Re: [RFC PATCH v4 5/8] KVM:VMX: Load Guest CET via VMCS when CET is enabled in Guest

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Mon, Mar 18, 2019 at 11:03:48PM +0800, Yang Weijiang wrote:
> "Load Guest CET state" bit controls whether guest CET states
> will be loaded at Guest entry. Before doing that, KVM needs
> to check if CPU CET feature is available.

Use the changelog to describe *why* the patch does what it does, for
something this simple it's obvious *what* the patch adds.  Specifically,
there was a decent amount of discussion regarding the virtualization
hole that gets introduced when SHSTK!=IBT, and that need to be captured
in the changelog.

> 
> Signed-off-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> ---
>  arch/x86/kvm/vmx.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 28b8ac027bd7..246467c12930 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"
> @@ -5414,6 +5415,23 @@ static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>  			return 1;
>  	}
>  
> +	/*
> +	 * To enable Guest CET, check whether CPU CET feature is
> +	 * available, if it's there, set Guest CET state loading bit
> +	 * per CR4.CET status, otherwise, return a fault to Guest.
> +	 */

No need for the comment, the code is straightforward.

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