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From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE
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>,
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        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>,
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        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>,
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Date:   Thu, 10 Dec 2020 12:48:20 +0100
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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