Re: [PATCH v4 10/10] x86: vmx: virtualize split lock detection

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 3/14/2020 3:34 PM, Xiaoyao Li wrote:
> Due to the fact that MSR_TEST_CTRL is per-core scope, i.e., the sibling
> threads in the same physical CPU core share the same MSR, only
> advertising feature split lock detection to guest when SMT is disabled
> or unsupported, for simplicitly.
> 
> Below summarizing how guest behaves of different host configuration:
> 
>    sld_fatal - hardware MSR_TEST_CTRL.SLD is always on when vcpu is running,
>                even though guest thinks it sets/clears MSR_TEST_CTRL.SLD
> 	      bit successfully. i.e., SLD is forced on for guest.
> 
>    sld_warn - hardware MSR_TEST_CTRL.SLD is left on until an #AC is
> 	     intercepted with MSR_TEST_CTRL.SLD=0 in the guest, at which
> 	     point normal sld_warn rules apply, i.e., clear
> 	     MSR_TEST_CTRL.SLD bit and set TIF_SLD.
> 	     If a vCPU associated with the task does VM-Enter with
> 	     virtual MSR_TEST_CTRL.SLD=1, TIF_SLD is reset, hardware
> 	     MSR_TEST_CTRL.SLD is re-set, and cycle begins anew.
> 
>    sld_kvm_only - hardware MSR_TEST_CTRL.SLD is set on VM-Entry and cleared
> 		 onVM-Exit if guest enables SLD, i.e., guest's virtual
> 	         MSR_TEST_CTRL.SLD is set.
> 
>    sld_disable - guest cannot see feature split lock detection.
> 
> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
> ---
>   arch/x86/include/asm/cpu.h  |  2 ++
>   arch/x86/kernel/cpu/intel.c |  7 ++++++
>   arch/x86/kvm/vmx/vmx.c      | 45 ++++++++++++++++++++++++++++++++-----
>   arch/x86/kvm/x86.c          | 17 +++++++++++---
>   4 files changed, 63 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
> index 2e17315b1fed..284be32aaf87 100644
> --- a/arch/x86/include/asm/cpu.h
> +++ b/arch/x86/include/asm/cpu.h
> @@ -64,6 +64,7 @@ extern void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c);
>   extern void switch_to_sld(unsigned long tifn);
>   extern bool handle_user_split_lock(unsigned long ip);
>   extern void sld_msr_set(bool on);
> +extern void sld_turn_back_on(void);
>   #else
>   static inline bool split_lock_detect_on(void) { return false; }
>   static inline bool split_lock_detect_disabled(void) { return true; }
> @@ -74,5 +75,6 @@ static inline bool handle_user_split_lock(unsigned long ip)
>   	return false;
>   }
>   static inline void sld_msr_set(bool on) {}
> +static inline void sld_turn_back_on(void) {}
>   #endif
>   #endif /* _ASM_X86_CPU_H */
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 8bfe8b07e06e..de46e1d3f1c7 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -1120,6 +1120,13 @@ void sld_msr_set(bool on)
>   }
>   EXPORT_SYMBOL_GPL(sld_msr_set);
>   
> +void sld_turn_back_on(void)
> +{
> +	__sld_msr_set(true);
> +	clear_tsk_thread_flag(current, TIF_SLD);
> +}
> +EXPORT_SYMBOL_GPL(sld_turn_back_on);
> +
>   /*
>    * This function is called only when switching between tasks with
>    * different split-lock detection modes. It sets the MSR for the
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 107c873b23c2..058dc6c478bd 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1819,6 +1819,22 @@ static int vmx_get_msr_feature(struct kvm_msr_entry *msr)
>   	}
>   }
>   
> +static inline u64 vmx_msr_test_ctrl_valid_bits(struct kvm_vcpu *vcpu)
> +{
> +	u64 valid_bits = 0;
> +
> +	/*
> +	 * Note: for guest, feature split lock detection can only be enumerated
> +	 * through MSR_IA32_CORE_CAPABILITIES bit.
> +	 * The FMS enumeration is invalid.
> +	 */
> +	if (vcpu->arch.core_capabilities &
> +	    MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT)
> +		valid_bits |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> +
> +	return valid_bits;
> +}
> +
>   /*
>    * Reads an msr value (of 'msr_index') into 'pdata'.
>    * Returns 0 on success, non-0 otherwise.
> @@ -1988,7 +2004,7 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   
>   	switch (msr_index) {
>   	case MSR_TEST_CTRL:
> -		if (data)
> +		if (data & ~vmx_msr_test_ctrl_valid_bits(vcpu))
>   			return 1;
>   
>   		vmx->msr_test_ctrl = data;
> @@ -4625,6 +4641,11 @@ static inline bool guest_cpu_alignment_check_enabled(struct kvm_vcpu *vcpu)
>   	       (kvm_get_rflags(vcpu) & X86_EFLAGS_AC);
>   }
>   
> +static inline bool guest_cpu_split_lock_detect_on(struct vcpu_vmx *vmx)
> +{
> +	return vmx->msr_test_ctrl & MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> +}
> +
>   static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>   {
>   	struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -4721,12 +4742,13 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>   	case AC_VECTOR:
>   		/*
>   		 * Reflect #AC to the guest if it's expecting the #AC, i.e. has
> -		 * legacy alignment check enabled.  Pre-check host split lock
> -		 * support to avoid the VMREADs needed to check legacy #AC,
> -		 * i.e. reflect the #AC if the only possible source is legacy
> -		 * alignment checks.
> +		 * legacy alignment check enabled or split lock detect enabled.
> +		 * Pre-check host split lock support to avoid further check of
> +		 * guest, i.e. reflect the #AC if host doesn't enable split lock
> +		 * detection.
>   		 */
>   		if (!split_lock_detect_on() ||
> +		    guest_cpu_split_lock_detect_on(vmx) ||
>   		    guest_cpu_alignment_check_enabled(vcpu)) {
>   			kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
>   			return 1;
> @@ -6619,6 +6641,14 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>   	 */
>   	x86_spec_ctrl_set_guest(vmx->spec_ctrl, 0);
>   
> +	if (!split_lock_detect_disabled() &&
> +	    guest_cpu_split_lock_detect_on(vmx)) {
> +		if (test_thread_flag(TIF_SLD))
> +			sld_turn_back_on();
> +		else if (!split_lock_detect_on())
> +			sld_msr_set(true);
> +	}
> +
>   	/* L1D Flush includes CPU buffer clear to mitigate MDS */
>   	if (static_branch_unlikely(&vmx_l1d_should_flush))
>   		vmx_l1d_flush(vcpu);
> @@ -6653,6 +6683,11 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>   
>   	x86_spec_ctrl_restore_host(vmx->spec_ctrl, 0);
>   
> +	if (!split_lock_detect_disabled() &&
> +	    guest_cpu_split_lock_detect_on(vmx) &&
> +	    !split_lock_detect_on())
> +		sld_msr_set(false);
> +
>   	/* All fields are clean at this point */
>   	if (static_branch_unlikely(&enable_evmcs))
>   		current_evmcs->hv_clean_fields |=
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 72d4bfea8864..c956aa180253 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1162,7 +1162,7 @@ static const u32 msrs_to_save_all[] = {
>   #endif
>   	MSR_IA32_TSC, MSR_IA32_CR_PAT, MSR_VM_HSAVE_PA,
>   	MSR_IA32_FEAT_CTL, MSR_IA32_BNDCFGS, MSR_TSC_AUX,
> -	MSR_IA32_SPEC_CTRL,
> +	MSR_IA32_SPEC_CTRL, MSR_TEST_CTRL,
>   	MSR_IA32_RTIT_CTL, MSR_IA32_RTIT_STATUS, MSR_IA32_RTIT_CR3_MATCH,
>   	MSR_IA32_RTIT_OUTPUT_BASE, MSR_IA32_RTIT_OUTPUT_MASK,
>   	MSR_IA32_RTIT_ADDR0_A, MSR_IA32_RTIT_ADDR0_B,
> @@ -1344,7 +1344,12 @@ static u64 kvm_get_arch_capabilities(void)
>   
>   static u64 kvm_get_core_capabilities(void)
>   {
> -	return 0;
> +	u64 data = 0;
> +
> +	if (!split_lock_detect_disabled() && !cpu_smt_possible())
> +		data |= MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT;

