Commit bfbcc81b authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
Browse files

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: default avatarSean Christopherson <seanjc@google.com>
Message-Id: <20220608224516.3788274-2-seanjc@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent ff81a90f

Documentation/virt/kvm/api.rst

+13 −0
Original line number Diff line number Diff line
@@ -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

arch/x86/include/asm/kvm_host.h

+2 −1
Original line number Diff line number Diff line
@@ -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 */

arch/x86/include/uapi/asm/kvm.h

+1 −0
Original line number Diff line number Diff line
@@ -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

arch/x86/kvm/x86.c

+19 −11
Original line number Diff line number Diff line
@@ -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;