From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Maciej S. Szmigiero" Date: Tue, 09 Nov 2021 16:29:45 +0000 Subject: Re: [PATCH v5.5 20/30] KVM: x86: Use nr_memslot_pages to avoid traversing the memslots array Message-Id: List-Id: References: <20211104002531.1176691-1-seanjc@google.com> <20211104002531.1176691-21-seanjc@google.com> <88d64cd0-4db1-34a8-96af-6661a55e971e@oracle.com> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Sean Christopherson Cc: James Morse , Alexandru Elisei , Suzuki K Poulose , Atish Patra , David Hildenbrand , Cornelia Huck , Claudio Imbrenda , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-mips@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, Ben Gardon , Marc Zyngier , Huacai Chen , Aleksandar Markovic , Anup Patel , Paul Mackerras , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Paolo Bonzini On 09.11.2021 02:34, Sean Christopherson wrote: > On Tue, Nov 09, 2021, Maciej S. Szmigiero wrote: >> On 04.11.2021 01:25, Sean Christopherson 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. Use KVM's >>> cached nr_memslot_pages to compute the default max number of MMU pages. >>> >>> Signed-off-by: Maciej S. Szmigiero >>> [sean: use common KVM field and rework changelog accordingly] > > Heh, and I forgot to add "and introduce bugs" > >>> Signed-off-by: Sean Christopherson >>> --- >>> arch/x86/include/asm/kvm_host.h | 1 - >>> arch/x86/kvm/mmu/mmu.c | 24 ------------------------ >>> arch/x86/kvm/x86.c | 11 ++++++++--- >>> 3 files changed, 8 insertions(+), 28 deletions(-) >>> >> (..) >>> --- a/arch/x86/kvm/x86.c >>> +++ b/arch/x86/kvm/x86.c >>> @@ -11837,9 +11837,14 @@ void kvm_arch_commit_memory_region(struct kvm *kvm, >>> enum kvm_mr_change change) >>> { >>> if (!kvm->arch.n_requested_mmu_pages && >>> - (change = KVM_MR_CREATE || change = KVM_MR_DELETE)) >>> - kvm_mmu_change_mmu_pages(kvm, >>> - kvm_mmu_calculate_default_mmu_pages(kvm)); >>> + (change = KVM_MR_CREATE || change = KVM_MR_DELETE)) { >>> + unsigned long nr_mmu_pages; >>> + >>> + nr_mmu_pages = kvm->nr_memslot_pages * KVM_PERMILLE_MMU_PAGES; >> >> Unfortunately, even if kvm->nr_memslot_pages is capped at ULONG_MAX then >> this value multiplied by 20 can still overflow an unsigned long variable. > > Doh. And that likely subtly avoided by the compiler collapsing the "* 20 / 1000" > into "/ 50". Unfortunately in this case (but fortunately in general) C has well-defined unsigned overflow behavior so if this code has an overflow by its naive reading the compiler is not allowed to transform it to a form which doesn't have this overflow (as it will result in a different value). > Any objection to adding a patch to cut out the multiplication entirely? Well, cut > it from the source code, looks like gcc generates some fancy SHR+MUL to do the > divide. > > I'm thinking this: > > #define KVM_MEMSLOT_PAGES_TO_MMU_PAGES_RATIO 50 > > > ... > > nr_mmu_pages = nr_pages / KVM_MEMSLOT_PAGES_TO_MMU_PAGES_RATIO; > > I agree this would be a good solution to our problem, since this macro is unlikely to be converted into something user-configurable in the future. So its units don't matter. Thanks, Maciej