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

Michael S. Tsirkin
2014-06-02T20:20:19Z

On Mon, Jun 02, 2014 at 09:48:19PM +0200, Alexander Graf wrote:
>
>
> > 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

Point is that.
Paolo here thinks it's safe to just make it a NOP unconditionally.
so module parameter would be there as a debugging tool:
as a means for users to test with old kvm behaviour if they see breakage.
Which we don't expect, so no need to waste cycles creating a pretty
interface for it.

> >
> > 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
> >>