Re: [PATCH 5/5] x86/feature: Detect the x86 feature Indirect Branch Prediction Barrier

[Tom Lendacky]

On 1/11/2018 7:32 PM, Ashok Raj wrote:
> cpuid ax=0x7, return rdx bit 26 to indicate presence of both
> IA32_SPEC_CTRL(MSR 0x48) and IA32_PRED_CMD(MSR 0x49)
> 
> BIT0: Indirect Branch Prediction Barrier
> 
> When this MSR is written with IBPB=1 it ensures that earlier code's behavior
> doesn't control later indirect branch predictions.
> 
> Note this MSR is only writable and does not carry any state. Its a barrier
> so the code should perform a wrmsr when the barrier is needed.
> 
> Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
> ---
>  arch/x86/include/asm/cpufeatures.h |  1 +
>  arch/x86/include/asm/msr-index.h   |  3 +++
>  arch/x86/kernel/cpu/spec_ctrl.c    |  7 +++++++
>  arch/x86/kvm/svm.c                 | 16 ++++++++++++++++
>  arch/x86/kvm/vmx.c                 | 10 ++++++++++
>  5 files changed, 37 insertions(+)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 624b58e..52f37fc 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -213,6 +213,7 @@
>  #define X86_FEATURE_MBA			( 7*32+18) /* Memory Bandwidth Allocation */
>  #define X86_FEATURE_SPEC_CTRL		( 7*32+19) /* Speculation Control */
>  #define X86_FEATURE_SPEC_CTRL_IBRS	( 7*32+20) /* Speculation Control, use IBRS */
> +#define X86_FEATURE_PRED_CMD	( 7*32+21) /* Indirect Branch Prediction Barrier */
>  
>  /* Virtualization flags: Linux defined, word 8 */
>  #define X86_FEATURE_TPR_SHADOW		( 8*32+ 0) /* Intel TPR Shadow */
> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> index 3e1cb18..1888e19 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -46,6 +46,9 @@
>  #define SPEC_CTRL_DISABLE_IBRS		(0 << 0)
>  #define SPEC_CTRL_ENABLE_IBRS		(1 << 0)
>  
> +#define MSR_IA32_PRED_CMD		0x00000049
> +#define FEATURE_SET_IBPB		(1<<0)
> +
>  #define MSR_IA32_PERFCTR0		0x000000c1
>  #define MSR_IA32_PERFCTR1		0x000000c2
>  #define MSR_FSB_FREQ			0x000000cd
> diff --git a/arch/x86/kernel/cpu/spec_ctrl.c b/arch/x86/kernel/cpu/spec_ctrl.c
> index 02fc630..6cfec19 100644
> --- a/arch/x86/kernel/cpu/spec_ctrl.c
> +++ b/arch/x86/kernel/cpu/spec_ctrl.c
> @@ -15,6 +15,13 @@ void spec_ctrl_scan_feature(struct cpuinfo_x86 *c)
>  			if (!c->cpu_index)
>  				static_branch_enable(&spec_ctrl_dynamic_ibrs);
>  		}
> +		/*
> +		 * For Intel CPU's this MSR is shared the same cpuid
> +		 * enumeration. When MSR_IA32_SPEC_CTRL is present
> +		 * MSR_IA32_SPEC_CTRL is also available
> +		 * TBD: AMD might have a separate enumeration for each.

AMD will follow the specification that if cpuid ax=0x7, return rdx[26]
is set, it will indicate both MSR registers and features are supported.

But AMD also has a separate bit for IBPB (X86_FEATURE_PRED_CMD) alone.
As all of the IBRS/IBPB stuff happens, that patch will follow.

> +		 */
> +		set_cpu_cap(c, X86_FEATURE_PRED_CMD);>  	}
>  }
>  
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 7c14471a..36924c9 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -251,6 +251,7 @@ static const struct svm_direct_access_msrs {
>  	{ .index = MSR_SYSCALL_MASK,			.always = true  },
>  #endif
>  	{ .index = MSR_IA32_SPEC_CTRL,          .always = true  },
> +	{ .index = MSR_IA32_PRED_CMD,           .always = false },

