Re: [RFC v2 2/2] KVM: VMX: Enable bus lock VM exit

[Vitaly Kuznetsov]

Chenyi Qiang <chenyi.qiang@xxxxxxxxx> writes:

> Virtual Machine can exploit bus locks to degrade the performance of
> system. Bus lock can be caused by split locked access to writeback(WB)
> memory or by using locks on uncacheable(UC) memory. The bus lock is
> typically >1000 cycles slower than an atomic operation within a cache
> line. It also disrupts performance on other cores (which must wait for
> the bus lock to be released before their memory operations can
> complete).
>
> To address the threat, bus lock VM exit is introduced to notify the VMM
> when a bus lock was acquired, allowing it to enforce throttling or other
> policy based mitigations.
>
> A VMM can enable VM exit due to bus locks by setting a new "Bus Lock
> Detection" VM-execution control(bit 30 of Secondary Processor-based VM
> execution controls). If delivery of this VM exit was preempted by a
> higher priority VM exit (e.g. EPT misconfiguration, EPT violation, APIC
> access VM exit, APIC write VM exit, exception bitmap exiting), bit 26 of
> exit reason in vmcs field is set to 1.
>
> In current implementation, the KVM exposes this capability through
> KVM_CAP_X86_BLD. The user can set it to enable the bus lock VM exit
> (disabled by default). If bus locks in guest are detected by KVM, exit
> to user space even when current exit reason is handled by KVM
> internally. Set a new field KVM_RUN_BUS_LOCK in vcpu->run->flags to
> inform the user space that there is a bus lock in guest and it is
> preempted by a higher priority VM exit.
>
> Every bus lock acquired in non-root mode will be recorded in
> vcpu->stat.bus_locks and exposed through debugfs when the bus lock
> VM exit is enabled.
>
> Document for Bus Lock VM exit is now available at the latest "Intel
> Architecture Instruction Set Extensions Programming Reference".
>
> Document Link:
> https://software.intel.com/content/www/us/en/develop/download/intel-architecture-instruction-set-extensions-programming-reference.html
>
> Co-developed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> Signed-off-by: Chenyi Qiang <chenyi.qiang@xxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h    |  9 ++++++++
>  arch/x86/include/asm/vmx.h         |  1 +
>  arch/x86/include/asm/vmxfeatures.h |  1 +
>  arch/x86/include/uapi/asm/kvm.h    |  1 +
>  arch/x86/include/uapi/asm/vmx.h    |  4 +++-
>  arch/x86/kvm/vmx/capabilities.h    |  6 +++++
>  arch/x86/kvm/vmx/vmx.c             | 33 ++++++++++++++++++++++++++-
>  arch/x86/kvm/vmx/vmx.h             |  2 +-
>  arch/x86/kvm/x86.c                 | 36 +++++++++++++++++++++++++++++-
>  arch/x86/kvm/x86.h                 |  5 +++++
>  include/uapi/linux/kvm.h           |  2 ++
>  11 files changed, 96 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index be5363b21540..bfabe2f15b30 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -829,6 +829,9 @@ struct kvm_vcpu_arch {
>  
>  	/* AMD MSRC001_0015 Hardware Configuration */
>  	u64 msr_hwcr;
> +
> +	/* Set when bus lock VM exit is preempted by a higher priority VM exit */
> +	bool bus_lock_detected;
>  };
