From: Marcelo Tosatti <mtosatti@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Maxim Levitsky <mlevitsk@redhat.com>,
kvm@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
Paolo Bonzini <pbonzini@redhat.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, 8 Dec 2020 15:11:07 -0300 [thread overview]
Message-ID: <20201208181107.GA31442@fuller.cnet> (raw)
In-Reply-To: <87sg8g2sn4.fsf@nanos.tec.linutronix.de>
On Tue, Dec 08, 2020 at 05:02:07PM +0100, Thomas Gleixner wrote:
> On Tue, Dec 08 2020 at 16:50, Maxim Levitsky wrote:
> > On Mon, 2020-12-07 at 20:29 -0300, Marcelo Tosatti wrote:
> >> > +This ioctl allows to reconstruct the guest's IA32_TSC and TSC_ADJUST value
> >> > +from the state obtained in the past by KVM_GET_TSC_STATE on the same vCPU.
> >> > +
> >> > +If 'KVM_TSC_STATE_TIMESTAMP_VALID' is set in flags,
> >> > +KVM will adjust the guest TSC value by the time that passed since the moment
> >> > +CLOCK_REALTIME timestamp was saved in the struct and current value of
> >> > +CLOCK_REALTIME, and set the guest's TSC to the new value.
> >>
> >> This introduces the wraparound bug in Linux timekeeping, doesnt it?
>
> Which bug?
max_cycles overflow. Sent a message to Maxim describing it.
>
> > It does.
> > Could you prepare a reproducer for this bug so I get a better idea about
> > what are you talking about?
> >
> > I assume you need very long (like days worth) jump to trigger this bug
> > and for such case we can either work around it in qemu / kernel
> > or fix it in the guest kernel and I strongly prefer the latter.
> >
> > Thomas, what do you think about it?
>
> For one I have no idea which bug you are talking about and if the bug is
> caused by the VMM then why would you "fix" it in the guest kernel.
1) Stop guest, save TSC value of cpu-0 = V.
2) Wait for some amount of time = W.
3) Start guest, load TSC value with V+W.
Can cause an overflow on Linux timekeeping.
> Aside of that I think I made it pretty clear what the right thing to do
> is.
Sure: the notion of a "unique TSC offset" already exists (it is detected
by write TSC logic, and not explicit in the interface, though).
But AFAIK it works pretty well.
Exposing a single TSC value on the interface level seems alright to
me...
next prev parent reply other threads:[~2020-12-08 18:16 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 [this message]
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
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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