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 16:34:54 -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:
> @@ -665,7 +665,7 @@ void kvm_set_cpu_caps(void)
>  		F(AVX512_VPOPCNTDQ) | F(UMIP) | F(AVX512_VBMI2) | F(GFNI) |
>  		F(VAES) | F(VPCLMULQDQ) | F(AVX512_VNNI) | F(AVX512_BITALG) |
>  		F(CLDEMOTE) | F(MOVDIRI) | F(MOVDIR64B) | 0 /*WAITPKG*/ |
> -		F(SGX_LC) | F(BUS_LOCK_DETECT)
> +		F(SGX_LC) | F(BUS_LOCK_DETECT) | F(SHSTK)
>  	);
>  	/* Set LA57 based on hardware capability. */
>  	if (cpuid_ecx(7) & F(LA57))
> @@ -683,7 +683,8 @@ void kvm_set_cpu_caps(void)
>  		F(SPEC_CTRL_SSBD) | F(ARCH_CAPABILITIES) | F(INTEL_STIBP) |
>  		F(MD_CLEAR) | F(AVX512_VP2INTERSECT) | F(FSRM) |
>  		F(SERIALIZE) | F(TSXLDTRK) | F(AVX512_FP16) |
> -		F(AMX_TILE) | F(AMX_INT8) | F(AMX_BF16) | F(FLUSH_L1D)
> +		F(AMX_TILE) | F(AMX_INT8) | F(AMX_BF16) | F(FLUSH_L1D) |
> +		F(IBT)
>  	);

..

> @@ -7977,6 +7993,18 @@ static __init void vmx_set_cpu_caps(void)
>  
>  	if (cpu_has_vmx_waitpkg())
>  		kvm_cpu_cap_check_and_set(X86_FEATURE_WAITPKG);
> +
> +	/*
> +	 * Disable CET if unrestricted_guest is unsupported as KVM doesn't
> +	 * enforce CET HW behaviors in emulator. On platforms with
> +	 * VMX_BASIC[bit56] == 0, inject #CP at VMX entry with error code
> +	 * fails, so disable CET in this case too.
> +	 */
> +	if (!cpu_has_load_cet_ctrl() || !enable_unrestricted_guest ||
> +	    !cpu_has_vmx_basic_no_hw_errcode()) {
> +		kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
> +		kvm_cpu_cap_clear(X86_FEATURE_IBT);
> +	}

Oh!  Almost missed it.  This patch should explicitly kvm_cpu_cap_clear()
X86_FEATURE_SHSTK and X86_FEATURE_IBT.  We *know* there are upcoming AMD CPUs
that support at least SHSTK, so enumerating support for common code would yield
a version of KVM that incorrectly advertises support for SHSTK.

I hope to land both Intel and AMD virtualization in the same kernel release, but
there are no guarantees that will happen.  And explicitly clearing both SHSTK and
IBT would guard against IBT showing up in some future AMD CPU in advance of KVM
gaining full support.