linux-kernel - Re: [PATCH v2 22/49] KVM: x86: Add a macro to precisely handle aliased 0x1.EDX CPUID features

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Message-ID: <43ef06aca700528d956c8f51101715df86f32a91.camel@redhat.com>
Date: Thu, 04 Jul 2024 21:25:39 -0400
From: Maxim Levitsky <mlevitsk@...hat.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>, Binbin Wu <binbin.wu@...ux.intel.com>, Yang
 Weijiang <weijiang.yang@...el.com>, Robert Hoo <robert.hoo.linux@...il.com>
Subject: Re: [PATCH v2 22/49] KVM: x86: Add a macro to precisely handle
 aliased 0x1.EDX CPUID features

On Fri, 2024-05-17 at 10:38 -0700, Sean Christopherson wrote:
> Add a macro to precisely handle CPUID features that AMD duplicated from
> CPUID.0x1.EDX into CPUID.0x8000_0001.EDX.  This will allow adding an
> assert that all features passed to kvm_cpu_cap_init() match the word being
> processed, e.g. to prevent passing a feature from CPUID 0x7 to CPUID 0x1.
> 
> Because the kernel simply reuses the X86_FEATURE_* definitions from
> CPUID.0x1.EDX, KVM's use of the aliased features would result in false
> positives from such an assert.
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/cpuid.c | 24 +++++++++++++++++-------
>  1 file changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 5e3b97d06374..f2bd2f5c4ea3 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -88,6 +88,16 @@ u32 xstate_required_size(u64 xstate_bv, bool compacted)
>  	F(name);						\
>  })
>  
> +/*
> + * Aliased Features - For features in 0x8000_0001.EDX that are duplicates of
> + * identical 0x1.EDX features, and thus are aliased from 0x1 to 0x8000_0001.
> + */
> +#define AF(name)								\
> +({										\
> +	BUILD_BUG_ON(__feature_leaf(X86_FEATURE_##name) != CPUID_1_EDX);	\
> +	feature_bit(name);							\
> +})
> +
>  /*
>   * Magic value used by KVM when querying userspace-provided CPUID entries and
>   * doesn't care about the CPIUD index because the index of the function in
> @@ -758,13 +768,13 @@ void kvm_set_cpu_caps(void)
>  	);
>  
>  	kvm_cpu_cap_init(CPUID_8000_0001_EDX,
> -		F(FPU) | F(VME) | F(DE) | F(PSE) |
> -		F(TSC) | F(MSR) | F(PAE) | F(MCE) |
> -		F(CX8) | F(APIC) | 0 /* Reserved */ | F(SYSCALL) |
> -		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) | X86_64_F(GBPAGES) | F(RDTSCP) |
> +		AF(FPU) | AF(VME) | AF(DE) | AF(PSE) |
> +		AF(TSC) | AF(MSR) | AF(PAE) | AF(MCE) |
> +		AF(CX8) | AF(APIC) | 0 /* Reserved */ | F(SYSCALL) |
> +		AF(MTRR) | AF(PGE) | AF(MCA) | AF(CMOV) |
> +		AF(PAT) | AF(PSE36) | 0 /* Reserved */ |
> +		F(NX) | 0 /* Reserved */ | F(MMXEXT) | AF(MMX) |
> +		AF(FXSR) | F(FXSR_OPT) | X86_64_F(GBPAGES) | F(RDTSCP) |
>  		0 /* Reserved */ | X86_64_F(LM) | F(3DNOWEXT) | F(3DNOW)
>  	);
>  

Hi,

What if we defined the aliased features instead.
Something like this:

#define __X86_FEATURE_8000_0001_ALIAS(feature) \
	(feature + (CPUID_8000_0001_EDX - CPUID_1_EDX) * 32)

#define KVM_X86_FEATURE_FPU_ALIAS	__X86_FEATURE_8000_0001_ALIAS(KVM_X86_FEATURE_FPU)
#define KVM_X86_FEATURE_VME_ALIAS	__X86_FEATURE_8000_0001_ALIAS(KVM_X86_FEATURE_VME)

And then just use for example the 'F(FPU_ALIAS)' in the CPUID_8000_0001_EDX


Best regards,
	Maxim Levitsky