Re: [PATCH v3 4/4] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL

From: Jim Mattson
Date: Tue Jan 30 2018 - 12:50:05 EST

On Mon, Jan 29, 2018 at 4:10 PM, KarimAllah Ahmed <karahmed@xxxxxxxxx> wrote:
> [ Based on a patch from Ashok Raj <ashok.raj@xxxxxxxxx> ]
>
> Add direct access to MSR_IA32_SPEC_CTRL for guests. This is needed for
> guests that will only mitigate Spectre V2 through IBRS+IBPB and will not
> be using a retpoline+IBPB based approach.
>
> To avoid the overhead of atomically saving and restoring the
> MSR_IA32_SPEC_CTRL for guests that do not actually use the MSR, only
> add_atomic_switch_msr when a non-zero is written to it.
>
> No attempt is made to handle STIBP here, intentionally. Filtering STIBP
> may be added in a future patch, which may require trapping all writes
> if we don't want to pass it through directly to the guest.
>
> [dwmw2: Clean up CPUID bits, save/restore manually, handle reset]
>
> Cc: Asit Mallick <asit.k.mallick@xxxxxxxxx>
> Cc: Arjan Van De Ven <arjan.van.de.ven@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Ashok Raj <ashok.raj@xxxxxxxxx>
> Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---
> v2:
> - remove 'host_spec_ctrl' in favor of only a comment (dwmw@).
> - special case writing '0' in SPEC_CTRL to avoid confusing live-migration
> when the instance never used the MSR (dwmw@).
> - depend on X86_FEATURE_IBRS instead of X86_FEATURE_SPEC_CTRL (dwmw@).
> - add MSR_IA32_SPEC_CTRL to the list of MSRs to save (dropped it by accident).
> v3:
> - Save/restore manually
> - Fix CPUID handling
> - Fix a copy & paste error in the name of SPEC_CTRL MSR in
> disable_intercept.
> - support !cpu_has_vmx_msr_bitmap()
> ---
> arch/x86/kvm/cpuid.c | 7 +++++--
> arch/x86/kvm/vmx.c | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> arch/x86/kvm/x86.c | 2 +-
> 3 files changed, 65 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 1909635..662d0c0 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -394,7 +394,8 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>
> /* cpuid 7.0.edx*/
> const u32 kvm_cpuid_7_0_edx_x86_features =
> - F(AVX512_4VNNIW) | F(AVX512_4FMAPS) | F(ARCH_CAPABILITIES);
> + F(AVX512_4VNNIW) | F(AVX512_4FMAPS) | F(SPEC_CTRL) |
> + F(ARCH_CAPABILITIES);
>
> /* all calls to cpuid_count() should be made on the same cpu */
> get_cpu();
> @@ -630,9 +631,11 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
> g_phys_as = phys_as;
> entry->eax = g_phys_as | (virt_as << 8);
> entry->edx = 0;
> - /* IBPB isn't necessarily present in hardware cpuid */
> + /* IBRS and IBPB aren't necessarily present in hardware cpuid */
> if (boot_cpu_has(X86_FEATURE_IBPB))
> entry->ebx |= F(IBPB);
> + if (boot_cpu_has(X86_FEATURE_IBRS))
> + entry->ebx |= F(IBRS);
> entry->ebx &= kvm_cpuid_8000_0008_ebx_x86_features;
> cpuid_mask(&entry->ebx, CPUID_8000_0008_EBX);
> break;
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 798a00b..9ac9747 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -582,6 +582,8 @@ struct vcpu_vmx {
> u64 msr_guest_kernel_gs_base;
> #endif
> u64 arch_capabilities;
> + u64 spec_ctrl;
> + bool save_spec_ctrl_on_exit;
>
> u32 vm_entry_controls_shadow;
> u32 vm_exit_controls_shadow;
> @@ -922,6 +924,8 @@ static void vmx_set_nmi_mask(struct kvm_vcpu *vcpu, bool masked);
> static bool nested_vmx_is_page_fault_vmexit(struct vmcs12 *vmcs12,
> u16 error_code);
> static void vmx_update_msr_bitmap(struct kvm_vcpu *vcpu);
> +static void __always_inline vmx_disable_intercept_for_msr(unsigned long *msr_bitmap,
> + u32 msr, int type);
>
> static DEFINE_PER_CPU(struct vmcs *, vmxarea);
> static DEFINE_PER_CPU(struct vmcs *, current_vmcs);
> @@ -3226,6 +3230,13 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> case MSR_IA32_TSC:
> msr_info->data = guest_read_tsc(vcpu);
> break;
> + case MSR_IA32_SPEC_CTRL:
> + if (!msr_info->host_initiated &&
> + !guest_cpuid_has(vcpu, X86_FEATURE_IBRS))
> + return 1;
> +
> + msr_info->data = to_vmx(vcpu)->spec_ctrl;
> + break;
> case MSR_IA32_ARCH_CAPABILITIES:
> if (!msr_info->host_initiated &&
> !guest_cpuid_has(vcpu, X86_FEATURE_ARCH_CAPABILITIES))
> @@ -3339,6 +3350,31 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> case MSR_IA32_TSC:
> kvm_write_tsc(vcpu, msr_info);
> break;
> + case MSR_IA32_SPEC_CTRL:
> + if (!msr_info->host_initiated &&
> + !guest_cpuid_has(vcpu, X86_FEATURE_IBRS))
> + return 1;
> +
> + /* The STIBP bit doesn't fault even if it's not advertised */
> + if (data & ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP))
> + return 1;
> +
> + vmx->spec_ctrl = data;
> +
> + /*
> + * When it's written (to non-zero) for the first time, pass
> + * it through. This means we don't have to take the perf
> + * hit of saving it on vmexit for the common case of guests
> + * that don't use it.
> + */
> + if (cpu_has_vmx_msr_bitmap() && data &&
> + !vmx->save_spec_ctrl_on_exit) {
> + vmx->save_spec_ctrl_on_exit = true;
> + vmx_disable_intercept_for_msr(vmx->vmcs01.msr_bitmap,
> + MSR_IA32_SPEC_CTRL,
> + MSR_TYPE_RW);
> + }

