linux-kernel - Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and advertise to userspace

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Date: Wed, 1 May 2024 16:15:28 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yang Weijiang <weijiang.yang@...el.com>
Cc: pbonzini@...hat.com, dave.hansen@...el.com, x86@...nel.org, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, peterz@...radead.org, 
	chao.gao@...el.com, rick.p.edgecombe@...el.com, mlevitsk@...hat.com, 
	john.allen@....com
Subject: Re: [PATCH v10 24/27] KVM: x86: Enable CET virtualization for VMX and
 advertise to userspace

On Sun, Feb 18, 2024, Yang Weijiang wrote:
> @@ -696,6 +697,20 @@ void kvm_set_cpu_caps(void)
>  		kvm_cpu_cap_set(X86_FEATURE_INTEL_STIBP);
>  	if (boot_cpu_has(X86_FEATURE_AMD_SSBD))
>  		kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL_SSBD);
> +	/*
> +	 * Don't use boot_cpu_has() to check availability of IBT because the
> +	 * feature bit is cleared in boot_cpu_data when ibt=off is applied
> +	 * in host cmdline.

I'm not convinced this is a good reason to diverge from the host kernel.  E.g.
PCID and many other features honor the host setup, I don't see what makes IBT
special.

LA57 is special because it's entirely reasonable, likely even, for a host to
only want to use 48-bit virtual addresses, but still want to let the guest enable
LA57.

> @@ -4934,6 +4935,14 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  
>  	vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, 0);  /* 22.2.1 */
>  
> +	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
> +		vmcs_writel(GUEST_SSP, 0);
> +		vmcs_writel(GUEST_S_CET, 0);
> +		vmcs_writel(GUEST_INTR_SSP_TABLE, 0);
> +	} else if (kvm_cpu_cap_has(X86_FEATURE_IBT)) {
> +		vmcs_writel(GUEST_S_CET, 0);
> +	}

Similar to my comments about MSR interception, I think it would be better to
explicitly handle the "common" field.  At first glance, code like the above makes
it look like IBT is mutually exclusive with SHSTK, e.g. a reader that isn't
looking closely could easily miss that both paths write GUEST_S_CET.

	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
		vmcs_writel(GUEST_SSP, 0);
		vmcs_writel(GUEST_INTR_SSP_TABLE, 0); 
	}
	if (kvm_cpu_cap_has(X86_FEATURE_IBT) ||
	    kvm_cpu_cap_has(X86_FEATURE_SHSTK))
		vmcs_writel(GUEST_S_CET, 0);