On 28/04/21 04:22, 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
[ 141.513496] TSC synchronization [CPU#0 -> CPU#1]:
[ 141.513496] Measured 235 cycles TSC warp between CPUs, turning off TSC clock.
[ 141.513496] tsc: Marking TSC unstable due to check_tsc_sync_source failed
[ 141.543996] KVM setup async PF for cpu 1
[ 141.544281] kvm-stealtime: cpu 1, msr 13bd2c080
[ 141.549381] Will online and init hotplugged CPU: 1
System time jumps from 85.101228 to 141.51.3496.
Guest: KVM
----- ------
check_tsc_sync_target()
wrmsrl(MSR_IA32_TSC_ADJUST,...)
kvm_set_msr_common(vcpu,...)
adjust_tsc_offset_guest(vcpu,...) //tsc_offset jumped
vcpu_enter_guest(vcpu) //tsc_timestamp was not changed
...
rdtsc() jumped, system time jumped
tsc_timestamp must be updated before go back to guest.
---
Zelin Deng (1):
KVM: x86: Update vCPU's hv_clock before back to guest when tsc_offset
is adjusted
arch/x86/kvm/x86.c | 4 ++++
1 file changed, 4 insertions(+)
While Thomas is right in general, what you found is indeed a bug with
the KVM->userspace API to set up the vCPU TSC adjust. So I'm queueing
the patch for 5.15.
Thanks,
Paolo
