From: Marcelo Tosatti <mtosatti@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Maxim Levitsky <mlevitsk@redhat.com>,
kvm@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
Jonathan Corbet <corbet@lwn.net>,
Jim Mattson <jmattson@google.com>,
Wanpeng Li <wanpengli@tencent.com>,
"open list:KERNEL SELFTEST FRAMEWORK"
<linux-kselftest@vger.kernel.org>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Sean Christopherson <sean.j.christopherson@intel.com>,
open list <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<x86@kernel.org>, Joerg Roedel <joro@8bytes.org>,
Borislav Petkov <bp@alien8.de>, Shuah Khan <shuah@kernel.org>,
Andrew Jones <drjones@redhat.com>,
Oliver Upton <oupton@google.com>,
"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE
Date: Tue, 15 Dec 2020 07:59:27 -0300 [thread overview]
Message-ID: <20201215105927.GA3321@fuller.cnet> (raw)
In-Reply-To: <d9063c37-a965-d5cf-e923-c0c9f6ddc044@redhat.com>
On Fri, Dec 11, 2020 at 10:59:59PM +0100, Paolo Bonzini wrote:
> On 11/12/20 22:04, Thomas Gleixner wrote:
> > > Its 100ms off with migration, and can be reduced further (customers
> > > complained about 5 seconds but seem happy with 0.1ms).
> > What is 100ms? Guaranteed maximum migration time?
>
> I suppose it's the length between the time from KVM_GET_CLOCK and
> KVM_GET_MSR(IA32_TSC) to KVM_SET_CLOCK and KVM_SET_MSR(IA32_TSC). But the
> VM is paused for much longer, the sequence for the non-live part of the
> migration (aka brownout) is as follows:
>
> pause
> finish sending RAM receive RAM ~1 sec
> send paused-VM state finish receiving RAM \
> receive paused-VM state ) 0.1 sec
> restart /
>
> The nanosecond and TSC times are sent as part of the paused-VM state at the
> very end of the live migration process.
>
> So it's still true that the time advances during live migration brownout;
> 0.1 seconds is just the final part of the live migration process. But for
> _live_ migration there is no need to design things according to "people are
> happy if their clock is off by 0.1 seconds only".
Agree. What would be a good way to fix this?
It seems to me using CLOCK_REALTIME as in the interface Maxim is
proposing is prone to difference in CLOCK_REALTIME itself.
Perhaps there is another way to measure that 0.1 sec which is
independent of the clock values of the source and destination hosts
(say by sending a packet once the clock stops counting).
Then on destination measure delta = clock_restart_time - packet_receival
and increase clock by that amount.
> Again, save-to-disk,
> reverse debugging and the like are a different story, which is why KVM
> should delegate policy to userspace (while documenting how to do it right).
>
> Paolo
>
> > CLOCK_REALTIME and CLOCK_TAI are off by the time the VM is paused and
> > this state persists up to the point where NTP corrects it with a time
> > jump.
> >
> > So if migration takes 5 seconds then CLOCK_REALTIME is not off by 100ms
> > it's off by 5 seconds.
> >
> > CLOCK_MONOTONIC/BOOTTIME might be off by 100ms between pause and resume.
