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

From: Paolo Bonzini
Date: Wed Feb 21 2018 - 06:41:53 EST


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;
> + }