linux-kernel - Re: [PATCH 36/44] KVM: x86: Do compatibility checks when onlining CPU

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Date:   Thu, 3 Nov 2022 22:34:10 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Eric Farman <farman@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Atish Patra <atishp@...shpatra.org>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        kvmarm@...ts.cs.columbia.edu, linux-mips@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        Fabiano Rosas <farosas@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Chao Gao <chao.gao@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yuan Yao <yuan.yao@...el.com>
Subject: Re: [PATCH 36/44] KVM: x86: Do compatibility checks when onlining CPU

On Thu, Nov 03, 2022, Isaku Yamahata wrote:
> On Wed, Nov 02, 2022 at 11:19:03PM +0000,
> Sean Christopherson <seanjc@...gle.com> wrote:
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index f223c845ed6e..c99222b71fcc 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1666,7 +1666,7 @@ struct kvm_x86_nested_ops {
> >  };
> >  
> >  struct kvm_x86_init_ops {
> > -	int (*check_processor_compatibility)(void);
> > +	int (*check_processor_compatibility)(int cpu);
> 
> Is this cpu argument used only for error message to include cpu number
> with avoiding repeating raw_smp_processor_id() in pr_err()?

Yep.

> The actual check is done on the current executing cpu.
> 
> If cpu != raw_smp_processor_id(), cpu is wrong. Although the function is called
> in non-preemptive context, it's a bit confusing. So voting to remove it and
> to use.

What if I rename the param is this_cpu?  I 100% agree the argument is confusing
as-is, but forcing all the helpers to manually grab the cpu is quite annoying.