linux-kernel - Re: [PATCH 43/61] KVM: x86: Use KVM cpu caps to mark CR4.LA57 as not-reserved

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Date:   Mon, 24 Feb 2020 23:08:30 +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 43/61] KVM: x86: Use KVM cpu caps to mark CR4.LA57 as not-reserved

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

> Add accessor(s) for KVM cpu caps and use said accessor to detect
> hardware support for LA57 instead of manually querying CPUID.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/cpuid.h | 13 +++++++++++++
>  arch/x86/kvm/x86.c   |  2 +-
>  2 files changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index 7b71ae0ca05e..5ce4219d465f 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -274,6 +274,19 @@ static __always_inline void kvm_cpu_cap_set(unsigned x86_feature)
>  	kvm_cpu_caps[x86_leaf] |= __feature_bit(x86_feature);
>  }
>  
> +static __always_inline u32 kvm_cpu_cap_get(unsigned x86_feature)
> +{
> +	unsigned x86_leaf = x86_feature / 32;
> +
> +	reverse_cpuid_check(x86_leaf);
> +	return kvm_cpu_caps[x86_leaf] & __feature_bit(x86_feature);
> +}
> +
> +static __always_inline bool kvm_cpu_cap_has(unsigned x86_feature)
> +{
> +	return kvm_cpu_cap_get(x86_feature);
> +}

I know this works (and I even checked C99 to make sure that it works not
by accident) but I have to admit that explicit '!!' conversion to bool
always makes me feel safer :-)

> +
>  static __always_inline void kvm_cpu_cap_check_and_set(unsigned x86_feature)
>  {
>  	if (boot_cpu_has(x86_feature))
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c5ed199d6cd9..cb40737187a1 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -912,7 +912,7 @@ static u64 kvm_host_cr4_reserved_bits(struct cpuinfo_x86 *c)
>  {
>  	u64 reserved_bits = __cr4_reserved_bits(cpu_has, c);
>  
> -	if (cpuid_ecx(0x7) & feature_bit(LA57))
> +	if (kvm_cpu_cap_has(X86_FEATURE_LA57))
>  		reserved_bits &= ~X86_CR4_LA57;
>  
>  	if (kvm_x86_ops->umip_emulated())

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

-- 
Vitaly