Re: [PATCH 4/5] x86/svm: Direct access to MSR_IA32_SPEC_CTRL

[Tom Lendacky]

On 1/11/2018 7:32 PM, Ashok Raj wrote:
> From: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> 
> Direct access to MSR_IA32_SPEC_CTRL is important
> for performance.  Allow load/store of MSR_IA32_SPEC_CTRL, restore guest
> IBRS on VM entry and set restore host values on VM exit.
> it yet).
> 
> TBD: need to check msr's can be passed through even if feature is not
> emuerated by the CPU.
> 
> [Ashok: Modified to reuse V3 spec-ctrl patches from Tim]
> 
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
> ---
>  arch/x86/kvm/svm.c | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 0e68f0b..7c14471a 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -183,6 +183,8 @@ struct vcpu_svm {
>  		u64 gs_base;
>  	} host;
>  
> +	u64 spec_ctrl;
> +
>  	u32 *msrpm;
>  
>  	ulong nmi_iret_rip;
> @@ -248,6 +250,7 @@ static const struct svm_direct_access_msrs {
>  	{ .index = MSR_CSTAR,				.always = true  },
>  	{ .index = MSR_SYSCALL_MASK,			.always = true  },
>  #endif
> +	{ .index = MSR_IA32_SPEC_CTRL,          .always = true  },
>  	{ .index = MSR_IA32_LASTBRANCHFROMIP,		.always = false },
>  	{ .index = MSR_IA32_LASTBRANCHTOIP,		.always = false },
>  	{ .index = MSR_IA32_LASTINTFROMIP,		.always = false },
> @@ -917,6 +920,9 @@ static void svm_vcpu_init_msrpm(u32 *msrpm)
>  
>  		set_msr_interception(msrpm, direct_access_msrs[i].index, 1, 1);
>  	}
> +
> +	if (boot_cpu_has(X86_FEATURE_SPEC_CTRL))
> +		set_msr_interception(msrpm, MSR_IA32_SPEC_CTRL, 1, 1);

This isn't needed.  The entry in the direct_access_msrs will do this in
the loop above.

Thanks,
Tom

>  }
>  
>  static void add_msr_offset(u32 offset)
> @@ -3576,6 +3582,9 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  	case MSR_VM_CR:
>  		msr_info->data = svm->nested.vm_cr_msr;
>  		break;
> +	case MSR_IA32_SPEC_CTRL:
> +		msr_info->data = svm->spec_ctrl;
> +		break;
>  	case MSR_IA32_UCODE_REV:
>  		msr_info->data = 0x01000065;
>  		break;
> @@ -3724,6 +3733,9 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>  	case MSR_VM_IGNNE:
>  		vcpu_unimpl(vcpu, "unimplemented wrmsr: 0x%x data 0x%llx\n", ecx, data);
>  		break;
> +	case MSR_IA32_SPEC_CTRL:
> +		svm->spec_ctrl = data;
> +		break;
>  	case MSR_IA32_APICBASE:
>  		if (kvm_vcpu_apicv_active(vcpu))
>  			avic_update_vapic_bar(to_svm(vcpu), data);
> @@ -4871,6 +4883,19 @@ static void svm_cancel_injection(struct kvm_vcpu *vcpu)
>  	svm_complete_interrupts(svm);
>  }
>  
> +
> +/*
> + * Save guest value of spec_ctrl and also restore host value
> + */
> +static void save_guest_spec_ctrl(struct vcpu_svm *svm)
> +{
> +	if (boot_cpu_has(X86_FEATURE_SPEC_CTRL)) {
> +		svm->spec_ctrl = spec_ctrl_get();
> +		spec_ctrl_restriction_on();
> +	} else
> +		rmb();
> +}
> +
>  static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
> @@ -4910,6 +4935,14 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>  
>  	clgi();
>  
> +	if (boot_cpu_has(X86_FEATURE_SPEC_CTRL)) {
> +		/*
> +		 * FIXME: lockdep_assert_irqs_disabled();
> +		 */
> +		WARN_ON_ONCE(!irqs_disabled());
> +		spec_ctrl_set(svm->spec_ctrl);
> +	}
> +
>  	local_irq_enable();
>  
>  	asm volatile (
> @@ -4985,6 +5018,8 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>  #endif
>  		);
>  
> +	save_guest_spec_ctrl(svm);
> +
>  #ifdef CONFIG_X86_64
>  	wrmsrl(MSR_GS_BASE, svm->host.gs_base);
>  #else
>