linux-kernel - Re: [PATCH v12 22/24] KVM: nVMX: Enable CET support for nested guest

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Message-ID: <aKSx4gisPrcHdq1h@google.com>
Date: Tue, 19 Aug 2025 10:18:26 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Chao Gao <chao.gao@...el.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, mlevitsk@...hat.com, 
	rick.p.edgecombe@...el.com, weijiang.yang@...el.com, xin@...or.com, 
	Mathias Krause <minipli@...ecurity.net>, John Allen <john.allen@....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>
Subject: Re: [PATCH v12 22/24] KVM: nVMX: Enable CET support for nested guest

On Mon, Aug 11, 2025, Chao Gao wrote:
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 618cc6c6425c..f20f205c6560 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -721,6 +721,27 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
>  	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
>  					 MSR_IA32_MPERF, MSR_TYPE_R);
>  
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
> +					 MSR_IA32_U_CET, MSR_TYPE_RW);
> +
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
> +					 MSR_IA32_S_CET, MSR_TYPE_RW);
> +
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
> +					 MSR_IA32_PL0_SSP, MSR_TYPE_RW);
> +
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
> +					 MSR_IA32_PL1_SSP, MSR_TYPE_RW);
> +
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
> +					 MSR_IA32_PL2_SSP, MSR_TYPE_RW);
> +
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
> +					 MSR_IA32_PL3_SSP, MSR_TYPE_RW);
> +
> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
> +					 MSR_IA32_INT_SSP_TAB, MSR_TYPE_RW);

As with the "regular" support, I don't think MSR_IA32_INT_SSP_TAB needs to be
passed through (though it's entirely possible I'm missing/forgetting something).

> +
>  	kvm_vcpu_unmap(vcpu, &map);
>  
>  	vmx->nested.force_msr_bitmap_recalc = false;
> @@ -2521,6 +2542,32 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct loaded_vmcs *vmcs0
>  	}
>  }
>  
> +static inline

Do not use "inline" for functions that are visible only to the local compilation
unit.  "inline" is just a hint, and modern compilers are smart enough to inline
functions when appropriate without a hint.

A longer explanation/rant here: https://lore.kernel.org/all/ZAdfX+S323JVWNZC@google.com


> void cet_vmcs_fields_get(struct kvm_vcpu *vcpu, u64 *s_cet,

I don't love the names, as it's not super clear what they are doing.  How about:

  vmcs_{read,write}_cet_state()

I like that vmcs_{read,write} matches the vmcs_{read,write}{l,16,32,64} helpers
(makes it clear they are reading/writing VMCS fields), and the cet_state connects
the dots to VM_EXIT_LOAD_CET_STATE (helps clarify which CET fields are accessed).