Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to the guest

[Ashok Raj]

Hi Paolo

Do you assume that host isn't using IBRS and only guest uses it?



On Mon, Jan 8, 2018 at 10:08 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
>  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);
> +

Do we even need to optimize this? what if host Linux enabled IBRS, but
guest has it turned off?
Thought it might be simpler to blindly update it with what
vmx->spec_ctrl value is?

>         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);
> +       }
> +

Same thing here.. if the host OS has enabled IBRS wouldn't you want to
keep the same value?

>         /* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
>         if (vmx->host_debugctlmsr)
>                 update_debugctlmsr(vmx->host_debugctlmsr);
> --
> 1.8.3.1
>
>