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AJvYcCUk0JxbZS3fbdL8DNRaf9l4UdcEY9HZMcL+MLL6iIbFRFyzFrK7iCuKdLhl1W00DaM6fMd8FtXXBDcypQI=@vger.kernel.org X-Gm-Message-State: AOJu0Yxvx9X+68oFnreuaBptED0qUgmkjxj42U2qXFWM6XzY/A5L+OY2 D4TtPYzZxRYJPmUGe+ynYaOBqgcsp/lndKOfR6HBgyC4vNWglNejRJpVMhCeiB1unM4zuvg2+nS 6iw== X-Google-Smtp-Source: AGHT+IE5X1SEViPqpTe89vzn6EtzphukY6AvSLniOTlThnRLrJgSHxS02dH03m+qXPZkT33RR5rviIXHEb8= X-Received: from pjbok14.prod.google.com ([2002:a17:90b:1d4e:b0:2ff:5df6:7e03]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90a:da87:b0:2ff:7b28:a519 with SMTP id 98e67ed59e1d1-3085efefbb2mr1188859a91.30.1744753963182; Tue, 15 Apr 2025 14:52:43 -0700 (PDT) Date: Tue, 15 Apr 2025 14:52:41 -0700 In-Reply-To: <20250415200635.GA210309.vipinsh@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250401155714.838398-1-seanjc@google.com> <20250401155714.838398-4-seanjc@google.com> <20250415200635.GA210309.vipinsh@google.com> Message-ID: Subject: Re: [PATCH v2 3/3] KVM: x86/mmu: Defer allocation of shadow MMU's hashed page list From: Sean Christopherson To: Vipin Sharma Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="us-ascii" On Tue, Apr 15, 2025, Vipin Sharma wrote: > On 2025-04-01 08:57:14, Sean Christopherson wrote: > > +static __ro_after_init HLIST_HEAD(empty_page_hash); > > + > > +static struct hlist_head *kvm_get_mmu_page_hash(struct kvm *kvm, gfn_t gfn) > > +{ > > + struct hlist_head *page_hash = READ_ONCE(kvm->arch.mmu_page_hash); > > + > > + if (!page_hash) > > + return &empty_page_hash; > > + > > + return &page_hash[kvm_page_table_hashfn(gfn)]; > > +} > > + > > > > @@ -2357,6 +2368,7 @@ static struct kvm_mmu_page *__kvm_mmu_get_shadow_page(struct kvm *kvm, > > struct kvm_mmu_page *sp; > > bool created = false; > > > > + BUG_ON(!kvm->arch.mmu_page_hash); > > sp_list = &kvm->arch.mmu_page_hash[kvm_page_table_hashfn(gfn)]; > > Why do we need READ_ONCE() at kvm_get_mmu_page_hash() but not here? We don't (need it in kvm_get_mmu_page_hash()). I suspect past me was thinking it could be accessed without holding mmu_lock, but that's simply not true. Unless I'm forgetting, something, I'll drop the READ_ONCE() and WRITE_ONCE() in kvm_mmu_alloc_page_hash(), and instead assert that mmu_lock is held for write. > My understanding is that it is in kvm_get_mmu_page_hash() to avoid compiler > doing any read tear. If yes, then the same condition is valid here, isn't it? The intent wasn't to guard against a tear, but to instead ensure mmu_page_hash couldn't be re-read and end up with a NULL pointer deref, e.g. if KVM set mmu_page_hash and then nullfied it because some later step failed. But if mmu_lock is held for write, that is simply impossible.