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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id x8sm5343865wru.46.2021.03.05.10.23.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 05 Mar 2021 10:23:19 -0800 (PST) Subject: Re: [PATCH v2 09/17] KVM: x86/mmu: Use '0' as the one and only value for an invalid PAE root To: Sean Christopherson Cc: Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Ben Gardon , Brijesh Singh , Tom Lendacky References: <20210305011101.3597423-1-seanjc@google.com> <20210305011101.3597423-10-seanjc@google.com> <63d2a610-f897-805c-23a7-488a65485f36@redhat.com> From: Paolo Bonzini Message-ID: Date: Fri, 5 Mar 2021 19:23:18 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/03/21 19:22, Sean Christopherson wrote: > On Fri, Mar 05, 2021, Paolo Bonzini wrote: >> On 05/03/21 02:10, Sean Christopherson wrote: >>> Use '0' to denote an invalid pae_root instead of '0' or INVALID_PAGE. >>> Unlike root_hpa, the pae_roots hold permission bits and thus are >>> guaranteed to be non-zero. Having to deal with both values leads to >>> bugs, e.g. failing to set back to INVALID_PAGE, warning on the wrong >>> value, etc... >> >> I don't dispute this is a good idea, but it deserves one or more comments. > > Agreed. What about adding macros? > > /* Comment goes here. */ > #define INVALID_PAE_ROOT 0 > #define IS_VALID_PAE_ROOT(x) (!!(x)) Yep, that's a nice solution. Paolo > > > Also, I missed this pattern in mmu_audit.c's mmu_spte_walk(): > > for (i = 0; i < 4; ++i) { > hpa_t root = vcpu->arch.mmu->pae_root[i]; > > if (root && VALID_PAGE(root)) { > root &= PT64_BASE_ADDR_MASK; > sp = to_shadow_page(root); > __mmu_spte_walk(vcpu, sp, fn, 2); > } > } >