linux-kernel - Re: [PATCH 46/61] KVM: x86: Remove the unnecessary loop on CPUID 0x7 sub-leafs

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Message-ID: <87tv3fmxm3.fsf@vitty.brq.redhat.com>
Date:   Mon, 24 Feb 2020 23:25:24 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...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 46/61] KVM: x86: Remove the unnecessary loop on CPUID 0x7 sub-leafs

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> Explicitly handle CPUID 0x7 sub-leaf 1.  The kernel is currently aware
> of exactly one feature in CPUID 0x7.1,  which means there is room for
> another 127 features before CPUID 0x7.2 will see the light of day, i.e.
> the looping is likely to be dead code for years to come.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/cpuid.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 7362e5238799..47f61f4497fb 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -533,11 +533,9 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  		if (boot_cpu_has(X86_FEATURE_AMD_SSBD))
>  			cpuid_entry_set(entry, X86_FEATURE_SPEC_CTRL_SSBD);
>  
> -		for (i = 1, max_idx = entry->eax; i <= max_idx; i++) {
> -			if (WARN_ON_ONCE(i > 1))
> -				break;
> -
> -			entry = do_host_cpuid(array, function, i);
> +		/* KVM only supports 0x7.0 and 0x7.1, capped above via min(). */
> +		if (entry->eax == 1) {
> +			entry = do_host_cpuid(array, function, 1);
>  			if (!entry)
>  				goto out;

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly