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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: <1619576521-81399-1-git-send-email-zelin.deng@linux.alibaba.com>
References: <1619576521-81399-1-git-send-email-zelin.deng@linux.alibaba.com>
Date:   Wed, 28 Apr 2021 11:00:18 +0200
Message-ID: <87lf92n5r1.ffs@nanos.tec.linutronix.de>
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X-Mailing-List: linux-kernel@vger.kernel.org

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.

Thanks,

        tglx