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    SubjectRe: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to the guest
    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@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 | 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
    >
    >

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