>  
>  struct kvm_lpage_info {
> @@ -1002,6 +1005,9 @@ struct kvm_arch {
>  	bool guest_can_read_msr_platform_info;
>  	bool exception_payload_enabled;
>  
> +	/* Set when bus lock vm exit is enabled by user */
> +	bool bus_lock_exit;
> +
>  	struct kvm_pmu_event_filter *pmu_event_filter;
>  	struct task_struct *nx_lpage_recovery_thread;
>  };
> @@ -1051,6 +1057,7 @@ struct kvm_vcpu_stat {
>  	u64 req_event;
>  	u64 halt_poll_success_ns;
>  	u64 halt_poll_fail_ns;
> +	u64 bus_locks;
>  };
>  
>  struct x86_instruction_info;
> @@ -1388,6 +1395,8 @@ extern u8   kvm_tsc_scaling_ratio_frac_bits;
>  extern u64  kvm_max_tsc_scaling_ratio;
>  /* 1ull << kvm_tsc_scaling_ratio_frac_bits */
>  extern u64  kvm_default_tsc_scaling_ratio;
> +/* bus lock detection supported */
> +extern bool kvm_has_bus_lock_exit;
>  
>  extern u64 kvm_mce_cap_supported;
>  
> diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
> index cd7de4b401fe..93a880bc31a7 100644
> --- a/arch/x86/include/asm/vmx.h
> +++ b/arch/x86/include/asm/vmx.h
> @@ -73,6 +73,7 @@
>  #define SECONDARY_EXEC_PT_USE_GPA		VMCS_CONTROL_BIT(PT_USE_GPA)
>  #define SECONDARY_EXEC_TSC_SCALING              VMCS_CONTROL_BIT(TSC_SCALING)
>  #define SECONDARY_EXEC_ENABLE_USR_WAIT_PAUSE	VMCS_CONTROL_BIT(USR_WAIT_PAUSE)
> +#define SECONDARY_EXEC_BUS_LOCK_DETECTION	VMCS_CONTROL_BIT(BUS_LOCK_DETECTION)
>  
>  #define PIN_BASED_EXT_INTR_MASK                 VMCS_CONTROL_BIT(INTR_EXITING)
>  #define PIN_BASED_NMI_EXITING                   VMCS_CONTROL_BIT(NMI_EXITING)
> diff --git a/arch/x86/include/asm/vmxfeatures.h b/arch/x86/include/asm/vmxfeatures.h
> index 9915990fd8cf..e80523346274 100644
> --- a/arch/x86/include/asm/vmxfeatures.h
> +++ b/arch/x86/include/asm/vmxfeatures.h
> @@ -83,5 +83,6 @@
>  #define VMX_FEATURE_TSC_SCALING		( 2*32+ 25) /* Scale hardware TSC when read in guest */
>  #define VMX_FEATURE_USR_WAIT_PAUSE	( 2*32+ 26) /* Enable TPAUSE, UMONITOR, UMWAIT in guest */
>  #define VMX_FEATURE_ENCLV_EXITING	( 2*32+ 28) /* "" VM-Exit on ENCLV (leaf dependent) */
> +#define VMX_FEATURE_BUS_LOCK_DETECTION	( 2*32+ 30) /* VM-Exit when bus lock caused */
>  
>  #endif /* _ASM_X86_VMXFEATURES_H */
> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
> index 0780f97c1850..a1471c05f7f9 100644
> --- a/arch/x86/include/uapi/asm/kvm.h
> +++ b/arch/x86/include/uapi/asm/kvm.h
> @@ -111,6 +111,7 @@ struct kvm_ioapic_state {
>  #define KVM_NR_IRQCHIPS          3
>  
>  #define KVM_RUN_X86_SMM		 (1 << 0)
> +#define KVM_RUN_BUS_LOCK         (1 << 1)
>  
>  /* for KVM_GET_REGS and KVM_SET_REGS */
>  struct kvm_regs {
> diff --git a/arch/x86/include/uapi/asm/vmx.h b/arch/x86/include/uapi/asm/vmx.h
> index b8ff9e8ac0d5..14c177c4afd5 100644
> --- a/arch/x86/include/uapi/asm/vmx.h
> +++ b/arch/x86/include/uapi/asm/vmx.h
> @@ -88,6 +88,7 @@
>  #define EXIT_REASON_XRSTORS             64
>  #define EXIT_REASON_UMWAIT              67
>  #define EXIT_REASON_TPAUSE              68
> +#define EXIT_REASON_BUS_LOCK            74
>  
>  #define VMX_EXIT_REASONS \
>  	{ EXIT_REASON_EXCEPTION_NMI,         "EXCEPTION_NMI" }, \
> @@ -148,7 +149,8 @@