This should be .always = true

>  	{ .index = MSR_IA32_LASTBRANCHFROMIP,		.always = false },
>  	{ .index = MSR_IA32_LASTBRANCHTOIP,		.always = false },
>  	{ .index = MSR_IA32_LASTINTFROMIP,		.always = false },
> @@ -531,6 +532,7 @@ struct svm_cpu_data {
>  	struct kvm_ldttss_desc *tss_desc;
>  
>  	struct page *save_area;
> +	struct vmcb *current_vmcb;
>  };
>  
>  static DEFINE_PER_CPU(struct svm_cpu_data *, svm_data);
> @@ -923,6 +925,8 @@ static void svm_vcpu_init_msrpm(u32 *msrpm)
>  
>  	if (boot_cpu_has(X86_FEATURE_SPEC_CTRL))
>  		set_msr_interception(msrpm, MSR_IA32_SPEC_CTRL, 1, 1);
> +	if (boot_cpu_has(X86_FEATURE_PRED_CMD))
> +		set_msr_interception(msrpm, MSR_IA32_PRED_CMD, 1, 1);

Similar to the comment about SPEC_CTRL, this should be removed as it will
be covered by the loop.

>  }
>  
>  static void add_msr_offset(u32 offset)
> @@ -1711,11 +1715,18 @@ static void svm_free_vcpu(struct kvm_vcpu *vcpu)
>  	__free_pages(virt_to_page(svm->nested.msrpm), MSRPM_ALLOC_ORDER);
>  	kvm_vcpu_uninit(vcpu);
>  	kmem_cache_free(kvm_vcpu_cache, svm);
> +    /* 
> +     * The VMCB could be recycled, causing a false negative in svm_vcpu_load;
> +     * block speculative execution.
> +     */
> +	if (boot_cpu_has(X86_FEATURE_PRED_CMD))
> +        native_wrmsrl(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);
>  }
>  
>  static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
> +	struct svm_cpu_data *sd = per_cpu(svm_data, cpu);
>  	int i;
>  
>  	if (unlikely(cpu != vcpu->cpu)) {
> @@ -1744,6 +1755,11 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  	if (static_cpu_has(X86_FEATURE_RDTSCP))
>  		wrmsrl(MSR_TSC_AUX, svm->tsc_aux);
>  
> +	if (sd->current_vmcb != svm->vmcb) {
> +		sd->current_vmcb = svm->vmcb;
> +		if (boot_cpu_has(X86_FEATURE_PRED_CMD))
> +			native_wrmsrl(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);
> +	}
>  	avic_vcpu_load(vcpu, cpu);
>  }
>  
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 1913896..caeb9ff 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2280,6 +2280,8 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  	if (per_cpu(current_vmcs, cpu) != vmx->loaded_vmcs->vmcs) {
>  		per_cpu(current_vmcs, cpu) = vmx->loaded_vmcs->vmcs;
>  		vmcs_load(vmx->loaded_vmcs->vmcs);
> +		if (boot_cpu_has(X86_FEATURE_SPEC_CTRL))

This should probably use X86_FEATURE_PRED_CMD.

> +			native_wrmsrl(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);>  	}
>  
>  	if (!already_loaded) {
> @@ -3837,6 +3839,12 @@ static void free_loaded_vmcs(struct loaded_vmcs *loaded_vmcs)
>  	free_vmcs(loaded_vmcs->vmcs);
>  	loaded_vmcs->vmcs = NULL;
>  	WARN_ON(loaded_vmcs->shadow_vmcs != NULL);
> +    /*
> +     * The VMCS could be recycled, causing a false negative in vmx_vcpu_load
> +     * block speculative execution.
> +     */
> +	if (boot_cpu_has(X86_FEATURE_SPEC_CTRL))

Again, X86_FEATURE_PRED_CMD.

> +        native_wrmsrl(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);
>  }
>  
>  static void free_kvm_area(void)
> @@ -6804,6 +6812,8 @@ static __init int hardware_setup(void)
>  	 */
>  	if (boot_cpu_has(X86_FEATURE_SPEC_CTRL))
>  		vmx_disable_intercept_for_msr(MSR_IA32_SPEC_CTRL, false);
> +	if (boot_cpu_has(X86_FEATURE_PRED_CMD))
> +		vmx_disable_intercept_for_msr(MSR_IA32_PRED_CMD, false);
>  
>  	vmx_disable_intercept_for_msr(MSR_FS_BASE, false);
>  	vmx_disable_intercept_for_msr(MSR_GS_BASE, false);
>