Re: [PATCH v2] kvm: x86: emulate monitor and mwait instructions as nop

Alexander Graf
2014-06-02T19:48:27Z

> Am 02.06.2014 um 21:25 schrieb "Gabriel L. Somlo" <gsomlo@gmail.com>:
>
>> On Wed, May 07, 2014 at 04:52:13PM -0400, Gabriel L. Somlo wrote:
>> Treat monitor and mwait instructions as nop, which is architecturally
>> correct (but inefficient) behavior. We do this to prevent misbehaving
>> guests (e.g. OS X <= 10.7) from crashing after they fail to check for
>> monitor/mwait availability via cpuid.
>>
>> Since mwait-based idle loops relying on these nop-emulated instructions
>> would keep the host CPU pegged at 100%, do NOT advertise their presence
>> via cpuid, to prevent compliant guests from using them inadvertently.
>>
>> Signed-off-by: Gabriel L. Somlo <somlo@cmu.edu>
>> ---
>>
>> New in v2: remove invalid_op handler functions which were only used to
>> handle exits caused by monitor and mwait
>>
>>> On Wed, May 07, 2014 at 08:31:27PM +0200, Alexander Graf wrote:
>>>> On 05/07/2014 08:15 PM, Michael S. Tsirkin wrote:
>>>> If we really want to be paranoid and worry about guests
>>>> that use this strange way to trigger invalid opcode,
>>>> we can make it possible for userspace to enable/disable
>>>> this hack, and teach qemu to set it.
>>>>
>>>> That would make it even safer than it was.
>>>>
>>>> Not sure it's worth it, just a thought.
>>>
>>> Since we don't trap on non-exposed other instructions (new SSE and
>>> whatdoiknow) I don't think it's really bad to just expose
>>> MONITOR/MWAIT as nops.
>
> Would it make sense to make this a module parameter,
> (e.g., "int emulate_mwait") ?
>
> Default would be 0 (no emulation). 1 would mean "emulate as nop", and
> if anyone ever figures out how to do proper page-locking based
> emulation we could use 2 to enable that, etc. ?
>
> Not sure we'd want qemu to enable/disable it automatically, though...
>
> What do you all think ?

I don't like module parameters - they're system global and there's a good chance you want to run non-osx in parallel ;).

I'd either link this to the cpuid bits or enable it forcefully through ENABLE_CAP per vcpu.

