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 17:36:27 +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 15:52, Tom Lendacky wrote:
> On 2/21/2018 8:47 AM, Tom Lendacky wrote:
>> 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

Yeah, it was referring to AMD hosts that don't have the MSR.  Sorry for
the confusion.

>> 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.
> 
> Hmm, actually I'll just leave the signature alone and pass in a local
> kvm_msr_entry struct variable for the call when initializing the list.

Sounds good!

Paolo