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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Juergen Gross <jgross@suse.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>, linux-kernel@vger.kernel.org,
        x86@kernel.org, linux-doc@vger.kernel.org, kvm@vger.kernel.org,
        Eduardo Habkost <ehabkost@redhat.com>
Subject: Re: [PATCH 6/6] x86/kvm: add boot parameter for setting max number
 of vcpus per guest
In-Reply-To: <bb4ebe24-1de5-82b8-001d-1c0f9f28861b@suse.com>
References: <20210701154105.23215-1-jgross@suse.com>
 <20210701154105.23215-7-jgross@suse.com>
 <87h7gx2lkt.fsf@vitty.brq.redhat.com>
 <1ddffb87-a6a2-eba3-3f34-cf606a2ecba2@suse.com>
 <878s292k75.fsf@vitty.brq.redhat.com>
 <62679c6a-2f23-c1d1-f54c-1872ec748965@suse.com>
 <8735sh2fr7.fsf@vitty.brq.redhat.com>
 <bb4ebe24-1de5-82b8-001d-1c0f9f28861b@suse.com>
Date:   Fri, 03 Sep 2021 09:40:53 +0200
Message-ID: <87lf4em7i2.fsf@vitty.brq.redhat.com>
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Juergen Gross <jgross@suse.com> writes:

> On 14.07.21 15:21, Vitaly Kuznetsov wrote:
>> Juergen Gross <jgross@suse.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

Sounds good to me! 

Also, there's an ongoing work to raise the default KVM_MAX_VCPUS number
by Eduardo (Cc):

https://lore.kernel.org/kvm/20210831204535.1594297-1-ehabkost@redhat.com/

It would be great if you could unify your efforts)

-- 
Vitaly