>  	{ EXIT_REASON_XSAVES,                "XSAVES" }, \
>  	{ EXIT_REASON_XRSTORS,               "XRSTORS" }, \
>  	{ EXIT_REASON_UMWAIT,                "UMWAIT" }, \
> -	{ EXIT_REASON_TPAUSE,                "TPAUSE" }
> +	{ EXIT_REASON_TPAUSE,                "TPAUSE" }, \
> +	{ EXIT_REASON_BUS_LOCK,              "BUS_LOCK" }
>  
>  #define VMX_EXIT_REASON_FLAGS \
>  	{ VMX_EXIT_REASONS_FAILED_VMENTRY,	"FAILED_VMENTRY" }
> diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
> index 4bbd8b448d22..aa94535e6705 100644
> --- a/arch/x86/kvm/vmx/capabilities.h
> +++ b/arch/x86/kvm/vmx/capabilities.h
> @@ -262,6 +262,12 @@ static inline bool cpu_has_vmx_tsc_scaling(void)
>  		SECONDARY_EXEC_TSC_SCALING;
>  }
>  
> +static inline bool cpu_has_vmx_bus_lock_detection(void)
> +{
> +	return vmcs_config.cpu_based_2nd_exec_ctrl &
> +	    SECONDARY_EXEC_BUS_LOCK_DETECTION;
> +}
> +
>  static inline bool cpu_has_vmx_apicv(void)
>  {
>  	return cpu_has_vmx_apic_register_virt() &&
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 89c131eaedf2..1560e51d2d8e 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2460,7 +2460,8 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
>  			SECONDARY_EXEC_ENABLE_USR_WAIT_PAUSE |
>  			SECONDARY_EXEC_PT_USE_GPA |
>  			SECONDARY_EXEC_PT_CONCEAL_VMX |
> -			SECONDARY_EXEC_ENABLE_VMFUNC;
> +			SECONDARY_EXEC_ENABLE_VMFUNC |
> +			SECONDARY_EXEC_BUS_LOCK_DETECTION;
>  		if (cpu_has_sgx())
>  			opt2 |= SECONDARY_EXEC_ENCLS_EXITING;
>  		if (adjust_vmx_controls(min2, opt2,
> @@ -4247,6 +4248,9 @@ static void vmx_compute_secondary_exec_control(struct vcpu_vmx *vmx)
>  		}
>  	}
>  
> +	if (!kvm_bus_lock_exit_enabled(vmx->vcpu.kvm))
> +		exec_control &= ~SECONDARY_EXEC_BUS_LOCK_DETECTION;
> +
>  	vmx->secondary_exec_control = exec_control;
>  }
>  
> @@ -5661,6 +5665,16 @@ static int handle_encls(struct kvm_vcpu *vcpu)
>  	return 1;
>  }
>  
> +static int handle_bus_lock(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_run *kvm_run = vcpu->run;
> +
> +	vcpu->stat.bus_locks++;
> +
> +	kvm_run->exit_reason = KVM_EXIT_BUS_LOCK;
> +	return 0;
> +}
> +
>  /*
>   * The exit handlers return 1 if the exit was handled fully and guest execution
>   * may resume.  Otherwise they set the kvm_run parameter to indicate what needs
> @@ -5717,6 +5731,7 @@ static int (*kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu) = {
>  	[EXIT_REASON_VMFUNC]		      = handle_vmx_instruction,
>  	[EXIT_REASON_PREEMPTION_TIMER]	      = handle_preemption_timer,
>  	[EXIT_REASON_ENCLS]		      = handle_encls,
> +	[EXIT_REASON_BUS_LOCK]                = handle_bus_lock,
>  };
>  
>  static const int kvm_vmx_max_exit_handlers =
> @@ -6809,6 +6824,19 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  	if (unlikely(vmx->exit_reason.failed_vmentry))
>  		return EXIT_FASTPATH_NONE;
>  
> +	/*
> +	 * check the exit_reason to see if there is a bus lock
> +	 * happened in guest.
> +	 */
> +	if (kvm_bus_lock_exit_enabled(vmx->vcpu.kvm)) {
> +		if (vmx->exit_reason.bus_lock_detected) {
> +			vcpu->stat.bus_locks++;
> +			vcpu->arch.bus_lock_detected = true;
> +		} else {
> +			vcpu->arch.bus_lock_detected = false;

