Re: [PATCH v2 40/49] KVM: x86: Initialize guest cpu_caps based on KVM support

[Maxim Levitsky <mlevitsk@redhat.com>]

On Fri, 2024-05-17 at 10:39 -0700, Sean Christopherson wrote:
> Constrain all guest cpu_caps based on KVM support instead of constraining
> only the few features that KVM _currently_ needs to verify are actually
> supported by KVM.  The intent of cpu_caps is to track what the guest is
> actually capable of using, not the raw, unfiltered CPUID values that the
> guest sees.
> 
> I.e. KVM should always consult it's only support when making decisions
> based on guest CPUID, and the only reason KVM has historically made the
> checks opt-in was due to lack of centralized tracking.
> 
> Suggested-by: Maxim Levitsky <mlevitsk@redhat.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/cpuid.c   | 14 +++++++++++++-
>  arch/x86/kvm/cpuid.h   |  7 -------
>  arch/x86/kvm/svm/svm.c | 11 -----------
>  arch/x86/kvm/vmx/vmx.c |  9 ++-------
>  4 files changed, 15 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index d1849fe874ab..8ada1cac8fcb 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -403,6 +403,8 @@ static u32 cpuid_get_reg_unsafe(struct kvm_cpuid_entry2 *entry, u32 reg)
>  	}
>  }
>  
> +static int cpuid_func_emulated(struct kvm_cpuid_entry2 *entry, u32 func);
> +
>  void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_lapic *apic = vcpu->arch.apic;
> @@ -421,6 +423,7 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  	 */
>  	for (i = 0; i < NR_KVM_CPU_CAPS; i++) {
>  		const struct cpuid_reg cpuid = reverse_cpuid[i];
> +		struct kvm_cpuid_entry2 emulated;
>  
>  		if (!cpuid.function)
>  			continue;
> @@ -429,7 +432,16 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  		if (!entry)
>  			continue;
>  
> -		vcpu->arch.cpu_caps[i] = cpuid_get_reg_unsafe(entry, cpuid.reg);
> +		cpuid_func_emulated(&emulated, cpuid.function);
> +
> +		/*
> +		 * A vCPU has a feature if it's supported by KVM and is enabled
> +		 * in guest CPUID.  Note, this includes features that are
> +		 * supported by KVM but aren't advertised to userspace!
> +		 */
> +		vcpu->arch.cpu_caps[i] = kvm_cpu_caps[i] | kvm_vmm_cpu_caps[i] |
> +					 cpuid_get_reg_unsafe(&emulated, cpuid.reg);
> +		vcpu->arch.cpu_caps[i] &= cpuid_get_reg_unsafe(entry, cpuid.reg);

Hi,

I have an idea. What if we get rid of kvm_vmm_cpu_caps, and instead advertise the
MWAIT in KVM_GET_EMULATED_CPUID?

MWAIT is sort of emulated as NOP after all, plus features in KVM_GET_EMULATED_CPUID are
sort of 'emulated inefficiently' and you can say that NOP is an inefficient emulation
of MWAIT sort of.

It just feels to me that kvm_vmm_cpu_caps, is somewhat an overkill, and its name is
somewhat confusing.

Other than that this code looks good.


>  	}
>  
>  	kvm_update_cpuid_runtime(vcpu);
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index c2c2b8aa347b..60da304db4e4 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -284,13 +284,6 @@ static __always_inline void guest_cpu_cap_change(struct kvm_vcpu *vcpu,
>  		guest_cpu_cap_clear(vcpu, x86_feature);
>  }
>  
> -static __always_inline void guest_cpu_cap_constrain(struct kvm_vcpu *vcpu,
> -						    unsigned int x86_feature)
> -{
> -	if (!kvm_cpu_cap_has(x86_feature))
> -		guest_cpu_cap_clear(vcpu, x86_feature);
> -}
> -
>  static __always_inline bool guest_cpu_cap_has(struct kvm_vcpu *vcpu,
>  					      unsigned int x86_feature)
>  {
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 1bc431a7e862..946a75771946 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4344,10 +4344,6 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  			     boot_cpu_has(X86_FEATURE_XSAVES) &&
>  			     guest_cpuid_has(vcpu, X86_FEATURE_XSAVE));
>  
> -	guest_cpu_cap_constrain(vcpu, X86_FEATURE_NRIPS);
> -	guest_cpu_cap_constrain(vcpu, X86_FEATURE_TSCRATEMSR);
> -	guest_cpu_cap_constrain(vcpu, X86_FEATURE_LBRV);
> -
>  	/*
>  	 * Intercept VMLOAD if the vCPU mode is Intel in order to emulate that
>  	 * VMLOAD drops bits 63:32 of SYSENTER (ignoring the fact that exposing
> @@ -4355,13 +4351,6 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  	 */
>  	if (guest_cpuid_is_intel(vcpu))
>  		guest_cpu_cap_clear(vcpu, X86_FEATURE_V_VMSAVE_VMLOAD);
> -	else
> -		guest_cpu_cap_constrain(vcpu, X86_FEATURE_V_VMSAVE_VMLOAD);
> -
> -	guest_cpu_cap_constrain(vcpu, X86_FEATURE_PAUSEFILTER);
> -	guest_cpu_cap_constrain(vcpu, X86_FEATURE_PFTHRESHOLD);
> -	guest_cpu_cap_constrain(vcpu, X86_FEATURE_VGIF);
> -	guest_cpu_cap_constrain(vcpu, X86_FEATURE_VNMI);

Finally, this code is gone.

>  
>  	svm_recalc_instruction_intercepts(vcpu, svm);
>  
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index d873386e1473..653c4b68ec7f 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7836,15 +7836,10 @@ void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  	 * to the guest.  XSAVES depends on CR4.OSXSAVE, and CR4.OSXSAVE can be
>  	 * set if and only if XSAVE is supported.
>  	 */
> -	if (boot_cpu_has(X86_FEATURE_XSAVE) &&
> -	    guest_cpuid_has(vcpu, X86_FEATURE_XSAVE))
> -		guest_cpu_cap_constrain(vcpu, X86_FEATURE_XSAVES);
> -	else
> +	if (!boot_cpu_has(X86_FEATURE_XSAVE) ||
> +	    !guest_cpuid_has(vcpu, X86_FEATURE_XSAVE))
>  		guest_cpu_cap_clear(vcpu, X86_FEATURE_XSAVES);
>  
> -	guest_cpu_cap_constrain(vcpu, X86_FEATURE_VMX);
> -	guest_cpu_cap_constrain(vcpu, X86_FEATURE_LAM);

Good riddance!
> -
>  	vmx_setup_uret_msrs(vmx);
>  
>  	if (cpu_has_secondary_exec_ctrls())


Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
	Maxim Levitsky