> >
next prev parent reply other threads:[~2020-12-15 11:37 UTC|newest]
Thread overview: 77+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-03 17:11 [PATCH v2 0/3] RFC: Precise TSC migration Maxim Levitsky
2020-12-03 17:11 ` [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE Maxim Levitsky
2020-12-06 16:19 ` Thomas Gleixner
2020-12-07 12:16 ` Maxim Levitsky
2020-12-07 13:16 ` Vitaly Kuznetsov
2020-12-07 17:41 ` Thomas Gleixner
2020-12-08 9:48 ` Peter Zijlstra
2020-12-10 11:42 ` Paolo Bonzini
2020-12-10 12:14 ` Peter Zijlstra
2020-12-10 12:22 ` Paolo Bonzini
2020-12-10 13:01 ` Peter Zijlstra
2020-12-10 20:20 ` Thomas Gleixner
2020-12-07 16:38 ` Thomas Gleixner
2020-12-07 16:53 ` Andy Lutomirski
2020-12-07 17:00 ` Maxim Levitsky
2020-12-07 18:04 ` Andy Lutomirski
2020-12-07 23:11 ` Marcelo Tosatti
2020-12-08 17:43 ` Andy Lutomirski
2020-12-08 19:24 ` Thomas Gleixner
2020-12-08 20:32 ` Andy Lutomirski
2020-12-09 0:19 ` Thomas Gleixner
2020-12-09 4:08 ` Andy Lutomirski
2020-12-09 10:14 ` Thomas Gleixner
2020-12-10 23:42 ` Andy Lutomirski
2020-12-08 11:24 ` Maxim Levitsky
2020-12-08 9:35 ` Peter Zijlstra
2020-12-07 23:34 ` Marcelo Tosatti
2020-12-07 17:29 ` Oliver Upton
2020-12-08 11:13 ` Maxim Levitsky
2020-12-08 15:57 ` Oliver Upton
2020-12-08 15:58 ` Oliver Upton
2020-12-08 17:10 ` Maxim Levitsky
2020-12-08 16:40 ` Thomas Gleixner
2020-12-08 17:08 ` Maxim Levitsky
2020-12-10 11:48 ` Paolo Bonzini
2020-12-10 14:25 ` Maxim Levitsky
2020-12-07 23:29 ` Marcelo Tosatti
2020-12-08 14:50 ` Maxim Levitsky
2020-12-08 16:02 ` Thomas Gleixner
2020-12-08 16:25 ` Maxim Levitsky
2020-12-08 17:33 ` Andy Lutomirski
2020-12-08 21:25 ` Thomas Gleixner
2020-12-08 18:12 ` Marcelo Tosatti
2020-12-08 21:35 ` Thomas Gleixner
2020-12-08 21:20 ` Thomas Gleixner
2020-12-10 11:48 ` Paolo Bonzini
2020-12-10 14:52 ` Maxim Levitsky
2020-12-10 15:16 ` Andy Lutomirski
2020-12-10 17:59 ` Oliver Upton
2020-12-10 18:05 ` Paolo Bonzini
2020-12-10 18:13 ` Oliver Upton
2020-12-10 21:25 ` Thomas Gleixner
2020-12-10 22:01 ` Andy Lutomirski
2020-12-10 22:28 ` Thomas Gleixner
2020-12-10 23:19 ` Andy Lutomirski
2020-12-11 0:03 ` Thomas Gleixner
2020-12-08 18:11 ` Marcelo Tosatti
2020-12-08 21:33 ` Thomas Gleixner
2020-12-09 16:34 ` Marcelo Tosatti
2020-12-09 20:58 ` Thomas Gleixner
2020-12-10 15:26 ` Marcelo Tosatti
2020-12-10 21:48 ` Thomas Gleixner
2020-12-11 0:27 ` Marcelo Tosatti
2020-12-11 13:30 ` Thomas Gleixner
2020-12-11 14:18 ` Marcelo Tosatti
2020-12-11 21:04 ` Thomas Gleixner
2020-12-11 21:59 ` Paolo Bonzini
2020-12-12 13:03 ` Thomas Gleixner
2020-12-15 10:59 ` Marcelo Tosatti [this message]
2020-12-15 16:55 ` Andy Lutomirski
2020-12-15 22:34 ` Thomas Gleixner
2020-12-11 13:37 ` Paolo Bonzini
2020-12-08 17:35 ` Marcelo Tosatti
2020-12-03 17:11 ` [PATCH v2 2/3] KVM: x86: introduce KVM_X86_QUIRK_TSC_HOST_ACCESS Maxim Levitsky
2020-12-03 17:11 ` [PATCH v2 3/3] kvm/selftests: update tsc_msrs_test to cover KVM_X86_QUIRK_TSC_HOST_ACCESS Maxim Levitsky
2020-12-07 23:16 ` [PATCH v2 0/3] RFC: Precise TSC migration Marcelo Tosatti
2020-12-10 11:48 ` Paolo Bonzini
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