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From:   Thomas Gleixner <tglx@linutronix.de>
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To:     Zelin Deng <zelin.deng@linux.alibaba.com>,
        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
In-Reply-To: <87lf92n5r1.ffs@nanos.tec.linutronix.de>
References: <1619576521-81399-1-git-send-email-zelin.deng@linux.alibaba.com> <87lf92n5r1.ffs@nanos.tec.linutronix.de>
Date:   Wed, 28 Apr 2021 11:09:50 +0200
Message-ID: <87im46n5b5.ffs@nanos.tec.linutronix.de>
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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