linux-kernel - Re: [PATCH v4 03/19] KVM: VMX: Disable FRED if FRED consistency checks fail

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Message-ID: <aFrCJzodXP0sT6Ny@google.com>
Date: Tue, 24 Jun 2025 08:20:07 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: "Xin Li (Intel)" <xin@...or.com>
Cc: pbonzini@...hat.com, kvm@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, corbet@....net, tglx@...utronix.de, 
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, 
	hpa@...or.com, andrew.cooper3@...rix.com, luto@...nel.org, 
	peterz@...radead.org, chao.gao@...el.com, xin3.li@...el.com
Subject: Re: [PATCH v4 03/19] KVM: VMX: Disable FRED if FRED consistency
 checks fail

On Fri, Mar 28, 2025, Xin Li (Intel) wrote:
> From: Xin Li <xin3.li@...el.com>
> 
> Do not virtualize FRED if FRED consistency checks fail.
> 
> Either on broken hardware, or when run KVM on top of another hypervisor
> before the underlying hypervisor implements nested FRED correctly.
> 
> Suggested-by: Chao Gao <chao.gao@...el.com>
> Signed-off-by: Xin Li <xin3.li@...el.com>
> Signed-off-by: Xin Li (Intel) <xin@...or.com>
> Tested-by: Shan Kang <shan.kang@...el.com>
> Reviewed-by: Chao Gao <chao.gao@...el.com>
> ---
> 
> Change in v4:
> * Call out the reason why not check FRED VM-exit controls in
>   cpu_has_vmx_fred() (Chao Gao).
> ---
>  arch/x86/kvm/vmx/capabilities.h | 11 +++++++++++
>  arch/x86/kvm/vmx/vmx.c          |  3 +++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
> index b2aefee59395..b4f49a4690ca 100644
> --- a/arch/x86/kvm/vmx/capabilities.h
> +++ b/arch/x86/kvm/vmx/capabilities.h
> @@ -400,6 +400,17 @@ static inline bool vmx_pebs_supported(void)
>  	return boot_cpu_has(X86_FEATURE_PEBS) && kvm_pmu_cap.pebs_ept;
>  }
>  
> +static inline bool cpu_has_vmx_fred(void)
> +{
> +	/*
> +	 * setup_vmcs_config() guarantees FRED VM-entry/exit controls
> +	 * are either all set or none.  So, no need to check FRED VM-exit
> +	 * controls.
> +	 */
> +	return cpu_feature_enabled(X86_FEATURE_FRED) &&

Drop the cpu_feature_enabled().  These helpers are all about checking raw CPU
support; whether or not the kernel is configured to support FRED is irrelevant.

[For these helpers; KVM obviously needs to account for FRED support in other
 paths, but that should be automagically handled by kvm_set_cpu_caps()]


> +		(vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_FRED);
> +}
> +
>  static inline bool cpu_has_notify_vmexit(void)
>  {
>  	return vmcs_config.cpu_based_2nd_exec_ctrl &
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index e38545d0dd17..ab84939ace96 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -8052,6 +8052,9 @@ static __init void vmx_set_cpu_caps(void)
>  		kvm_cpu_cap_check_and_set(X86_FEATURE_DTES64);
>  	}
>  
> +	if (!cpu_has_vmx_fred())
> +		kvm_cpu_cap_clear(X86_FEATURE_FRED);
> +
>  	if (!enable_pmu)
>  		kvm_cpu_cap_clear(X86_FEATURE_PDCM);
>  	kvm_caps.supported_perf_cap = vmx_get_perf_capabilities();
> -- 
> 2.48.1
>