linux-kernel - 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
From:   Juergen Gross <jgross@...e.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-doc@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 6/6] x86/kvm: add boot parameter for setting max number of
 vcpus per guest

On 14.07.21 15:21, Vitaly Kuznetsov wrote:
> Juergen Gross <jgross@...e.com> writes:
> 
>> 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).
> 
> I'd suggest we default to '4' for both Intel and AMD as we haven't given
> 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 comes
> from of course.
> 

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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