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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id z13sm217243pfq.130.2021.10.19.15.24.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 19 Oct 2021 15:24:15 -0700 (PDT) Date: Tue, 19 Oct 2021 22:24:11 +0000 From: Sean Christopherson To: "Maciej S. Szmigiero" Cc: Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Igor Mammedov , Marc Zyngier , James Morse , Julien Thierry , Suzuki K Poulose , Huacai Chen , Aleksandar Markovic , Paul Mackerras , Christian Borntraeger , Janosch Frank , David Hildenbrand , Cornelia Huck , Claudio Imbrenda , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v5 01/13] KVM: x86: Cache total page count to avoid traversing the memslot array Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 20, 2021, Maciej S. Szmigiero wrote: > From: "Maciej S. Szmigiero" > > There is no point in recalculating from scratch the total number of pages > in all memslots each time a memslot is created or deleted. > > Just cache the value and update it accordingly on each such operation so > the code doesn't need to traverse the whole memslot array each time. > > Signed-off-by: Maciej S. Szmigiero > --- > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 28ef14155726..65fdf27b9423 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -11609,9 +11609,23 @@ void kvm_arch_commit_memory_region(struct kvm *kvm, > const struct kvm_memory_slot *new, > enum kvm_mr_change change) > { > - if (!kvm->arch.n_requested_mmu_pages) > - kvm_mmu_change_mmu_pages(kvm, > - kvm_mmu_calculate_default_mmu_pages(kvm)); > + if (change == KVM_MR_CREATE) > + kvm->arch.n_memslots_pages += new->npages; > + else if (change == KVM_MR_DELETE) { > + WARN_ON(kvm->arch.n_memslots_pages < old->npages); > + kvm->arch.n_memslots_pages -= old->npages; > + } > + > + if (!kvm->arch.n_requested_mmu_pages) { Hmm, once n_requested_mmu_pages is set it can't be unset. That means this can be further optimized to skip avoid taking mmu_lock on flags-only changes (and memslot movement). E.g. if (!kvm->arch.n_requested_mmu_pages && (change == KVM_MR_CREATE || change == KVM_MR_DELETE)) { } It's a little risky, but kvm_vm_ioctl_set_nr_mmu_pages() would need to be modified to allow clearing n_requested_mmu_pages and it already takes slots_lock, so IMO it's ok to force kvm_vm_ioctl_set_nr_mmu_pages() to recalculate pages if it wants to allow reverting back to the default. > + u64 memslots_pages; > + unsigned long nr_mmu_pages; > + > + memslots_pages = kvm->arch.n_memslots_pages * KVM_PERMILLE_MMU_PAGES; > + do_div(memslots_pages, 1000); > + nr_mmu_pages = max_t(typeof(nr_mmu_pages), > + memslots_pages, KVM_MIN_ALLOC_MMU_PAGES); "memslots_pages" is a bit of a misnomer. Any objection to avoiding naming problems by explicitly casting to an "unsigned long" and simply operating on nr_mmu_pages? nr_mmu_pages = (unsigned long)kvm->arch.n_memslots_pages; nr_mmu_pages *= (KVM_PERMILLE_MMU_PAGES / 1000); nr_mmu_pages = max(nr_mmu_pages, KVM_MIN_ALLOC_MMU_PAGES); kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages); E.g. the whole thing can be if (!kvm->arch.n_requested_mmu_pages && (change == KVM_MR_CREATE || change == KVM_MR_DELETE)) { unsigned long nr_mmu_pages; if (change == KVM_MR_CREATE) { kvm->arch.n_memslots_pages += new->npages; } else { WARN_ON(kvm->arch.n_memslots_pages < old->npages); kvm->arch.n_memslots_pages -= old->npages; } nr_mmu_pages = (unsigned long)kvm->arch.n_memslots_pages; nr_mmu_pages *= (KVM_PERMILLE_MMU_PAGES / 1000); nr_mmu_pages = max(nr_mmu_pages, KVM_MIN_ALLOC_MMU_PAGES); kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages); } > + kvm_mmu_change_mmu_pages(kvm, nr_mmu_pages); > + } > > kvm_mmu_slot_apply_flags(kvm, old, new, change); >