This is a fast path so I'm wondering if we can move bus_lock_detected
clearing somewhere else.

> +		}
> +	}
> +
>  	vmx->loaded_vmcs->launched = 1;
>  	vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
>  
> @@ -8060,6 +8088,9 @@ static __init int hardware_setup(void)
>  		kvm_tsc_scaling_ratio_frac_bits = 48;
>  	}
>  
> +	if (cpu_has_vmx_bus_lock_detection())
> +		kvm_has_bus_lock_exit = true;
> +
>  	set_bit(0, vmx_vpid_bitmap); /* 0 is reserved for host */
>  
>  	if (enable_ept)
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 06a91b224ef3..7e6d63a8589a 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -104,7 +104,7 @@ union vmx_exit_reason {
>  		u32	reserved23		: 1;
>  		u32	reserved24		: 1;
>  		u32	reserved25		: 1;
> -		u32	reserved26		: 1;
> +		u32	bus_lock_detected	: 1;
>  		u32	enclave_mode		: 1;
>  		u32	smi_pending_mtf		: 1;
>  		u32	smi_from_vmx_root	: 1;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 88c593f83b28..00a54d0cd7da 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -133,6 +133,8 @@ u64  __read_mostly kvm_max_tsc_scaling_ratio;
>  EXPORT_SYMBOL_GPL(kvm_max_tsc_scaling_ratio);
>  u64 __read_mostly kvm_default_tsc_scaling_ratio;
>  EXPORT_SYMBOL_GPL(kvm_default_tsc_scaling_ratio);
> +bool __read_mostly kvm_has_bus_lock_exit;
> +EXPORT_SYMBOL_GPL(kvm_has_bus_lock_exit);
>  
>  /* tsc tolerance in parts per million - default to 1/2 of the NTP threshold */
>  static u32 __read_mostly tsc_tolerance_ppm = 250;
> @@ -220,6 +222,7 @@ struct kvm_stats_debugfs_item debugfs_entries[] = {
>  	VCPU_STAT("l1d_flush", l1d_flush),
>  	VCPU_STAT("halt_poll_success_ns", halt_poll_success_ns),
>  	VCPU_STAT("halt_poll_fail_ns", halt_poll_fail_ns),
> +	VCPU_STAT("bus_locks", bus_locks),
>  	VM_STAT("mmu_shadow_zapped", mmu_shadow_zapped),
>  	VM_STAT("mmu_pte_write", mmu_pte_write),
>  	VM_STAT("mmu_pte_updated", mmu_pte_updated),
> @@ -3538,6 +3541,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_HYPERV_ENLIGHTENED_VMCS:
>  		r = kvm_x86_ops.nested_ops->enable_evmcs != NULL;
>  		break;
> +	case KVM_CAP_X86_BUS_LOCK_EXIT:
> +		r = kvm_has_bus_lock_exit;
> +		break;

I think we can always report KVM_CAP_X86_BUS_LOCK_EXIT as supported
(meaning KVM itself supports it) ...

>  	default:
>  		break;
>  	}
> @@ -4990,6 +4996,12 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>  		kvm->arch.exception_payload_enabled = cap->args[0];
>  		r = 0;
>  		break;
> +	case KVM_CAP_X86_BUS_LOCK_EXIT:
> +		if (!kvm_has_bus_lock_exit)
> +			return -EINVAL;

... because userspace can check for -EINVAL when enabling the cap. Or we
can return e.g. -EOPNOTSUPP here. I don't have a strong opinion on the matter..

