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Subject: Re: [PATCH] Guest system time jumps when new vCPUs is hot-added
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
References: <1619576521-81399-1-git-send-email-zelin.deng@linux.alibaba.com>
 <87lf92n5r1.ffs@nanos.tec.linutronix.de>
 <e33920a0-24bc-fa40-0a23-c2eb5693f85d@linux.alibaba.com>
 <875z057a12.ffs@nanos.tec.linutronix.de>
 <2df3de0e-670a-ba28-fdd2-0002cebde545@linux.alibaba.com>
 <87o8dxf597.ffs@nanos.tec.linutronix.de>
From:   Zelin Deng <zelin.deng@linux.alibaba.com>
Message-ID: <1cdd15f4-1242-a21e-e2e5-cecfc93a1219@linux.alibaba.com>
Date:   Fri, 30 Apr 2021 06:40:05 +0800
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Got it. Many thanks, Thomas.

On 2021/4/30 上午12:02, Thomas Gleixner wrote:

> On Thu, Apr 29 2021 at 17:38, Zelin Deng wrote:
>> On 2021/4/29 下午4:46, Thomas Gleixner wrote:
>>> And that validation expects that the CPUs involved run in a tight loop
>>> concurrently so the TSC readouts which happen on both can be reliably
>>> compared.
>>>
>>> But this cannot be guaranteed on vCPUs at all, because the host can
>>> schedule out one or both at any point during that synchronization
>>> check.
>> Is there any plan to fix this?
> The above cannot be fixed.
>
> As I said before the solution is:
>
>>> A two socket guest setup needs to have information from the host that
>>> TSC is usable and that the socket sync check can be skipped. Anything
>>> else is just doomed to fail in hard to diagnose ways.
>> Yes, I had tried to add "tsc=unstable" to skip tsc sync.  However if a
> tsc=unstable? Oh well.
>
>> user process which is not pined to vCPU is using rdtsc, it can get tsc
>> warp, because it can be scheduled among vCPUs.  Does it mean user
> Only if the hypervisor is not doing the right thing and makes sure that
> all vCPUs have the same tsc offset vs. the host TSC.
>
>> applications have to guarantee itself to use rdtsc only when TSC is
>> reliable?
> If the TSCs of CPUs are not in sync then the kernel does the right thing
> and uses some other clocksource for the various time interfaces, e.g.
> the kernel provides clock_getttime() which guarantees to be correct
> whether TSC is usable or not.
>
> Any application using RDTSC directly is own their own and it's not a
> kernel problem.
>
> The host kernel cannot make guarantees that the hardware is sane neither
> can a guest kernel make guarantees that the hypervisor is sane.
>
> Thanks,
>
>          tglx
>
>
>