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

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Message-ID: <94f6ed44-07ab-ff0f-2a6d-b3d8ff4ff4b1@redhat.com>
Date:   Wed, 21 Feb 2018 15:32:30 +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 21/02/2018 15:15, Tom Lendacky wrote:
> On 2/21/2018 5:41 AM, Paolo Bonzini wrote:
>> 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.
> 
> Ok, that's what I had originally and then you said to just return the full
> list and let KVM_GET_MSR return a 0 or 1 if it was supported. I can switch
> it back.

Hmm, I cannot find this remark (I would have been very confused, so I
tried to look for it).  I commented on removing kvm_valid_msr_feature,
but not kvm_init_msr_list.

>>
>>> +	}
>>> +	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.
> 
> If I go back to trimming the list based on support, then KVM_GET_MSRS can
> be used.

No problem, renaming is enough---I should have made a better suggestion
in the previous review.

Paolo