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

Sean Christopherson
2022-06-08T22:45:30Z

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