linux-kernel - Re: [PATCH v2 31/50] KVM: x86: Do CPU compatibility checks in x86 code

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Message-ID: <Y45ex/CyTY7jYa5D@google.com>
Date:   Mon, 5 Dec 2022 21:12:39 +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>,
        David Woodhouse <dwmw2@...radead.org>,
        Paul Durrant <paul@....org>, 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, Yuan Yao <yuan.yao@...el.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        Philippe Mathieu-Daudé <philmd@...aro.org>,
        Fabiano Rosas <farosas@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Kai Huang <kai.huang@...el.com>, Chao Gao <chao.gao@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 31/50] KVM: x86: Do CPU compatibility checks in x86
 code

On Mon, Dec 05, 2022, Isaku Yamahata wrote:
> On Wed, Nov 30, 2022 at 11:09:15PM +0000,
> > index 66f16458aa97..3571bc968cf8 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -9277,10 +9277,36 @@ static inline void kvm_ops_update(struct kvm_x86_init_ops *ops)
> >  	kvm_pmu_ops_update(ops->pmu_ops);
> >  }
> >  
> > +struct kvm_cpu_compat_check {
> > +	struct kvm_x86_init_ops *ops;
> > +	int *ret;
> 
> minor nitpick: just int ret. I don't see the necessity of the pointer.
> Anyway overall it looks good to me.

...

> > @@ -9360,6 +9386,14 @@ static int __kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
> >  	if (r != 0)
> >  		goto out_mmu_exit;
> >  
> > +	c.ret = &r;
> > +	c.ops = ops;
> > +	for_each_online_cpu(cpu) {
> > +		smp_call_function_single(cpu, kvm_x86_check_cpu_compat, &c, 1);
> > +		if (r < 0)
> 
> Here it can be "c.ret < 0".

No, because the below goto leads to "return r", i.e. "c.ret" needs to be propagated
to "r".  That's why the code does the admittedly funky "int *ret" thing.

FWIW, this gets cleanup in the end.  "struct kvm_cpu_compat_check" goes away and
"&r" is passed directly to kvm_x86_check_cpu_compat.

> > +			goto out_hardware_unsetup;