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