This code seems to assume that L1 is currently active. What if L2 is
currently active?

> + break;
> case MSR_IA32_PRED_CMD:
> if (!msr_info->host_initiated &&
> !guest_cpuid_has(vcpu, X86_FEATURE_IBPB))
> @@ -5644,6 +5680,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> u64 cr0;
>
> vmx->rmode.vm86_active = 0;
> + vmx->spec_ctrl = 0;
>
> vmx->vcpu.arch.regs[VCPU_REGS_RDX] = get_rdx_init_val();
> kvm_set_cr8(vcpu, 0);
> @@ -9314,6 +9351,15 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>
> vmx_arm_hv_timer(vcpu);
>
> + /*
> + * If this vCPU has touched SPEC_CTRL, restore the guest's value if
> + * it's non-zero. Since vmentry is serialising on affected CPUs, there
> + * is no need to worry about the conditional branch over the wrmsr
> + * being speculatively taken.
> + */
> + if (vmx->spec_ctrl)
> + wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
> +
> vmx->__launched = vmx->loaded_vmcs->launched;
> asm(
> /* Store host registers */
> @@ -9420,6 +9466,19 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
> #endif
> );
>
> + /*
> + * We do not use IBRS in the kernel. If this vCPU has used the
> + * SPEC_CTRL MSR it may have left it on; save the value and
> + * turn it off. This is much more efficient than blindly adding
> + * it to the atomic save/restore list. Especially as the former
> + * (Saving guest MSRs on vmexit) doesn't even exist in KVM.
> + */
> + if (vmx->save_spec_ctrl_on_exit)
> + rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
> +
> + if (vmx->spec_ctrl)
> + wrmsrl(MSR_IA32_SPEC_CTRL, 0);
> +
> /* Eliminate branch target predictions from guest mode */
> vmexit_fill_RSB();
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8e889dc..fc9724c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1006,7 +1006,7 @@ static u32 msrs_to_save[] = {
> #endif
> MSR_IA32_TSC, MSR_IA32_CR_PAT, MSR_VM_HSAVE_PA,
> MSR_IA32_FEATURE_CONTROL, MSR_IA32_BNDCFGS, MSR_TSC_AUX,
> - MSR_IA32_ARCH_CAPABILITIES
> + MSR_IA32_SPEC_CTRL, MSR_IA32_ARCH_CAPABILITIES
> };
>
> static unsigned num_msrs_to_save;
> --
> 2.7.4
>