linux-kernel - [PATCH 4/5] KVM: x86: Query X86_FEATURE_MWAIT iff userspace owns the CPUID feature bit

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Message-ID: <20241211013302.1347853-5-seanjc@google.com>
Date: Tue, 10 Dec 2024 17:33:01 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Jim Mattson <jmattson@...gle.com>
Subject: [PATCH 4/5] KVM: x86: Query X86_FEATURE_MWAIT iff userspace owns the
 CPUID feature bit

Rework MONITOR/MWAIT emulation to query X86_FEATURE_MWAIT if and only if
the MISC_ENABLE_NO_MWAIT quirk is enabled, in which case MWAIT is not a
dynamic, KVM-controlled CPUID feature.  KVM's funky ABI for that quirk is
to emulate MONITOR/MWAIT as nops if userspace sets MWAIT in guest CPUID.

For the case where KVM owns the MWAIT feature bit, check MISC_ENABLES
itself, i.e. check the actual control, not its reflection in guest CPUID.

Avoiding consumption of dynamic CPUID features will allow KVM to defer
runtime CPUID updates until kvm_emulate_cpuid(), i.e. until the updates
become visible to the guest.  Alternatively, KVM could play other games
with runtime CPUID updates, e.g. by precisely specifying which feature
bits to update, but doing so adds non-trivial complexity and doesn't solve
the underlying issue of unnecessary updates causing meaningful overhead
for nested virtualization roundtrips.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/x86.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 320764e5f798..dc8829712edd 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2080,10 +2080,20 @@ EXPORT_SYMBOL_GPL(kvm_handle_invalid_op);

 static int kvm_emulate_monitor_mwait(struct kvm_vcpu *vcpu, const char *insn)
 {
-	if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MWAIT_NEVER_UD_FAULTS) &&
-	    !guest_cpu_cap_has(vcpu, X86_FEATURE_MWAIT))
+	bool enabled;
+
+	if (kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MWAIT_NEVER_UD_FAULTS))
+		goto emulate_as_nop;
+
+	if (kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT))
+		enabled = guest_cpu_cap_has(vcpu, X86_FEATURE_MWAIT);
+	else
+		enabled = vcpu->arch.ia32_misc_enable_msr & MSR_IA32_MISC_ENABLE_MWAIT;
+
+	if (!enabled)
 		return kvm_handle_invalid_op(vcpu);

+emulate_as_nop:
 	pr_warn_once("%s instruction emulated as NOP!\n", insn);
 	return kvm_emulate_as_nop(vcpu);
 }
-- 
2.47.0.338.g60cca15819-goog