KVM: SVM: Manually recalc all MSR intercepts on userspace MSR filter change

On a userspace MSR filter change, recalculate all MSR intercepts using the
filter-agnostic logic instead of maintaining a "shadow copy" of KVM's
desired intercepts.  The shadow bitmaps add yet another point of failure,
are confusing (e.g. what does "handled specially" mean!?!?), an eyesore,
and a maintenance burden.

Given that KVM *must* be able to recalculate the correct intercepts at any
given time, and that MSR filter updates are not hot paths, there is zero
benefit to maintaining the shadow bitmaps.

Opportunistically switch from boot_cpu_has() to cpu_feature_enabled() as
appropriate.

Link: https://lore.kernel.org/all/aCdPbZiYmtni4Bjs@google.com
Link: https://lore.kernel.org/all/20241126180253.GAZ0YNTdXH1UGeqsu6@fat_crate.local
Cc: Francesco Lavra <francescolavra.fl@gmail.com>
Link: https://lore.kernel.org/r/20250610225737.156318-20-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/sev.c

@@ -4386,9 +4386,12 @@ int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in)
 				    count, in);
 }
 
-static void sev_es_vcpu_after_set_cpuid(struct vcpu_svm *svm)
+void sev_es_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
 {
-	struct kvm_vcpu *vcpu = &svm->vcpu;
+	/* Clear intercepts on MSRs that are context switched by hardware. */
+	svm_disable_intercept_for_msr(vcpu, MSR_AMD64_SEV_ES_GHCB, MSR_TYPE_RW);
+	svm_disable_intercept_for_msr(vcpu, MSR_EFER, MSR_TYPE_RW);
+	svm_disable_intercept_for_msr(vcpu, MSR_IA32_CR_PAT, MSR_TYPE_RW);
 
 	if (boot_cpu_has(X86_FEATURE_V_TSC_AUX))
 		svm_set_intercept_for_msr(vcpu, MSR_TSC_AUX, MSR_TYPE_RW,
@@ -4423,16 +4426,12 @@ void sev_vcpu_after_set_cpuid(struct vcpu_svm *svm)
 	best = kvm_find_cpuid_entry(vcpu, 0x8000001F);
 	if (best)
 		vcpu->arch.reserved_gpa_bits &= ~(1UL << (best->ebx & 0x3f));
-
-	if (sev_es_guest(svm->vcpu.kvm))
-		sev_es_vcpu_after_set_cpuid(svm);
 }
 
 static void sev_es_init_vmcb(struct vcpu_svm *svm)
 {
 	struct kvm_sev_info *sev = to_kvm_sev_info(svm->vcpu.kvm);
 	struct vmcb *vmcb = svm->vmcb01.ptr;
-	struct kvm_vcpu *vcpu = &svm->vcpu;
 
 	svm->vmcb->control.nested_ctl |= SVM_NESTED_CTL_SEV_ES_ENABLE;
 
@@ -4486,11 +4485,6 @@ static void sev_es_init_vmcb(struct vcpu_svm *svm)
 
 	/* Can't intercept XSETBV, HV can't modify XCR0 directly */
 	svm_clr_intercept(svm, INTERCEPT_XSETBV);
-
-	/* Clear intercepts on MSRs that are context switched by hardware. */
-	svm_disable_intercept_for_msr(vcpu, MSR_AMD64_SEV_ES_GHCB, MSR_TYPE_RW);
-	svm_disable_intercept_for_msr(vcpu, MSR_EFER, MSR_TYPE_RW);
-	svm_disable_intercept_for_msr(vcpu, MSR_IA32_CR_PAT, MSR_TYPE_RW);
 }
 
 void sev_init_vmcb(struct vcpu_svm *svm)