linux-kernel - Re: [PATCH 1/5] KVM: arm64: Cap KVM_CAP_NR_VCPUS by KVM_CAP_MAX

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Message-ID: <87y25onsj6.fsf@redhat.com>
Date:   Tue, 16 Nov 2021 14:23:25 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>
Cc:     kvm@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Eduardo Habkost <ehabkost@...hat.com>,
        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>, kvm-ppc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        kvm-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] KVM: arm64: Cap KVM_CAP_NR_VCPUS by KVM_CAP_MAX_VCPUS

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 11/12/21 15:02, Marc Zyngier wrote:
>>> I'd like KVM to be consistent across architectures and have the same
>>> (similar) meaning for KVM_CAP_NR_VCPUS.
>> Sure, but this is a pretty useless piece of information anyway. As
>> Andrew pointed out, the information is available somewhere else, and
>> all we need to do is to cap it to the number of supported vcpus, which
>> is effectively a KVM limitation.
>> 
>> Also, we are talking about representing the architecture to userspace.
>> No amount of massaging is going to make an arm64 box look like an x86.
>
> Not sure what you mean?  The API is about providing a piece of 
> information independent of the architecture, while catering for a ppc 
> weirdness.  Yes it's mostly useless if you don't care about ppc, but 
> it's not about making arm64 look like x86 or ppc; it's about not having 
> to special case ppc in userspace.
>
> If anything, if KVM_CAP_NR_VCPUS returns the same for kvm and !kvm, then 
> *that* is making an arm64 box look like an x86.  On ARM the max vCPUs 
> depends on VM's GIC configuration, so KVM_CAP_NR_VCPUS should take that 
> into account.

(I'm about to send v2 as we have s390 sorted out.)

So what do we decide about ARM? 
- Current approach (kvm->arch.max_vcpus/kvm_arm_default_max_vcpus()
 depending on 'if (kvm)') - that would be my preference.
- Always kvm_arm_default_max_vcpus to make the output independent on 'if
 (kvm)'.
- keep the status quo (drop the patch).

Please advise)

-- 
Vitaly