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 08:47:44 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Paolo Bonzini <pbonzini@...hat.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 2/21/2018 8:32 AM, Paolo Bonzini wrote:
> 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.

I think this is the reply that sent me off on that track:
  https://marc.info/?l=linux-kernel&m=151862648123153&w=2

I'll make it consistent with the other MSR-related items and initialize
the list in kvm_init_msr_list().  I'll change the signature of the
msr_feature() kvm_x86_ops callback to take an index and optionally return
a data value so it can be used to check for support when building the
list and return a value when needed.

Thanks,
Tom

> 
>>>
>>>> +	}
>>>> +	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
>