From: Paolo Bonzini <pbonzini@redhat.com>
To: Andy Lutomirski <luto@kernel.org>, Maxim Levitsky <mlevitsk@redhat.com>
Cc: kvm list <kvm@vger.kernel.org>, Oliver Upton <oupton@google.com>,
Ingo Molnar <mingo@redhat.com>,
Sean Christopherson <sean.j.christopherson@intel.com>,
Thomas Gleixner <tglx@linutronix.de>,
open list <linux-kernel@vger.kernel.org>,
Marcelo Tosatti <mtosatti@redhat.com>,
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: Mon, 30 Nov 2020 17:59:08 +0100 [thread overview]
Message-ID: <26444d65-083b-5df4-52c9-c1cfad556b10@redhat.com> (raw)
In-Reply-To: <CALCETrVr2bM4yJTVpQULN+EYVQJuWGCvjX0SMFsCRy6BwqZc0w@mail.gmail.com>
On 30/11/20 17:54, Andy Lutomirski wrote:
>> I*do think* however that we should redefine KVM_CLOCK_TSC_STABLE
>> in the documentation to state that it only guarantees invariance if the guest
>> doesn't mess with its own TSC.
>>
>> Also I think we should consider enabling the X86_FEATURE_TSC_RELIABLE
>> in the guest kernel, when kvm is detected to avoid the guest even from trying
>> to sync TSC on newly hotplugged vCPUs.
>>
>> (The guest doesn't end up touching TSC_ADJUST usually, but it still might
>> in some cases due to scheduling of guest vCPUs)
>>
>> (X86_FEATURE_TSC_RELIABLE short circuits tsc synchronization on CPU hotplug,
>> and TSC clocksource watchdog, and the later we might want to keep).
> If you're going to change the guest behavior to be more trusting of
> the host, I think
> the host should probably signal this to the guest using a new bit.
>
Yes, a new CPUID bit takes longer to propagate to existing setups, but
it is more future proof.
Paolo
next prev parent reply other threads:[~2020-11-30 17:00 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 [this message]
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
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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