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SubjectRe: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to the guest


On 2018/1/9 20:03, Paolo Bonzini wrote:

> Direct access to MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD is important
> for performance. Allow load/store of MSR_IA32_SPEC_CTRL, restore guest
> IBRS on VM entry and set it to 0 on VM exit (because Linux does not use
> it yet).
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/kvm/vmx.c | 42 ++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 42 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 669f5f74857d..ef603692aa98 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -120,6 +120,8 @@
> module_param_named(preemption_timer, enable_preemption_timer, bool, S_IRUGO);
> #endif
>
> +static bool __read_mostly have_spec_ctrl;
> +
> #define KVM_GUEST_CR0_MASK (X86_CR0_NW | X86_CR0_CD)
> #define KVM_VM_CR0_ALWAYS_ON_UNRESTRICTED_GUEST (X86_CR0_WP | X86_CR0_NE)
> #define KVM_VM_CR0_ALWAYS_ON \
> @@ -609,6 +611,8 @@ struct vcpu_vmx {
> u64 msr_host_kernel_gs_base;
> u64 msr_guest_kernel_gs_base;
> #endif
> + u64 spec_ctrl;
> +
> u32 vm_entry_controls_shadow;
> u32 vm_exit_controls_shadow;
> u32 secondary_exec_control;
> @@ -3361,6 +3365,9 @@ 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:
> + msr_info->data = to_vmx(vcpu)->spec_ctrl;
> + break;
> case MSR_IA32_SYSENTER_CS:
> msr_info->data = vmcs_read32(GUEST_SYSENTER_CS);
> break;
> @@ -3500,6 +3507,9 @@ 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:
> + to_vmx(vcpu)->spec_ctrl = data;
> + break;
> case MSR_IA32_CR_PAT:
> if (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PAT) {
> if (!kvm_mtrr_valid(vcpu, MSR_IA32_CR_PAT, data))
> @@ -7062,6 +7072,17 @@ static __init int hardware_setup(void)
> goto out;
> }
>
> + /*
> + * FIXME: this is only needed until SPEC_CTRL is supported
> + * by upstream Linux in cpufeatures, then it can be replaced
> + * with static_cpu_has.
> + */
> + have_spec_ctrl = cpu_has_spec_ctrl();
> + if (have_spec_ctrl)
> + pr_info("kvm: SPEC_CTRL available\n");
> + else
> + pr_info("kvm: SPEC_CTRL not available\n");
> +

In this approach, we must reload these modules if we update the microcode later ?

> if (boot_cpu_has(X86_FEATURE_NX))
> kvm_enable_efer_bits(EFER_NX);
>
> @@ -7131,6 +7152,8 @@ static __init int hardware_setup(void)
> vmx_disable_intercept_for_msr(MSR_IA32_SYSENTER_CS, false);
> vmx_disable_intercept_for_msr(MSR_IA32_SYSENTER_ESP, false);
> vmx_disable_intercept_for_msr(MSR_IA32_SYSENTER_EIP, false);
> + vmx_disable_intercept_for_msr(MSR_IA32_SPEC_CTRL, false);
> + vmx_disable_intercept_for_msr(MSR_IA32_PRED_CMD, false);
>
> memcpy(vmx_msr_bitmap_legacy_x2apic_apicv,
> vmx_msr_bitmap_legacy, PAGE_SIZE);
> @@ -9601,6 +9624,13 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>
> vmx_arm_hv_timer(vcpu);
>
> + /*
> + * MSR_IA32_SPEC_CTRL is restored after the last indirect branch
> + * before vmentry.
> + */
> + if (have_spec_ctrl && vmx->spec_ctrl != 0)
> + wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
> +
> vmx->__launched = vmx->loaded_vmcs->launched;
> asm(
> /* Store host registers */
> @@ -9707,6 +9737,18 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
> #endif
> );
>
> + if (have_spec_ctrl) {
> + rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
> + if (vmx->spec_ctrl != 0)
> + wrmsrl(MSR_IA32_SPEC_CTRL, 0);
> + }
> + /*
> + * Speculative execution past the above wrmsrl might encounter
> + * an indirect branch and use guest-controlled contents of the
> + * indirect branch predictor; block it.
> + */
> + asm("lfence");
> +
> /* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
> if (vmx->host_debugctlmsr)
> update_debugctlmsr(vmx->host_debugctlmsr);


--
Regards,
Longpeng(Mike)

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