On Wed, May 07, 2014 at 02:10:59PM -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 receiving invalid opcode faults after
> failing 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, preventing compliant guests from ever using them inadvertently.
>
> Signed-off-by: Gabriel L. Somlo <somlo@xxxxxxx>
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.
> ---
>
> On Wed, May 07, 2014 at 05:30:47PM +0200, Paolo Bonzini wrote:
> > Il 07/05/2014 17:05, Michael S. Tsirkin ha scritto:
> > >>> 2. Emulate monitor and mwait as "nop", but continue to claim they are
> > >>> not supported via CPUID. That's the patch you cited. Not sure
> > >>> though whether that sort of "undocumented" functionality would be
> > >>> OK with the KVM crowd, though :)
> > >I'd go for this one. It seems unlikely a guest wants to get
> > >an exception intentionally.
> > >Paolo?
> >
> > That's okay, but please add a printk_once the first time mwait is called.
>
> OK, here's a first pass at an official submission. I have two questions:
>
> 1. I can't test svm.c (on AMD). As such, I'm not sure the
> skip_emulated_instruction() call in my own version of nop_interception()
> is necessary. If not, I could probably just call the already existing
> nop_on_interception() (line 1926 or thereabouts in svm.c), which
> just returns returns 1 without skipping anything.
>
> 2. I get "defined but not used" warnings on invalid_op_interception() (svm.c)
> and handle_invalid_op() (vmx.c). Apparently monitor/mwait are currently
> the only VM exit reasons which lead to an "invalid opcode" exception.
> Should my patch just nuke those functions (so that if anyone needs them
> in the future they'd have to re-add them), or comment them out, or
> call them after the "return 1;" statement in the monitor/mwait functions
> to shut up gcc, or ??? :)
>
> Thanks much,
> Gabriel
>
> arch/x86/kvm/cpuid.c | 2 ++
> arch/x86/kvm/svm.c | 22 ++++++++++++++++++++--
> arch/x86/kvm/vmx.c | 22 ++++++++++++++++++++--
> 3 files changed, 42 insertions(+), 4 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..1976488 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -3287,6 +3287,24 @@ 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 +3362,8 @@ 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..060b384 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -5669,6 +5669,24 @@ static int handle_pause(struct kvm_vcpu *vcpu)
> return 1;
> }
>
> +static int handle_nop(struct kvm_vcpu *vcpu)
> +{
> + 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);
> +}
> +
> static int handle_invalid_op(struct kvm_vcpu *vcpu)
> {
> kvm_queue_exception(vcpu, UD_VECTOR);
> @@ -6571,8 +6589,8 @@ 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
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