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From: Andy Lutomirski <luto@amacapital.net>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Marcelo Tosatti <mtosatti@redhat.com>,
	Maxim Levitsky <mlevitsk@redhat.com>,
	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, 1 Dec 2020 08:19:29 -0800	[thread overview]
Message-ID: <FB43C4E2-D7C4-430B-9D6B-15FA59BB5286@amacapital.net> (raw)
In-Reply-To: <874kl5hbgp.fsf@nanos.tec.linutronix.de>



> On Dec 1, 2020, at 6:01 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> 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.

This reminds me: if you’re adding a new kvm feature that tells the guest that the TSC works well, could you perhaps only have one structure for all vCPUs in the same guest?

> 
> Thanks,
> 
>        tglx

  reply	other threads:[~2020-12-01 16:20 UTC|newest]

Thread overview: 32+ 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
2020-12-01 16:19     ` Andy Lutomirski [this message]
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
2020-11-30 13:38 Maxim Levitsky
2020-11-30 13:38 ` Maxim Levitsky

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