linux-kernel - Re: [PATCH 33/44] KVM: x86: Do VMX/SVM support checks directly in vendor code

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Message-ID: <Y3RDPOerOIf6SwI0@google.com>
Date:   Wed, 16 Nov 2022 01:56:12 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "imbrenda@...ux.ibm.com" <imbrenda@...ux.ibm.com>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "mjrosato@...ux.ibm.com" <mjrosato@...ux.ibm.com>,
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        "oliver.upton@...ux.dev" <oliver.upton@...ux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
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Subject: Re: [PATCH 33/44] KVM: x86: Do VMX/SVM support checks directly in
 vendor code

On Tue, Nov 15, 2022, Huang, Kai wrote:
> On Wed, 2022-11-02 at 23:19 +0000, Sean Christopherson wrote:
> > +static bool __init kvm_is_vmx_supported(void)
> > +{
> > +	if (!cpu_has_vmx()) {
> > +		pr_err("CPU doesn't support VMX\n");
> > +		return false;
> > +	}
> > +
> > +	if (!boot_cpu_has(X86_FEATURE_MSR_IA32_FEAT_CTL) ||
> > +	    !boot_cpu_has(X86_FEATURE_VMX)) {
> > +		pr_err("VMX not enabled in MSR_IA32_FEAT_CTL\n");
> > +		return false;
> > +	}
> > +
> > +	return true;
> > +}
> > +
> >  static int __init vmx_check_processor_compat(void)
> >  {
> >  	struct vmcs_config vmcs_conf;
> >  	struct vmx_capability vmx_cap;
> >  
> > -	if (!this_cpu_has(X86_FEATURE_MSR_IA32_FEAT_CTL) ||
> > -	    !this_cpu_has(X86_FEATURE_VMX)) {
> > -		pr_err("VMX is disabled on CPU %d\n", smp_processor_id());
> > +	if (!kvm_is_vmx_supported())
> >  		return -EIO;
> > -	}
> >  
> 
> Looks there's a functional change here -- the old code checks local cpu's
> feature bits but the new code always checks bsp's feature bits.  Should have no
> problem I think, though.

Ouch.  The bad check will defeat the purpose of doing compat checks.  Nice catch!