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
next prev parent reply other threads:[~2020-12-01 16:20 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
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
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