On Wed, Apr 28 2021 at 11:00, 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?
>
> That's broken.
Aside of that the TSC synchronization check in guests cannot work
reliably at all. Simply because there is no guarantee that vCPU0 and
vCPU1 are running in parallel.
Thanks,
tglx
