linux-kernel - Re: [PATCH v2 21/49] KVM: x86: Add a macro to init CPUID features that are 64-bit only

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Message-ID: <8c783615-1e79-471d-b853-d654696fb782@intel.com>
Date: Wed, 17 Jul 2024 21:31:02 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Hou Wenlong <houwenlong.hwl@...group.com>, Kechen Lu <kechenl@...dia.com>,
 Oliver Upton <oliver.upton@...ux.dev>, Maxim Levitsky <mlevitsk@...hat.com>,
 Binbin Wu <binbin.wu@...ux.intel.com>,
 Yang Weijiang <weijiang.yang@...el.com>,
 Robert Hoo <robert.hoo.linux@...il.com>
Subject: Re: [PATCH v2 21/49] KVM: x86: Add a macro to init CPUID features
 that are 64-bit only

On 5/18/2024 1:38 AM, Sean Christopherson wrote:
> Add a macro to mask-in feature flags that are supported only on 64-bit
> kernels/KVM.  In addition to reducing overall #ifdeffery, using a macro
> will allow hardening the kvm_cpu_cap initialization sequences to assert
> that the features being advertised are indeed included in the word being
> initialized.  And arguably using *F() macros through is more readable.
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Very nice patch!

Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>

> ---
>   arch/x86/kvm/cpuid.c | 22 ++++++++++------------
>   1 file changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 5a4d6138c4f1..5e3b97d06374 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -70,6 +70,12 @@ u32 xstate_required_size(u64 xstate_bv, bool compacted)
>   	(boot_cpu_has(X86_FEATURE_##name) ? F(name) : 0);	\
>   })
>   
> +/* Features that KVM supports only on 64-bit kernels. */
> +#define X86_64_F(name)						\
> +({								\
> +	(IS_ENABLED(CONFIG_X86_64) ? F(name) : 0);		\
> +})
> +
>   /*
>    * Raw Feature - For features that KVM supports based purely on raw host CPUID,
>    * i.e. that KVM virtualizes even if the host kernel doesn't use the feature.
> @@ -639,15 +645,6 @@ static __always_inline void kvm_cpu_cap_init(enum cpuid_leafs leaf, u32 mask)
>   
>   void kvm_set_cpu_caps(void)
>   {
> -#ifdef CONFIG_X86_64
> -	unsigned int f_gbpages = F(GBPAGES);
> -	unsigned int f_lm = F(LM);
> -	unsigned int f_xfd = F(XFD);
> -#else
> -	unsigned int f_gbpages = 0;
> -	unsigned int f_lm = 0;
> -	unsigned int f_xfd = 0;
> -#endif
>   	memset(kvm_cpu_caps, 0, sizeof(kvm_cpu_caps));
>   
>   	BUILD_BUG_ON(sizeof(kvm_cpu_caps) - (NKVMCAPINTS * sizeof(*kvm_cpu_caps)) >
> @@ -744,7 +741,8 @@ void kvm_set_cpu_caps(void)
>   	);
>   
>   	kvm_cpu_cap_init(CPUID_D_1_EAX,
> -		F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | F(XSAVES) | f_xfd
> +		F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | F(XSAVES) |
> +		X86_64_F(XFD)
>   	);
>   
>   	kvm_cpu_cap_init_kvm_defined(CPUID_12_EAX,
> @@ -766,8 +764,8 @@ void kvm_set_cpu_caps(void)
>   		F(MTRR) | F(PGE) | F(MCA) | F(CMOV) |
>   		F(PAT) | F(PSE36) | 0 /* Reserved */ |
>   		F(NX) | 0 /* Reserved */ | F(MMXEXT) | F(MMX) |
> -		F(FXSR) | F(FXSR_OPT) | f_gbpages | F(RDTSCP) |
> -		0 /* Reserved */ | f_lm | F(3DNOWEXT) | F(3DNOW)
> +		F(FXSR) | F(FXSR_OPT) | X86_64_F(GBPAGES) | F(RDTSCP) |
> +		0 /* Reserved */ | X86_64_F(LM) | F(3DNOWEXT) | F(3DNOW)
>   	);
>   
>   	if (!tdp_enabled && IS_ENABLED(CONFIG_X86_64))