linux-kernel - Re: [RFC PATCH v2 03/11] KVM: x86: Advertise BHI

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Date:   Mon, 15 May 2023 15:14:02 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Chao Gao <chao.gao@...el.com>, kvm@...r.kernel.org
Cc:     Jiaan Lu <jiaan.lu@...el.com>, Zhang Chen <chen.zhang@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 03/11] KVM: x86: Advertise BHI_CTRL support

On 4/14/2023 2:25 PM, Chao Gao wrote:
> From: Zhang Chen <chen.zhang@...el.com>
> 
> Add 100% kvm-only feature for BHI_CTRL because the kernel doesn't use it
> at all.
> 
> BHI_CTRL is enumerated by CPUID.7.2.EDX[4]. If supported, BHI_DIS_S (bit
> 10) of IA32_SPEC_CTRL MSR can be used to enable BHI_DIS_S behavior.
> 
> Note that KVM does not intercept guests' IA32_SPEC_CTRL MSR accesses
> after a non-zero is written to the MSR. Therefore, guests can already
> toggle the BHI_DIS_S bit if the host supports BHI_CTRL, and no extra
> code is needed to allow guests to toggle the bit.

Same as Patch 2, please first fix virtualization of MSR_IA32_SPEC_CTRL. 
Otherwise the this patch makes no sense. E.g, if only 
X86_FEATURE_BHI_CTRL is exposed to guest without any other CPUID bits 
related to MSR_IA32_SPEC_CTRL, then guest cannot write 
MSR_IA32_SPEC_CTRL at all.

> Signed-off-by: Zhang Chen <chen.zhang@...el.com>
> Signed-off-by: Chao Gao <chao.gao@...el.com>
> Tested-by: Jiaan Lu <jiaan.lu@...el.com>
> ---
>   arch/x86/kvm/cpuid.c         | 2 +-
>   arch/x86/kvm/reverse_cpuid.h | 1 +
>   2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index f024c3ac2203..7cdd859d09a2 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -686,7 +686,7 @@ void kvm_set_cpu_caps(void)
>   	);
>   
>   	kvm_cpu_cap_init_kvm_defined(CPUID_7_2_EDX,
> -		SF(RRSBA_CTRL)
> +		SF(RRSBA_CTRL) | F(BHI_CTRL)
>   	);
>   
>   	kvm_cpu_cap_mask(CPUID_8000_0001_ECX,
> diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
> index 72bad8314a9c..e7e70c9aa384 100644
> --- a/arch/x86/kvm/reverse_cpuid.h
> +++ b/arch/x86/kvm/reverse_cpuid.h
> @@ -50,6 +50,7 @@ enum kvm_only_cpuid_leafs {
>   
>   /* Intel-defined sub-features, CPUID level 0x00000007:2 (EDX) */
>   #define KVM_X86_FEATURE_RRSBA_CTRL	KVM_X86_FEATURE(CPUID_7_2_EDX, 2)
> +#define X86_FEATURE_BHI_CTRL		KVM_X86_FEATURE(CPUID_7_2_EDX, 4)
>   
>   struct cpuid_reg {
>   	u32 function;