linux-kernel - Re: [PATCH v2 2/7] KVM: X86: Go on updating other CPUID leaves when leaf 1 is absent

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Date:   Thu, 2 Jul 2020 12:02:37 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/7] KVM: X86: Go on updating other CPUID leaves when
 leaf 1 is absent

On Thu, Jul 02, 2020 at 11:54:03AM -0700, Sean Christopherson wrote:
> On Tue, Jun 23, 2020 at 07:58:11PM +0800, Xiaoyao Li wrote:
> > As handling of bits other leaf 1 added over time, kvm_update_cpuid()
> > should not return directly if leaf 1 is absent, but should go on
> > updateing other CPUID leaves.
> > 
> > Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> 
> This should probably be marked for stable.
> 
> > ---
> >  arch/x86/kvm/cpuid.c | 23 +++++++++++------------
> >  1 file changed, 11 insertions(+), 12 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 1d13bad42bf9..0164dac95ef5 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -60,22 +60,21 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
> >  	struct kvm_lapic *apic = vcpu->arch.apic;
> >  
> >  	best = kvm_find_cpuid_entry(vcpu, 1, 0);
> > -	if (!best)
> > -		return 0;
> 
> Rather than wrap the existing code, what about throwing it in a separate
> helper?  That generates an easier to read diff and also has the nice
> property of getting 'apic' out of the common code.

Hrm, that'd be overkill once the apic code is moved in a few patches.
What if you keep the cpuid updates wrapped (as in this patch), but then
do

	if (best && apic) {
	}

for the apic path?  That'll minimize churn for code that is disappearing,
e.g. will make future git archaeologists happy :-).

> > -
> > -	/* Update OSXSAVE bit */
> > -	if (boot_cpu_has(X86_FEATURE_XSAVE) && best->function == 0x1)
> > -		cpuid_entry_change(best, X86_FEATURE_OSXSAVE,
> > +	if (best) {
> > +		/* Update OSXSAVE bit */
> > +		if (boot_cpu_has(X86_FEATURE_XSAVE))
> > +			cpuid_entry_change(best, X86_FEATURE_OSXSAVE,
> >  				   kvm_read_cr4_bits(vcpu, X86_CR4_OSXSAVE));
> >  
> > -	cpuid_entry_change(best, X86_FEATURE_APIC,
> > +		cpuid_entry_change(best, X86_FEATURE_APIC,
> >  			   vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE);
> >  
> > -	if (apic) {
> > -		if (cpuid_entry_has(best, X86_FEATURE_TSC_DEADLINE_TIMER))
> > -			apic->lapic_timer.timer_mode_mask = 3 << 17;
> > -		else
> > -			apic->lapic_timer.timer_mode_mask = 1 << 17;
> > +		if (apic) {
> > +			if (cpuid_entry_has(best, X86_FEATURE_TSC_DEADLINE_TIMER))
> > +				apic->lapic_timer.timer_mode_mask = 3 << 17;
> > +			else
> > +				apic->lapic_timer.timer_mode_mask = 1 << 17;
> > +		}
> >  	}
> >  
> >  	best = kvm_find_cpuid_entry(vcpu, 7, 0);
> > -- 
> > 2.18.2
> >