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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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



[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]

  Powered by Linux