linux-kernel - Re: [PATCH v2 6/6] KVM: x86: Drop arbitraty KVM_SOFT_MAX

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Date:   Wed, 17 Nov 2021 13:12:00 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Eduardo Habkost <ehabkost@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Andrew Jones <drjones@...hat.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Anup Patel <anup.patel@....com>,
        Paul Mackerras <paulus@...abs.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>, kvm-ppc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        kvm-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/6] KVM: x86: Drop arbitraty KVM_SOFT_MAX_VCPUS

On 11/16/21 17:34, Vitaly Kuznetsov wrote:
> KVM_CAP_NR_VCPUS is used to get the "recommended" maximum number of
> VCPUs and arm64/mips/riscv report num_online_cpus(). Powerpc reports
> either num_online_cpus() or num_present_cpus(), s390 has multiple
> constants depending on hardware features. On x86, KVM reports an
> arbitrary value of '710' which is supposed to be the maximum tested
> value but it's possible to test all KVM_MAX_VCPUS even when there are
> less physical CPUs available.
> 
> Drop the arbitrary '710' value and return num_online_cpus() on x86 as
> well. The recommendation will match other architectures and will mean
> 'no CPU overcommit'.
> 
> For reference, QEMU only queries KVM_CAP_NR_VCPUS to print a warning
> when the requested vCPU number exceeds it. The static limit of '710'
> is quite weird as smaller systems with just a few physical CPUs should
> certainly "recommend" less.
> 
> Suggested-by: Eduardo Habkost<ehabkost@...hat.com>
> Signed-off-by: Vitaly Kuznetsov<vkuznets@...hat.com>

Given that KVM_SOFT_MAX_VCPUS has already been dropped in 5.16, I 
changed the commit message to

     KVM: x86: Cap KVM_CAP_NR_VCPUS by KVM_CAP_MAX_VCPUS

     It doesn't make sense to return the recommended maximum number of
     vCPUs which exceeds the maximum possible number of vCPUs.

Paolo