linux-kernel - Re: [PATCH v2 6/7] KVM:VMX: Load Guest CET via VMCS when CET is enabled in Guest

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Date:   Fri, 25 Jan 2019 14:56:25 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, rkrcmar@...hat.com, jmattson@...gle.com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org, mst@...hat.com,
        yu-cheng.yu@...el.com, yi.z.zhang@...el.com, hjl.tools@...il.com,
        Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Subject: Re: [PATCH v2 6/7] KVM:VMX: Load Guest CET via VMCS when CET is
 enabled in Guest

On Wed, Jan 23, 2019 at 04:59:08AM +0800, Yang Weijiang wrote:
> "Load Guest CET state" bit controls whether guest CET states
> will be loaded on Guest entry. Before doing that, KVM needs
> to check if CET feature is exposed to Guest.
> 
> Signed-off-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> ---
>  arch/x86/kvm/vmx.c | 33 +++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 68c0e5e41cb1..9c8cecac80ea 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -55,6 +55,7 @@
>  #include <asm/mmu_context.h>
>  #include <asm/spec-ctrl.h>
>  #include <asm/mshyperv.h>
> +#include <asm/cet.h>
>  
>  #include "trace.h"
>  #include "pmu.h"
> @@ -4065,6 +4066,18 @@ static inline bool vmx_feature_control_msr_valid(struct kvm_vcpu *vcpu,
>  	return !(val & ~valid_bits);
>  }
>  
> +static int vmx_guest_cet_cap(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_cpuid_entry2 *best;
> +	int r = 0;
> +
> +	best = kvm_find_cpuid_entry(vcpu, 7, 0);
> +	if (best && best->function == 0x7)
> +		r = (best->ecx & bit(X86_FEATURE_SHSTK)) |
> +		    (best->edx & bit(X86_FEATURE_IBT)) ? 1 : 0;
> +	return r;
> +}
> +
>  static int vmx_get_msr_feature(struct kvm_msr_entry *msr)
>  {
>  	switch (msr->index) {
> @@ -5409,6 +5422,26 @@ static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>  			return 1;
>  	}
>  
> +	/*
> +	 * To enable Guest CET, first check if Guest CET feature is
> +	 * available, if it's not available but its CR4.CET is being set,
> +	 * return a fault to Guest; then check if Host CET is enabled and
> +	 * CR4.CET is toggled, if they are, then enable loading CET state

Comment doesn't match the code.  Comment says "toggled", code is just
looking at "enabled".

> +	 * bit in entry control, otherwise, clear the bit to
> +	 * disable guest CET state loading.

What happens to CET state if the control is clear?  Is host state
retained but inaccessible?

> +	 */
> +	if (vmx_guest_cet_cap(vcpu)) {

Why not?

	if (guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) ||
	    guest_cpuid_has(vcpu, X86_FEATURE_IBT)) {

> +		if (hw_cr4 & cr4 & X86_CR4_CET) {
> +			vmcs_set_bits(VM_ENTRY_CONTROLS,
> +				      VM_ENTRY_LOAD_GUEST_CET_STATE);
> +		} else {
> +			vmcs_clear_bits(VM_ENTRY_CONTROLS,
> +					VM_ENTRY_LOAD_GUEST_CET_STATE);
> +		}
> +	} else if (cr4 & X86_CR4_CET) {
> +		return 1;
> +	}
> +
>  	if (to_vmx(vcpu)->nested.vmxon && !nested_cr4_valid(vcpu, cr4))
>  		return 1;
>  
> -- 
> 2.17.1
>