Guest usage of MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD should be
predicated on guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL).
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;
I have:
if (!have_spec_ctrl ||
(!msr_info->host_initiated &&
!guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
return 1;
msr_info->data = to_vmx(vcpu)->msr_ia32_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;
I have:
if (!have_spec_ctrl ||
(!msr_info->host_initiated &&
!guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
return 1;
to_vmx(vcpu)->msr_ia32_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);
I have a lot of changes to MSR permission bitmap handling, but these
intercepts should only be disabled when guest_cpuid_has(vcpu,
X86_FEATURE_SPEC_CTRL).
> 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);
> +
Here, I have:
/*
* If the guest is allowed to write to MSR_IA32_SPEC_CTRL,
* store it on VM-exit.
*/
if (guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
add_autostore_msr(vmx, MSR_IA32_SPEC_CTRL);
else
clear_autostore_msr(vmx, MSR_IA32_SPEC_CTRL);
/*
* If the guest's IA32_SPEC_CTRL MSR doesn't match the host's
* IA32_SPEC_CTRL MSR, then add the MSR to the atomic switch
* MSRs, so that the guest value will be loaded on VM-entry
* and the host value will be loaded on VM-exit.
*/
if (vmx->msr_ia32_spec_ctrl != spec_ctrl_enabled())
add_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL,
vmx->msr_ia32_spec_ctrl,
spec_ctrl_enabled());
else
clear_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL);
> 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);
> + }
> +
I know the VM-exit MSR load and store lists are probably slower, but
I'm a little uncomfortable restoring the host's IA32_SPEC_CTRL MSR
late if the guest has it clear and the host has it set.
> /* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
> if (vmx->host_debugctlmsr)
> update_debugctlmsr(vmx->host_debugctlmsr);
> --
> 1.8.3.1
>
>
