linux-kernel - Re: [PATCH 29/61] KVM: x86: Add Kconfig-controlled auditing of reverse CPUID lookups

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Date:   Tue, 25 Feb 2020 16:02:47 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 29/61] KVM: x86: Add Kconfig-controlled auditing of
 reverse CPUID lookups

On 24/02/20 23:46, Sean Christopherson wrote:
>>>  static __always_inline u32 *__cpuid_entry_get_reg(struct kvm_cpuid_entry2 *entry,
>>>  						  const struct cpuid_reg *cpuid)
>>>  {
>>> +#ifdef CONFIG_KVM_CPUID_AUDIT
>>> +	WARN_ON_ONCE(entry->function != cpuid->function);
>>> +	WARN_ON_ONCE(entry->index != cpuid->index);
>>> +#endif
>>> +
>>>  	switch (cpuid->reg) {
>>>  	case CPUID_EAX:
>>>  		return &entry->eax;
>>
>> Honestly, I was thinking we should BUG_ON() and even in production builds
>> but not everyone around is so rebellious I guess, so
> 
> LOL.  It's a waste of cycles for something that will "never" be hit, i.e.
> we _really_ dropped the ball if a bug of this natures makes it into a
> kernel release.

There are quite a few WARN_ONs like that already.  I'd say each
non-constant-folded call to __cpuid_enty_get_reg is a waste of cycles,
if you're counting them. :)

Paolo