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From: Paolo Bonzini <pbonzini@redhat.com>
To: Maxim Levitsky <mlevitsk@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Oliver Upton <oupton@google.com>
Cc: kvm list <kvm@vger.kernel.org>, "H. Peter Anvin" <hpa@zytor.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>,
	Marcelo Tosatti <mtosatti@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>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE
Date: Thu, 10 Dec 2020 12:48:20 +0100	[thread overview]
Message-ID: <7c25e8c0-a7d4-8906-ae47-20714e6699fe@redhat.com> (raw)
In-Reply-To: <047afdde655350a6701803aa8ae739a8bd1c1c14.camel@redhat.com>

On 08/12/20 18:08, Maxim Levitsky wrote:
>> Even if you support TSCADJUST and let the guest write to it does not
>> change the per guest offset at all. TSCADJUST is per [v]CPU and adds on
>> top:
>>
>>      tscvcpu = tsc_host + guest_offset + TSC_ADJUST
>>
>> Scaling is just orthogonal and does not change any of this.
>
> I agree with this, and I think that this is what we will end up doing.
> Paulo, what do you think about this?

Yes, you can have a VM ioctl that saves/restores cur_tsc_nsec and 
cur_tsc_write.  The restore side would loop on all vcpus.

However, it is not so easy: 1) it would have to be usable only if 
KVM_X86_QUIRK_TSC_HOST_ACCESS is false, 2) it would fail if 
kvm->arch.nr_vcpus_matched_tsc == kvm->online_vcpus (which basically 
means that userspace didn't mess up the TSC configuration).  If not, it 
would return -EINVAL.

Also, while at it let's burn and pour salt on the support for 
KVM_SET_TSC_KHZ unless TSC scaling is supported, together with 
vcpu->tsc_catchup and all the "tolerance" crap that is in 
kvm_set_tsc_khz.  And initialize vcpu->arch.virtual_tsc_khz to 
kvm->arch.last_tsc_khz before calling kvm_synchronize_tsc.

Paolo


  reply	other threads:[~2020-12-10 11:50 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 [this message]
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
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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