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

Michael S. Tsirkin
2014-06-02T20:24:27Z

On Mon, Jun 02, 2014 at 03:25:30PM -0400, Gabriel L. Somlo wrote:
> 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. ?

If it's a module parameter, default should be one that's
safe for all guests, that would be 1 in the list above.

> Not sure we'd want qemu to enable/disable it automatically, though...
>
> What do you all think ?
>
> 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
> >