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

[Liran Alon <liran.alon@oracle.com>]

----- pbonzini@redhat.com 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");
> +
>  	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);
> -- 
> 1.8.3.1

Reviewed-by: Liran Alon <liran.alon@oracle.com>