[PATCH 1/5] KVM: x86: Add a quirk for KVM's "MONITOR/MWAIT are NOPs!" behavior

[Sean Christopherson <seanjc@google.com>]

Add a quirk for KVM's behavior of emulating intercepted MONITOR/MWAIT
instructions a NOPs regardless of whether or not they are supported in
guest CPUID.  KVM's current behavior was likely motiviated by a certain
fruity operating system that expects MONITOR/MWAIT to be supported
unconditionally and blindly executes MONITOR/MWAIT without first checking
CPUID.  And because KVM does NOT advertise MONITOR/MWAIT to userspace,
that's effectively the default setup for any VMM that regurgitates
KVM_GET_SUPPORTED_CPUID to KVM_SET_CPUID2.

Note, this quirk interacts with KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT.  The
behavior is actually desirable, as userspace VMMs that want to
unconditionally hide MONITOR/MWAIT from the guest can leave the
MISC_ENABLE quirk enabled.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 Documentation/virt/kvm/api.rst  | 13 +++++++++++++
 arch/x86/include/asm/kvm_host.h |  3 ++-
 arch/x86/include/uapi/asm/kvm.h |  1 +
 arch/x86/kvm/x86.c              | 26 +++++++++++++++++---------
 4 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 42a1984fafc8..5c6cd8f7975f 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7374,6 +7374,19 @@ The valid bits in cap.args[0] are:
                                     hypercall instructions. Executing the
                                     incorrect hypercall instruction will
                                     generate a #UD within the guest.
+
+KVM_X86_QUIRK_MWAIT_NEVER_FAULTS    By default, KVM emulates MONITOR/MWAIT (if
+                                    they are intercepted) as NOPs regardless of
+                                    whether or not MONITOR/MWAIT are supported
+                                    according to guest CPUID.  When this quirk
+                                    is disabled and KVM_X86_DISABLE_EXITS_MWAIT
+                                    is not set (MONITOR/MWAIT are intercepted),
+                                    KVM will inject a #UD on MONITOR/MWAIT if
+                                    they're unsupported per guest CPUID.  Note,
+                                    KVM will modify MONITOR/MWAIT support in
+                                    guest CPUID on writes to MISC_ENABLE if
+                                    KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT is
+                                    disabled.
 =================================== ============================================
 
 7.32 KVM_CAP_MAX_VCPU_ID
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 6cf5d77d7896..bc3e85a81f41 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2011,6 +2011,7 @@ int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages);
 	 KVM_X86_QUIRK_LAPIC_MMIO_HOLE |	\
 	 KVM_X86_QUIRK_OUT_7E_INC_RIP |		\
 	 KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT |	\
-	 KVM_X86_QUIRK_FIX_HYPERCALL_INSN)
+	 KVM_X86_QUIRK_FIX_HYPERCALL_INSN |	\
+	 KVM_X86_QUIRK_MWAIT_NEVER_FAULTS)
 
 #endif /* _ASM_X86_KVM_HOST_H */
diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 24c807c8d5f7..b7c92844550b 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -438,6 +438,7 @@ struct kvm_sync_regs {
 #define KVM_X86_QUIRK_OUT_7E_INC_RIP		(1 << 3)
 #define KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT	(1 << 4)
 #define KVM_X86_QUIRK_FIX_HYPERCALL_INSN	(1 << 5)
+#define KVM_X86_QUIRK_MWAIT_NEVER_FAULTS	(1 << 6)
 
 #define KVM_STATE_NESTED_FORMAT_VMX	0
 #define KVM_STATE_NESTED_FORMAT_SVM	1
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2db6f0373fa3..14abcd5c2714 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2034,13 +2034,6 @@ int kvm_emulate_invd(struct kvm_vcpu *vcpu)
 }
 EXPORT_SYMBOL_GPL(kvm_emulate_invd);
 
-int kvm_emulate_mwait(struct kvm_vcpu *vcpu)
-{
-	pr_warn_once("kvm: MWAIT instruction emulated as NOP!\n");
-	return kvm_emulate_as_nop(vcpu);
-}
-EXPORT_SYMBOL_GPL(kvm_emulate_mwait);
-
 int kvm_handle_invalid_op(struct kvm_vcpu *vcpu)
 {
 	kvm_queue_exception(vcpu, UD_VECTOR);
@@ -2048,10 +2041,25 @@ int kvm_handle_invalid_op(struct kvm_vcpu *vcpu)
 }
 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_FAULTS) &&
+	    !guest_cpuid_has(vcpu, X86_FEATURE_MWAIT))
+		return kvm_handle_invalid_op(vcpu);
+
+	pr_warn_once("kvm: %s instruction emulated as NOP!\n", insn);
+	return kvm_emulate_as_nop(vcpu);
+}
+int kvm_emulate_mwait(struct kvm_vcpu *vcpu)
+{
+	return kvm_emulate_monitor_mwait(vcpu, "MWAIT");
+}
+EXPORT_SYMBOL_GPL(kvm_emulate_mwait);
+
 int kvm_emulate_monitor(struct kvm_vcpu *vcpu)
 {
-	pr_warn_once("kvm: MONITOR instruction emulated as NOP!\n");
-	return kvm_emulate_as_nop(vcpu);
+	return kvm_emulate_monitor_mwait(vcpu, "MONITOR");
 }
 EXPORT_SYMBOL_GPL(kvm_emulate_monitor);
 
-- 
2.36.1.255.ge46751e96f-goog