember that with CPR there's a live migration going on and a brief
pause; so it's always a fun question about what happens with clocks; however
with CPR I guess there's a better chance of it being solvable than general
live migration.

> That is, suppose the TSC is still good enough, it is marked unstable because the
> kernel uses an inaccurate kvm-clock to measure tsc.
> 
> How about the guest kernel version? Does it have the below patch?

It's debian's 6.12.12 as the guest, and    git describe b50db7095
v5.16-rc2-5-gb50db7095fe0

so I'd say yes, should have that patch.

> Or is this an
> AMD server (by default X86_FEATURE_CONSTANT_TSC isn't set)?

It's an AMD single socket desktop (3950X) [with 6.14.2 kernel]

> 
> x86/tsc: Disable clocksource watchdog for TSC on qualified platorms
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b50db7095fe002fa3e16605546cba66bf1b68a3e
> 
> In addition, I assume the cpr-transfer doesn't re-create a new KVM instance (fd).
> 
> I used to encounter similar issue during vCPU hotplug.
> 
> KVM: x86: Don't unnecessarily force masterclock update on vCPU hotplug
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c52ffadc65e28ab461fd055e9991e8d8106a0056
> 
> David Woodhouse has a patchset related to kvmclock and live migration.
> 
> [RFC PATCH v3 00/21] Cleaning up the KVM clock mess
> https://lore.kernel.org/all/20240522001817.619072-1-dwmw2@infradead.org/
> 
> Maciej also fixed a similar clock unstable issue.
> 
> target/i386: Reset TSCs of parked vCPUs too on VM reset
> https://gitlab.com/qemu-project/qemu/-/commit/3f2a05b31ee9ce2ddb6c75a9bc3f5e7f7af9a76f

Thanks.

Dave

> Dongli Zhang
> 
> > 
> >   (That was hand copied, probably with some typos - who knew the
> >    GUI doesn't let you copy/paste from serial0...)
> > 
> > 
> > The source commandline was:
> > ./try/qemu-system-x86_64  -object memory-backend-file,id=ram0,size=4G,mem-path=/dev/shm/qemuram0,share=on -m 4G -machine memory-backend=ram0,aux-ram-share=on -cpu host --enable-kvm -smp 16 -drive if=virtio,file=/discs/more/images/debian-13-nocloud-amd64-daily.qcow2 -qmp stdio
> > 
> > The dest commandline was:
> > ./try/qemu-system-x86_64 -object memory-backend-file,id=ram0,size=4G,mem-path=/dev/shm/qemuram0,share=on -m 4G -machine memory-backend=ram0,aux-ram-share=on -cpu host --enable-kvm -smp 16 -drive if=virtio,file=/discs/more/images/debian-13-nocloud-amd64-daily.qcow2 -incoming tcp:0:44444 -incoming '{"channel-type": "cpr", "addr": { "transport": "socket", "type": "unix", "path": "cpr.sock"}}'
> > 
> > Dave
> 
-- 
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