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 11:54:03 -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 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.

> -
> -	/* 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
>