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

[Liran Alon]

----- liran.alon@xxxxxxxxxx wrote:

> ----- 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 | 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)

As I said also on the AMD patch, I think this is a bug.
Intel specify that we should set IBRS bit even if it was already set on every #VMExit.

-Liran

> > +			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@xxxxxxxxxx>