From bfbcc81bb82cbbad8bf4e40cea274f42b50674e2 Mon Sep 17 00:00:00 2001
From: Sean Christopherson <seanjc@google.com>
Date: Wed, 8 Jun 2022 22:45:12 +0000
Subject: [PATCH] KVM: x86: Add a quirk for KVM's "MONITOR/MWAIT are NOPs!"
behavior
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>
Message-Id: <20220608224516.3788274-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.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 | 30 +++++++++++++++++++-----------
4 files changed, 35 insertions(+), 12 deletions(-)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 84c486ce6279c..320cb04f7bd9f 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7522,6 +7522,19 @@ 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 1038ccb7056a3..e37727a74d0a4 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2076,6 +2076,7 @@ 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 50a4e787d5e6e..ee3896416c68a 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -439,6 +439,7 @@ 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 b70fe85608811..9b9f9f65c5484 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2036,13 +2036,6 @@ 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);
@@ -2050,11 +2043,26 @@ int kvm_handle_invalid_op(struct kvm_vcpu *vcpu)
}
EXPORT_SYMBOL_GPL(kvm_handle_invalid_op);
-int kvm_emulate_monitor(struct kvm_vcpu *vcpu)
+
+static int kvm_emulate_monitor_mwait(struct kvm_vcpu *vcpu, const char *insn)
{
- pr_warn_once("kvm: MONITOR instruction emulated as NOP!\n");
+ 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)
+{
+ return kvm_emulate_monitor_mwait(vcpu, "MONITOR");
+}
EXPORT_SYMBOL_GPL(kvm_emulate_monitor);
static inline bool kvm_vcpu_exit_request(struct kvm_vcpu *vcpu)
@@ -3884,8 +3892,8 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
return kvm_pmu_get_msr(vcpu, msr_info);
/*
- * Userspace is allowed to read MSRs that KVM reports in
- * KVM_GET_MSR_INDEX_LIST, even if an MSR isn't fully supported.
+ * Userspace is allowed to read MSRs that KVM reports as
+ * to-be-saved, even if an MSR isn't fully supported.
*/
if (!msr_info->host_initiated)
return 1;
--
2.30.2