Just realize that cannot use !split_lock_detect_disabled() to replace 
the X86_FEATURE_SPLIT_LOCK_DETECT check, because it doesn't exclude 
"sld_not_exist" case. Also vmx_vcpu_run() has the same issue.

It should use boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT) to check, and 
drop the split_lock_detect_disabled() helper entirely.

I'll spin a v5, sorry for bothering everyone.

> +
> +	return data;
>   }
>   
>   static int kvm_get_msr_feature(struct kvm_msr_entry *msr)
> @@ -2729,7 +2734,8 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		vcpu->arch.arch_capabilities = data;
>   		break;
>   	case MSR_IA32_CORE_CAPS:
> -		if (!msr_info->host_initiated)
> +		if (!msr_info->host_initiated ||
> +		    data & ~kvm_get_core_capabilities())
>   			return 1;
>   		vcpu->arch.core_capabilities = data;
>   		break;
> @@ -5276,6 +5282,11 @@ static void kvm_init_msr_list(void)
>   		 * to the guests in some cases.
>   		 */
>   		switch (msrs_to_save_all[i]) {
> +		case MSR_TEST_CTRL:
> +			if (!(kvm_get_core_capabilities() &
> +			      MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT))
> +				continue;
> +			break;
>   		case MSR_IA32_BNDCFGS:
>   			if (!kvm_mpx_supported())
>   				continue;
>