Re: [PATCH 01/12] KVM: x86: Add a framework for enabling KVM-governed x86 features

[Sean Christopherson <seanjc@google.com>]

On Fri, Jun 30, 2023, Chao Gao wrote:
> On Fri, Feb 17, 2023 at 03:10:11PM -0800, Sean Christopherson wrote:
> >+static __always_inline void kvm_governed_feature_set(struct kvm_vcpu *vcpu,
> >+						     unsigned int x86_feature)
> >+{
> >+	BUILD_BUG_ON(KVM_NR_GOVERNED_FEATURES >
> >+		     sizeof(vcpu->arch.governed_features.enabled) * BITS_PER_BYTE);
> >+
> >+	vcpu->arch.governed_features.enabled |= kvm_governed_feature_bit(x86_feature);
> >+}
> >+
> >+static __always_inline void kvm_governed_feature_check_and_set(struct kvm_vcpu *vcpu,
> >+							       unsigned int x86_feature)
> >+{
> >+	if (guest_cpuid_has(vcpu, x86_feature))
> 
> Most callers in this series are conditional on either boot_cpu_has() or some
> local variables. Can we convert them to kvm_cpu_cap_has() and incorporate them
> within this function? i.e.,
> 
> 	if (kvm_cpu_cap_has(x86_feature) && guest_cpuid_has(vcpu, x86_feature))

Hmm, I was going to say "no", as most callers don't check kvm_cpu_cap_has() verbatim,
but it doesn't have to be that way.   The majority of SVM features factor in module
params, but KVM should set the kvm_cpu capability if and only if a feature is supported
in hardware *and* enabled by its module param.

And arguably that's kinda sorta a bug fix, because this

	if (lbrv)
		kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_LBRV);

technically should be 

	if (lbrv && nested)
		kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_LBRV);	

Heh, and it's kinda sorta a bug fix for XSAVES on VMX, because this

	if (cpu_has_vmx_xsaves() && boot_cpu_has(X86_FEATURE_XSAVE) &&
	    guest_cpuid_has(vcpu, X86_FEATURE_XSAVE))
		kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_XSAVES);

should technically be

	if (kvm_cpu_cap_has(X86_FEATURE_XSAVES) &&
	    boot_cpu_has(X86_FEATURE_XSAVE) &&
	    guest_cpuid_has(vcpu, X86_FEATURE_XSAVE))
		kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_XSAVES);

> The benefits of doing so are
> 1. callers needn't repeat
> 
> 	if (kvm_cpu_cap_has(x86_feature))
> 		kvm_governed_feature_check_and_set(x86_feature)
> 
> 2. this fits the idea better that guests can use a governed feature only if host
>    supports it _and_ QEMU exposes it to the guest.

Agreed, especially since we'll still have kvm_governed_feature_set() for the
extra special cases.

Thanks for the input!