linux-kernel - Re: [PATCH v2 1/2] KVM: x86: Add a framework for supporting MSR-based features

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Date:   Wed, 21 Feb 2018 12:41:45 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Tom Lendacky <thomas.lendacky@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v2 1/2] KVM: x86: Add a framework for supporting MSR-based
 features

On 16/02/2018 00:12, Tom Lendacky wrote:
> +static u32 msr_based_features[] = {
> +};
> +
> +static unsigned int num_msr_based_features = ARRAY_SIZE(msr_based_features);
> +
>  bool kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer)
>  {
>  	if (efer & efer_reserved_bits)
> @@ -2785,6 +2794,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	case KVM_CAP_SET_BOOT_CPU_ID:
>   	case KVM_CAP_SPLIT_IRQCHIP:
>  	case KVM_CAP_IMMEDIATE_EXIT:
> +	case KVM_CAP_GET_MSR_FEATURES:
>  		r = 1;
>  		break;
>  	case KVM_CAP_ADJUST_CLOCK:
> @@ -4410,6 +4420,47 @@ long kvm_arch_vm_ioctl(struct file *filp,
>  			r = kvm_x86_ops->mem_enc_unreg_region(kvm, &region);
>  		break;
>  	}
> +	case KVM_GET_MSR_INDEX_LIST: {
> +		struct kvm_msr_list __user *user_msr_list = argp;
> +		struct kvm_msr_list msr_list;
> +		unsigned int n;
> +
> +		r = -EFAULT;
> +		if (copy_from_user(&msr_list, user_msr_list, sizeof(msr_list)))
> +			goto out;
> +		n = msr_list.nmsrs;
> +		msr_list.nmsrs = num_msr_based_features;
> +		if (copy_to_user(user_msr_list, &msr_list, sizeof(msr_list)))
> +			goto out;
> +		r = -E2BIG;
> +		if (n < msr_list.nmsrs)
> +			goto out;
> +		r = -EFAULT;
> +		if (copy_to_user(user_msr_list->indices, &msr_based_features,
> +				 num_msr_based_features * sizeof(u32)))
> +			goto out;
> +		r = 0;
> +		break;

I think it's better to have some logic in kvm_init_msr_list, to filter
the MSR list based on whatever MSRs the backend provides.

> +	}
> +	case KVM_GET_MSR: {

It's not that the API isn't usable, KVM_GET_MSR is fine for what we need
here (it's not a fast path), but it's a bit confusing to have
KVM_GET_MSR and KVM_GET_MSRS.

I see two possibilities:

1) reuse KVM_GET_MSRS as in the previous version.  It's okay to
cut-and-paste code from msr_io.

2) find a name for KVM_GET_MSR that is better and different from
KVM_GET_MSRS.  KVM_GET_HOST_MSR or KVM_GET_HOST_FEATURE_MSR come to
mind, but I'm obviously open to other suggestions.

Thanks!

Paolo

> +		struct kvm_msr_entry __user *user_msr = argp;
> +		struct kvm_msr_entry msr;
> +
> +		r = -EFAULT;
> +		if (copy_from_user(&msr, user_msr, sizeof(msr)))
> +			goto out;
> +
> +		r = 1;
> +		if (!kvm_x86_ops->msr_feature || kvm_x86_ops->msr_feature(&msr))
> +			goto out;
> +
> +		r = -EFAULT;
> +		if (copy_to_user(user_msr, &msr, sizeof(msr)))
> +			goto out;
> +
> +		r = 0;
> +		break;
> +	}