> +		kvm->arch.bus_lock_exit = cap->args[0];
> +		r = 0;
> +		break;
>  	default:
>  		r = -EINVAL;
>  		break;
> @@ -7732,12 +7744,23 @@ static void post_kvm_run_save(struct kvm_vcpu *vcpu)
>  	struct kvm_run *kvm_run = vcpu->run;
>  
>  	kvm_run->if_flag = (kvm_get_rflags(vcpu) & X86_EFLAGS_IF) != 0;
> -	kvm_run->flags = is_smm(vcpu) ? KVM_RUN_X86_SMM : 0;
>  	kvm_run->cr8 = kvm_get_cr8(vcpu);
>  	kvm_run->apic_base = kvm_get_apic_base(vcpu);
>  	kvm_run->ready_for_interrupt_injection =
>  		pic_in_kernel(vcpu->kvm) ||
>  		kvm_vcpu_ready_for_interrupt_injection(vcpu);
> +
> +	if (is_smm(vcpu))
> +		kvm_run->flags |= KVM_RUN_X86_SMM;
> +	else
> +		kvm_run->flags &= ~KVM_RUN_X86_SMM;
> +
> +	if (vcpu->arch.bus_lock_detected &&
> +	    kvm_run->exit_reason != KVM_EXIT_BUS_LOCK)
> +		kvm_run->flags |= KVM_RUN_BUS_LOCK;
> +	else
> +		kvm_run->flags &= ~KVM_RUN_BUS_LOCK;
> +
>  }
>  
>  static void update_cr8_intercept(struct kvm_vcpu *vcpu)
> @@ -8597,6 +8620,17 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		kvm_lapic_sync_from_vapic(vcpu);
>  
>  	r = kvm_x86_ops.handle_exit(vcpu, exit_fastpath);
> +
> +	/*
> +	 * Even when current exit reason is handled by KVM
> +	 * internally, we still needs to exit to user space
> +	 * when bus lock detected to inform that there is a
> +	 * bus lock in guest.
> +	 */
> +	if (r > 0 && vcpu->arch.bus_lock_detected) {
> +		vcpu->run->exit_reason = KVM_EXIT_BUS_LOCK;
> +		r = 0;

Can we maybe reset bus_lock_detected here?

> +	}
>  	return r;
>  
>  cancel_injection:
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 6eb62e97e59f..d3b1095cc8fb 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -334,6 +334,11 @@ static inline bool kvm_cstate_in_guest(struct kvm *kvm)
>  	return kvm->arch.cstate_in_guest;
>  }
>  
> +static inline bool kvm_bus_lock_exit_enabled(struct kvm *kvm)
> +{
> +	return kvm->arch.bus_lock_exit;
> +}
> +
>  DECLARE_PER_CPU(struct kvm_vcpu *, current_vcpu);
>  
>  static inline void kvm_before_interrupt(struct kvm_vcpu *vcpu)
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 4fdf30316582..e66aa4bdaf24 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -248,6 +248,7 @@ struct kvm_hyperv_exit {
>  #define KVM_EXIT_IOAPIC_EOI       26
>  #define KVM_EXIT_HYPERV           27
>  #define KVM_EXIT_ARM_NISV         28
> +#define KVM_EXIT_BUS_LOCK         29
>  
>  /* For KVM_EXIT_INTERNAL_ERROR */
>  /* Emulate instruction failed. */
> @@ -1031,6 +1032,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_PPC_SECURE_GUEST 181
>  #define KVM_CAP_HALT_POLL 182
>  #define KVM_CAP_ASYNC_PF_INT 183
> +#define KVM_CAP_X86_BUS_LOCK_EXIT 184
>  
>  #ifdef KVM_CAP_IRQ_ROUTING

-- 
Vitaly