Re: [PATCH] Guest system time jumps when new vCPUs is hot-added

[Zelin Deng]

On 2021/4/28 下午5:00, Thomas Gleixner wrote:
> On Wed, Apr 28 2021 at 10:22, Zelin Deng wrote:
>
> > Hello,
> > I have below VM configuration:
> > ...
> >      <vcpu placement='static' current='1'>2</vcpu>
> >      <cpu mode='host-passthrough'>
> >      </cpu>
> >      <clock offset='utc'>
> >          <timer name='tsc' frequency='3000000000'/>
> >      </clock>
> > ...
> > After VM has been up for a few minutes, I use "virsh setvcpus" to hot-add
> > second vCPU into VM, below dmesg is observed:
> > [   53.273484] CPU1 has been hot-added
> > [   85.067135] SMP alternatives: switching to SMP code
> > [   85.078409] x86: Booting SMP configuration:
> > [   85.079027] smpboot: Booting Node 0 Processor 1 APIC 0x1
> > [   85.080240] kvm-clock: cpu 1, msr 77601041, secondary cpu clock
> > [   85.080450] smpboot: CPU 1 Converting physical 0 to logical die 1
> > [   85.101228] TSC ADJUST compensate: CPU1 observed 169175101528 warp. Adjust: 169175101528
> > [  141.513496] TSC ADJUST compensate: CPU1 observed 166 warp. Adjust: 169175101694
> Why is TSC_ADJUST on CPU1 different from CPU0 in the first place?

Per my understanding when vCPU is created by KVM, it's tsc_offset = 0 - 
host rdtsc() meanwhile TSC_ADJUST is 0.

Assume vCPU0 boots up with tsc_offset0, after 10000 tsc cycles, hotplug 
via "virsh setvcpus" creates a new vCPU1 whose tsc_offset1 should be 
about tsc_offset0 - 10000.  Therefore there's 10000 tsc warp between 
rdtsc() in guest of vCPU0 and vCPU1, check_tsc_sync_target() when vCPU1 
gets online will set TSC_ADJUST for vCPU1.

Did I miss something?

> That's broken.
>
> Thanks,
>
>          tglx