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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        "Durrant, Paul" <pdurrant@amazon.co.uk>,
        Peter Zijlstra <peterz@infradead.org>,
        "Allister, Jack" <jalliste@amazon.com>
Cc:     "bp@alien8.de" <bp@alien8.de>,
        "diapop@amazon.co.uk" <diapop@amazon.co.uk>,
        "hpa@zytor.com" <hpa@zytor.com>,
        "jmattson@google.com" <jmattson@google.com>,
        "joro@8bytes.org" <joro@8bytes.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "metikaya@amazon.co.uk" <metikaya@amazon.co.uk>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "rkrcmar@redhat.com" <rkrcmar@redhat.com>,
        "sean.j.christopherson@intel.com" <sean.j.christopherson@intel.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "wanpengli@tencent.com" <wanpengli@tencent.com>,
        "x86@kernel.org" <x86@kernel.org>
Subject: Re: ...\n
In-Reply-To: <307f19cc-322e-c900-2894-22bdee1e248a@redhat.com>
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Date:   Wed, 01 Jun 2022 09:57:14 +0200
Message-ID: <87tu94olyd.fsf@redhat.com>
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Paolo Bonzini <pbonzini@redhat.com> writes:

> On 5/31/22 16:52, Durrant, Paul wrote:
>>> -----Original Message-----
>>> From: Peter Zijlstra <peterz@infradead.org>
>>> Sent: 31 May 2022 15:44
>>> To: Allister, Jack <jalliste@amazon.com>
>>> Cc: bp@alien8.de; diapop@amazon.co.uk; hpa@zytor.com; jmattson@google.com; joro@8bytes.org;
>>> kvm@vger.kernel.org; linux-kernel@vger.kernel.org; metikaya@amazon.co.uk; mingo@redhat.com;
>>> pbonzini@redhat.com; rkrcmar@redhat.com; sean.j.christopherson@intel.com; tglx@linutronix.de;
>>> vkuznets@redhat.com; wanpengli@tencent.com; x86@kernel.org
>>> Subject: RE: [EXTERNAL]...\n
>>>
>>>
>>> On Tue, May 31, 2022 at 02:02:36PM +0000, Jack Allister wrote:
>>>> The reasoning behind this is that you may want to run a guest at a
>>>> lower CPU frequency for the purposes of trying to match performance
>>>> parity between a host of an older CPU type to a newer faster one.
>>>
>>> That's quite ludicrus. Also, then it should be the host enforcing the
>>> cpufreq, not the guest.
>> 
>> I'll bite... What's ludicrous about wanting to run a guest at a lower CPU freq to minimize observable change in whatever workload it is running?
>
> Well, the right API is cpufreq, there's no need to make it a KVM 
> functionality.

KVM may probably use the cpufreq API to run each vCPU at the desired
frequency: I don't quite see how this can be done with a VMM today when
it's not a 1-vCPU-per-1-pCPU setup.

-- 
Vitaly