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

[Thomas Gleixner]

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?

That's broken.

Thanks,

        tglx