linux-kernel - Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED

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Date:   Mon, 08 Jan 2018 21:22:27 +0200
From:   Liran Alon <LIRAN.ALON@...CLE.COM>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
CC:     jmattson@...gle.com, aliguori@...zon.com, thomas.lendacky@....com,
        dwmw@...zon.co.uk, bp@...en8.de
Subject: Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD
 down to the guest



On 08/01/18 20:08, 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@...hat.com>
> ---
>   arch/x86/kvm/vmx.c | 32 ++++++++++++++++++++++++++++++++
>   1 file changed, 32 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 669f5f74857d..d00bcad7336e 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 = msr_info->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);
> @@ -9597,6 +9620,9 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>
>   	pt_guest_enter(vmx);
>
> +	if (have_spec_ctrl && vmx->spec_ctrl != 0)
> +		wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
> +

Intel specifies that the restore of MSR_IA32_SPEC_CTRL to guest value 
using WRMSR should be done after the last indirect branch before VMEntry.
However, atomic_switch_perf_msrs() calls perf_guest_get_msrs() which 
calls x86_pmu.guest_get_msrs() which is an indirect branch.

Therefore, it seems that this block should be done after the call to 
vmx_arm_hv_timer().

>   	atomic_switch_perf_msrs(vmx);
>
>   	vmx_arm_hv_timer(vcpu);
> @@ -9707,6 +9733,12 @@ 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)
> +			wrmsrl(MSR_IA32_SPEC_CTRL, 0);
> +	}
> +
>   	/* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
>   	if (vmx->host_debugctlmsr)
>   		update_debugctlmsr(vmx->host_debugctlmsr);
>