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To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
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From:   Juergen Gross <jgross@suse.com>
Subject: Re: [PATCH 6/6] x86/kvm: add boot parameter for setting max number of
 vcpus per guest
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Date:   Fri, 3 Sep 2021 09:01:11 +0200
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From: Juergen Gross <jgross@suse.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>, Paolo Bonzini <pbonzini@redhat.com>,
 Sean Christopherson <seanjc@google.com>, Wanpeng Li <wanpengli@tencent.com>,
 Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
 Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
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 kvm@vger.kernel.org
Message-ID: <bb4ebe24-1de5-82b8-001d-1c0f9f28861b@suse.com>
Subject: Re: [PATCH 6/6] x86/kvm: add boot parameter for setting max number of
 vcpus per guest
References: <20210701154105.23215-1-jgross@suse.com>
 <20210701154105.23215-7-jgross@suse.com>
 <87h7gx2lkt.fsf@vitty.brq.redhat.com>
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In-Reply-To: <8735sh2fr7.fsf@vitty.brq.redhat.com>

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On 14.07.21 15:21, Vitaly Kuznetsov wrote:
> Juergen Gross <jgross@suse.com> writes:
>=20
>> On 14.07.21 13:45, Vitaly Kuznetsov wrote:
>>
>>> Personally, I'd vote for introducing a 'ratio' parameter then so
>>> generally users will only have to set 'kvm.max_vcpus'.
>>
>> Okay.
>>
>> Default '4' then? Or '2 ^ (topology_levels - 2)' (assuming a
>> topology_level of 3 on Intel: thread/core/socket and 4 on EPYC:
>> thread/core/package/socket).
>=20
> I'd suggest we default to '4' for both Intel and AMD as we haven't give=
n
> up completely on cross-vendor VMs (running AMD VMs on Intel CPUs and
> vice versa). It would be great to leave a comment where the number come=
s
> from of course.
>=20

Thinking more about it I believe it would be better to make the
parameter something like "additional vcpu-id bits" with a default of
topology_levels - 2 (cross-vendor VMs are so special that I think the
need to specify another value explicitly in this case is acceptable).

Reasons are:

- the ability to specify factor values not being a power of 2 is weird
- just specifying the additional number of bits would lead to compatible
   behavior (e.g. a max vcpu-id of 1023 with max_vcpus being 288 and the
   default value of 1)
- the max vcpu-id should (normally) be 2^n - 1


Juergen

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