linux-kernel - [PATCH v2 44/49] KVM: x86: Update guest cpu_caps at runtime for dynamic CPUID-based features

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Date: Fri, 17 May 2024 10:39:21 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>, 
	Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Hou Wenlong <houwenlong.hwl@...group.com>, Kechen Lu <kechenl@...dia.com>, 
	Oliver Upton <oliver.upton@...ux.dev>, Maxim Levitsky <mlevitsk@...hat.com>, 
	Binbin Wu <binbin.wu@...ux.intel.com>, Yang Weijiang <weijiang.yang@...el.com>, 
	Robert Hoo <robert.hoo.linux@...il.com>
Subject: [PATCH v2 44/49] KVM: x86: Update guest cpu_caps at runtime for
 dynamic CPUID-based features

When updating guest CPUID entries to emulate runtime behavior, e.g. when
the guest enables a CR4-based feature that is tied to a CPUID flag, also
update the vCPU's cpu_caps accordingly.  This will allow replacing all
usage of guest_cpuid_has() with guest_cpu_cap_has().

Note, this relies on kvm_set_cpuid() taking a snapshot of cpu_caps before
invoking kvm_update_cpuid_runtime(), i.e. when KVM is updating CPUID
entries that *may* become the vCPU's CPUID, so that unwinding to the old
cpu_caps is possible if userspace tries to set bogus CPUID information.

Note #2, none of the features in question use guest_cpu_cap_has() at this
time, i.e. aside from settings bits in cpu_caps, this is a glorified nop.

Cc: Yang Weijiang <weijiang.yang@...el.com>
Cc: Robert Hoo <robert.hoo.linux@...il.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/cpuid.c | 28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 552e65ba5efa..1424a9d4eb17 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -330,28 +330,38 @@ static u64 cpuid_get_supported_xcr0(struct kvm_vcpu *vcpu)
 	return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
 }
 
+static __always_inline void kvm_update_feature_runtime(struct kvm_vcpu *vcpu,
+						       struct kvm_cpuid_entry2 *entry,
+						       unsigned int x86_feature,
+						       bool has_feature)
+{
+	cpuid_entry_change(entry, x86_feature, has_feature);
+	guest_cpu_cap_change(vcpu, x86_feature, has_feature);
+}
+
 void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
 {
 	struct kvm_cpuid_entry2 *best;
 
 	best = kvm_find_cpuid_entry(vcpu, 1);
 	if (best) {
-		cpuid_entry_change(best, X86_FEATURE_OSXSAVE,
-				   kvm_is_cr4_bit_set(vcpu, X86_CR4_OSXSAVE));
+		kvm_update_feature_runtime(vcpu, best, X86_FEATURE_OSXSAVE,
+					   kvm_is_cr4_bit_set(vcpu, X86_CR4_OSXSAVE));
 
-		cpuid_entry_change(best, X86_FEATURE_APIC,
-			   vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE);
+		kvm_update_feature_runtime(vcpu, best, X86_FEATURE_APIC,
+					   vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE);
 
 		if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT))
-			cpuid_entry_change(best, X86_FEATURE_MWAIT,
-					   vcpu->arch.ia32_misc_enable_msr &
-					   MSR_IA32_MISC_ENABLE_MWAIT);
+			kvm_update_feature_runtime(vcpu, best, X86_FEATURE_MWAIT,
+						   vcpu->arch.ia32_misc_enable_msr &
+						   MSR_IA32_MISC_ENABLE_MWAIT);
 	}
 
 	best = kvm_find_cpuid_entry_index(vcpu, 7, 0);
 	if (best)
-		cpuid_entry_change(best, X86_FEATURE_OSPKE,
-				   kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE));
+		kvm_update_feature_runtime(vcpu, best, X86_FEATURE_OSPKE,
+					   kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE));
+
 
 	best = kvm_find_cpuid_entry_index(vcpu, 0xD, 0);
 	if (best)
-- 
2.45.0.215.g3402c0e53f-goog