linux-kernel - Re: [PATCH v7 2/3] KVM: vmx: Emulate MSR IA32_UMWAIT

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Date:   Fri, 12 Jul 2019 08:52:02 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Tao Xu <tao3.xu@...el.com>
Cc:     pbonzini@...hat.com, rkrcmar@...hat.com, corbet@....net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        fenghua.yu@...el.com, xiaoyao.li@...ux.intel.com,
        jingqi.liu@...el.com
Subject: Re: [PATCH v7 2/3] KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL

On Fri, Jul 12, 2019 at 04:29:06PM +0800, Tao Xu wrote:
> diff --git a/arch/x86/kernel/cpu/umwait.c b/arch/x86/kernel/cpu/umwait.c
> index 6a204e7336c1..631152a67c6e 100644
> --- a/arch/x86/kernel/cpu/umwait.c
> +++ b/arch/x86/kernel/cpu/umwait.c
> @@ -15,7 +15,8 @@
>   * Cache IA32_UMWAIT_CONTROL MSR. This is a systemwide control. By default,
>   * umwait max time is 100000 in TSC-quanta and C0.2 is enabled
>   */
> -static u32 umwait_control_cached = UMWAIT_CTRL_VAL(100000, UMWAIT_C02_ENABLE);
> +u32 umwait_control_cached = UMWAIT_CTRL_VAL(100000, UMWAIT_C02_ENABLE);
> +EXPORT_SYMBOL_GPL(umwait_control_cached);

It'd probably be better to add an accessor to expose umwait_control_cached
given that umwait.c is using {READ,WRITE}_ONCE() and there shouldn't be a
need to write it outside of umwait.c.

>  /*
>   * Serialize access to umwait_control_cached and IA32_UMWAIT_CONTROL MSR in
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index f411c9ae5589..0787f140d155 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1676,6 +1676,12 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  #endif
>  	case MSR_EFER:
>  		return kvm_get_msr_common(vcpu, msr_info);
> +	case MSR_IA32_UMWAIT_CONTROL:
> +		if (!msr_info->host_initiated && !vmx_has_waitpkg(vmx))
> +			return 1;
> +
> +		msr_info->data = vmx->msr_ia32_umwait_control;
> +		break;
>  	case MSR_IA32_SPEC_CTRL:
>  		if (!msr_info->host_initiated &&
>  		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
> @@ -1838,6 +1844,16 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  			return 1;
>  		vmcs_write64(GUEST_BNDCFGS, data);
>  		break;
> +	case MSR_IA32_UMWAIT_CONTROL:
> +		if (!msr_info->host_initiated && !vmx_has_waitpkg(vmx))
> +			return 1;
> +
> +		/* The reserved bit IA32_UMWAIT_CONTROL[1] should be zero */
> +		if (data & BIT_ULL(1))
> +			return 1;
> +
> +		vmx->msr_ia32_umwait_control = data;

The SDM only defines bits 31:0, and the kernel uses a u32 to cache its
value.  I assume bits 63:32 are reserved?  I'm guessing we also need an
SDM update...

> +		break;
>  	case MSR_IA32_SPEC_CTRL:
>  		if (!msr_info->host_initiated &&
>  		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
> @@ -4139,6 +4155,8 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  	vmx->rmode.vm86_active = 0;
>  	vmx->spec_ctrl = 0;
>  
> +	vmx->msr_ia32_umwait_control = 0;
> +
>  	vcpu->arch.microcode_version = 0x100000000ULL;
>  	vmx->vcpu.arch.regs[VCPU_REGS_RDX] = get_rdx_init_val();
>  	kvm_set_cr8(vcpu, 0);
> @@ -6352,6 +6370,19 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
>  					msrs[i].host, false);
>  }
>