Alex

>
> Thanks,
> --Gabriel
>
>>
>> So AFAICT, linux prefers to use mwait for idling if cpuid tells it that
>> it's available. If we keep telling everyone that we do NOT have monitor
>> and mwait available, compliant guests will never end up using them, and
>> this hack would remain completely invisible to them, which is good
>> (better to use hlt-based idle loops when you're a vm guest, that would
>> actually allow the host to relax while you're halted :)
>>
>> So the only time anyone would be able to tell we have this hack would be
>> when they're about to receive an invalid opcode for using monitor/mwait
>> in violation of what CPUID (would have) told them. That's what happens
>> to OS X prior to 10.8, which is when I'm hypothesizing the Apple devs
>> begain to seriously think about their OS running as a vm guest (on fusion
>> and parallels)...
>>
>> Instead of killing the misbehaving guest with an invalid opcode, we'd
>> allow them to peg the host CPU with their monitor == mwait == nop idle
>> loop instead, which, at least on OS X, should be tolerable long enough
>> to run 'rm -rf System/Library/Extensions/AppleIntelCPUPowerManagement.kext'
>> and reboot the guest, after which things would settle down by reverting
>> the guest to a hlt-based idle loop.
>>
>> The only reason I can think of to add functionality for enabling/disabling
>> this hack would be to protect against a malicious guest which would use
>> mwait *on purpose* to peg the host CPU. But a malicious guest could just
>> run "for(;;);" in ring 0 and accomplish the same goal, so we wouldn't
>> really gain anything in exchange for the added complexity...
>>
>> Thanks,
>> Gabriel
>>
>> arch/x86/kvm/cpuid.c | 2 ++
>> arch/x86/kvm/svm.c | 28 ++++++++++++++++++++--------
>> arch/x86/kvm/vmx.c | 20 ++++++++++++++++----
>> 3 files changed, 38 insertions(+), 12 deletions(-)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index f47a104..d094fc6 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -283,6 +283,8 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>> 0 /* Reserved */ | f_lm | F(3DNOWEXT) | F(3DNOW);
>> /* cpuid 1.ecx */
>> const u32 kvm_supported_word4_x86_features =
>> + /* NOTE: MONITOR (and MWAIT) are emulated as NOP,
>> + * but *not* advertised to guests via CPUID ! */
>> F(XMM3) | F(PCLMULQDQ) | 0 /* DTES64, MONITOR */ |
>> 0 /* DS-CPL, VMX, SMX, EST */ |
>> 0 /* TM2 */ | F(SSSE3) | 0 /* CNXT-ID */ | 0 /* Reserved */ |
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> index 7f4f9c2..0b7d58d 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -2770,12 +2770,6 @@ static int xsetbv_interception(struct vcpu_svm *svm)
>> return 1;
>> }
>>
>> -static int invalid_op_interception(struct vcpu_svm *svm)
>> -{
>> - kvm_queue_exception(&svm->vcpu, UD_VECTOR);
>> - return 1;
>> -}
>> -
>> static int task_switch_interception(struct vcpu_svm *svm)
>> {
>> u16 tss_selector;
>> @@ -3287,6 +3281,24 @@ static int pause_interception(struct vcpu_svm *svm)
>> return 1;
>> }
>>
>> +static int nop_interception(struct vcpu_svm *svm)
>> +{
>> + skip_emulated_instruction(&(svm->vcpu));
>> + return 1;
>> +}
>> +
>> +static int monitor_interception(struct vcpu_svm *svm)
>> +{
>> + printk_once(KERN_WARNING "kvm: MONITOR instruction emulated as NOP!\n");
>> + return nop_interception(svm);
>> +}
>> +
>> +static int mwait_interception(struct vcpu_svm *svm)
>> +{
>> + printk_once(KERN_WARNING "kvm: MWAIT instruction emulated as NOP!\n");
>> + return nop_interception(svm);
>> +}
>> +
>> static int (*const svm_exit_handlers[])(struct vcpu_svm *svm) = {
>> [SVM_EXIT_READ_CR0] = cr_interception,
>> [SVM_EXIT_READ_CR3] = cr_interception,
>> @@ -3344,8 +3356,8 @@ static int (*const svm_exit_handlers[])(struct vcpu_svm *svm) = {
>> [SVM_EXIT_CLGI] = clgi_interception,
>> [SVM_EXIT_SKINIT] = skinit_interception,
>> [SVM_EXIT_WBINVD] = emulate_on_interception,
>> - [SVM_EXIT_MONITOR] = invalid_op_interception,
>> - [SVM_EXIT_MWAIT] = invalid_op_interception,
>> + [SVM_EXIT_MONITOR] = monitor_interception,
>> + [SVM_EXIT_MWAIT] = mwait_interception,
>> [SVM_EXIT_XSETBV] = xsetbv_interception,
>> [SVM_EXIT_NPF] = pf_interception,
>> };
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 33e8c02..3ccbcb1 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -5669,12 +5669,24 @@ static int handle_pause(struct kvm_vcpu *vcpu)
>> return 1;
>> }
>>
>> -static int handle_invalid_op(struct kvm_vcpu *vcpu)
>> +static int handle_nop(struct kvm_vcpu *vcpu)
>> {
>> - kvm_queue_exception(vcpu, UD_VECTOR);
>> + skip_emulated_instruction(vcpu);
>> return 1;
>> }
>>
>> +static int handle_mwait(struct kvm_vcpu *vcpu)
>> +{
>> + printk_once(KERN_WARNING "kvm: MWAIT instruction emulated as NOP!\n");
>> + return handle_nop(vcpu);
>> +}
>> +
>> +static int handle_monitor(struct kvm_vcpu *vcpu)
>> +{
>> + printk_once(KERN_WARNING "kvm: MONITOR instruction emulated as NOP!\n");
>> + return handle_nop(vcpu);
>> +}
>> +
>> /*
>> * To run an L2 guest, we need a vmcs02 based on the L1-specified vmcs12.
>> * We could reuse a single VMCS for all the L2 guests, but we also want the
>> @@ -6571,8 +6583,8 @@ static int (*const kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu) = {
>> [EXIT_REASON_EPT_VIOLATION] = handle_ept_violation,
>> [EXIT_REASON_EPT_MISCONFIG] = handle_ept_misconfig,
>> [EXIT_REASON_PAUSE_INSTRUCTION] = handle_pause,
>> - [EXIT_REASON_MWAIT_INSTRUCTION] = handle_invalid_op,
>> - [EXIT_REASON_MONITOR_INSTRUCTION] = handle_invalid_op,
>> + [EXIT_REASON_MWAIT_INSTRUCTION] = handle_mwait,
>> + [EXIT_REASON_MONITOR_INSTRUCTION] = handle_monitor,
>> [EXIT_REASON_INVEPT] = handle_invept,
>> };
>>
>> --
>> 1.9.0
>>