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([2001:b07:6468:f312:3577:1cfe:d98a:5fb6]) by smtp.gmail.com with ESMTPSA id x6sm24495600wrr.6.2020.02.25.07.02.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 25 Feb 2020 07:02:48 -0800 (PST) Subject: Re: [PATCH 29/61] KVM: x86: Add Kconfig-controlled auditing of reverse CPUID lookups To: Sean Christopherson , Vitaly Kuznetsov Cc: Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org References: <20200201185218.24473-1-sean.j.christopherson@intel.com> <20200201185218.24473-30-sean.j.christopherson@intel.com> <87a758oztt.fsf@vitty.brq.redhat.com> <20200224224613.GO29865@linux.intel.com> From: Paolo Bonzini Message-ID: <7f944dd3-aed3-be94-624b-35e7d35cb2db@redhat.com> Date: Tue, 25 Feb 2020 16:02:47 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <20200224224613.GO29865@linux.intel.com> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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