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From: Thomas Gleixner <tglx@linutronix.de>
To: Marcelo Tosatti <mtosatti@redhat.com>,
	Maxim Levitsky <mlevitsk@redhat.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Oliver Upton <oupton@google.com>, Ingo Molnar <mingo@redhat.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	open list <linux-kernel@vger.kernel.org>,
	Jonathan Corbet <corbet@lwn.net>,
	Wanpeng Li <wanpengli@tencent.com>,
	Borislav Petkov <bp@alien8.de>, Jim Mattson <jmattson@google.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"open list\:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	Joerg Roedel <joro@8bytes.org>,
	"maintainer\:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)"
	<x86@kernel.org>, Vitaly Kuznetsov <vkuznets@redhat.com>
Subject: Re: [PATCH 0/2] RFC: Precise TSC migration
Date: Tue, 01 Dec 2020 15:01:26 +0100	[thread overview]
Message-ID: <874kl5hbgp.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20201130191643.GA18861@fuller.cnet>

On Mon, Nov 30 2020 at 16:16, Marcelo Tosatti wrote:
> Not really. The synchronization logic tries to sync TSCs during
> BIOS boot (and CPU hotplug), because the TSC values are loaded
> sequentially, say:
>
> CPU		realtime	TSC val
> vcpu0		0 usec		0
> vcpu1		100 usec	0
> vcpu2		200 usec	0

That's nonsense, really.

> And we'd like to see all vcpus to read the same value at all times.

Providing guests with a synchronized and stable TSC on a host with a
synchronized and stable TSC is trivial.

Write the _same_ TSC offset to _all_ vcpu control structs and be done
with it. It's not rocket science.

The guest TSC read is:

    hostTSC + vcpu_offset

So if the host TSC is synchronized then the guest TSCs are synchronized
as well.

If the host TSC is not synchronized, then don't even try.

Thanks,

        tglx

  parent reply	other threads:[~2020-12-01 14:02 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-30 13:35 [PATCH 0/2] RFC: Precise TSC migration Maxim Levitsky
2020-11-30 13:35 ` [PATCH 1/2] KVM: x86: implement KVM_SET_TSC_PRECISE/KVM_GET_TSC_PRECISE Maxim Levitsky
2020-11-30 14:33   ` Paolo Bonzini
2020-11-30 15:58     ` Maxim Levitsky
2020-11-30 17:01       ` Paolo Bonzini
2020-12-01 19:43   ` Thomas Gleixner
2020-12-03 11:11     ` Maxim Levitsky
2020-11-30 13:35 ` [PATCH 2/2] KVM: x86: introduce KVM_X86_QUIRK_TSC_HOST_ACCESS Maxim Levitsky
2020-11-30 13:54   ` Paolo Bonzini
2020-11-30 14:11     ` Maxim Levitsky
2020-11-30 14:15       ` Paolo Bonzini
2020-11-30 15:33         ` Maxim Levitsky
2020-11-30 13:38 ` [PATCH 0/2] RFC: Precise TSC migration (summary) Maxim Levitsky
2020-11-30 16:54 ` [PATCH 0/2] RFC: Precise TSC migration Andy Lutomirski
2020-11-30 16:59   ` Paolo Bonzini
2020-11-30 19:16 ` Marcelo Tosatti
2020-12-01 12:30   ` Maxim Levitsky
2020-12-01 19:48     ` Marcelo Tosatti
2020-12-03 11:39       ` Maxim Levitsky
2020-12-03 20:18         ` Marcelo Tosatti
2020-12-07 13:00           ` Maxim Levitsky
2020-12-01 13:48   ` Thomas Gleixner
2020-12-01 15:02     ` Marcelo Tosatti
2020-12-03 11:51       ` Maxim Levitsky
2020-12-01 14:01   ` Thomas Gleixner [this message]
2020-12-01 16:19     ` Andy Lutomirski
2020-12-03 11:57       ` Maxim Levitsky
2020-12-01 19:35 ` Thomas Gleixner
2020-12-03 11:41   ` Paolo Bonzini
2020-12-03 12:47   ` Maxim Levitsky

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