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KVM: x86: Add a quirk for KVM's "MONITOR/MWAIT are NOPs!" behavior

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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>
This commit is contained in:
Sean Christopherson 2022-06-08 22:45:12 +00:00 committed by Paolo Bonzini
parent ff81a90f45
commit bfbcc81bb8
4 changed files with 35 additions and 12 deletions
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13
Documentation/virt/kvm/api.rst
View File

@ -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

3
arch/x86/include/asm/kvm_host.h
View File

@ -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 */

1
arch/x86/include/uapi/asm/kvm.h
View File

@ -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

30
arch/x86/kvm/x86.c
View File

@ -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);
@ -2050,10 +2043,25 @@ 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);
@ -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;