Messages in this thread | | | Subject | Re: [PATCH 5/5] x86/feature: Detect the x86 feature Indirect Branch Prediction Barrier | From | Tom Lendacky <> | Date | Fri, 12 Jan 2018 09:31:02 -0600 |
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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@intel.com> > --- > 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); >
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