From: Zelin Deng <zelin.deng@linux.alibaba.com>
To: Thomas Gleixner <tglx@linutronix.de>,
Paolo Bonzini <pbonzini@redhat.com>,
Sean Christopherson <seanjc@google.com>,
Wanpeng Li <wanpengli@tencent.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH] Guest system time jumps when new vCPUs is hot-added
Date: Thu, 29 Apr 2021 07:24:55 +0800 [thread overview]
Message-ID: <e33920a0-24bc-fa40-0a23-c2eb5693f85d@linux.alibaba.com> (raw)
In-Reply-To: <87lf92n5r1.ffs@nanos.tec.linutronix.de>
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
next prev parent reply other threads:[~2021-04-28 23:25 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-28 2:22 [PATCH] Guest system time jumps when new vCPUs is hot-added Zelin Deng
2021-04-28 2:22 ` [PATCH] KVM: x86: Update vCPU's hv_clock before back to guest when tsc_offset is adjusted Zelin Deng
2021-04-28 9:00 ` [PATCH] Guest system time jumps when new vCPUs is hot-added Thomas Gleixner
2021-04-28 9:09 ` Thomas Gleixner
2021-04-28 23:24 ` Zelin Deng [this message]
2021-04-29 8:46 ` Thomas Gleixner
2021-04-29 9:38 ` Zelin Deng
2021-04-29 16:02 ` Thomas Gleixner
2021-04-29 22:40 ` Zelin Deng
2021-09-06 11:22 ` Paolo Bonzini
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