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

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Date:   Wed, 14 Feb 2018 17:41:06 +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: [RFC PATCH 1/2] KVM: x86: Add a framework for supporting
 MSR-based features

On 14/02/2018 05:42, Tom Lendacky wrote:
>>> +bool kvm_valid_msr_feature(u32 msr, u64 data)
>>> +{
>>> +	unsigned int i;
>>> +
>>> +	for (i = 0; i < num_msr_based_features; i++) {
>>> +		struct kvm_msr_based_features *m = msr_based_features + i;
>>> +
>>> +		if (msr != m->msr)
>>> +			continue;
>>> +
>>> +		/* Make sure not trying to change unsupported bits */
>>> +		return (data & ~m->mask) ? false : true;
>>> +	}
>>> +
>>> +	return false;
>>> +}
>>> +EXPORT_SYMBOL_GPL(kvm_valid_msr_feature);
>>> +
>>
>> This is probably unnecessary too (the allowed values are a bit more
>> complicated for, you just guessed it, VMX capability MSRs) and you can
>> just check bits other than LFENCE in svm_set_msr.
>
> The whole routine or just the bit checking?  I can see still needing the
> check to be sure the "feature" is present.

You can return the MSR unconditionally from KVM_GET_MSR_INDEX_LIST.
Then KVM_GET_MSR would return 0 or 1 depending on whether the feature is
present.

Paolo