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AJvYcCXP1tc253IEhezgZwsVltp03Lkqfg1nRzhM+dEQy1ZZ6Aeqj7M7im76xLaL3D9YLOdp/hJ5YHZ44t5eQEbX0J4muZ8= X-Gm-Message-State: AOJu0YwiWUM+Dw2R+5KYx4a4B6orARzLOFPY8caW8XDmBjsq7IPQ7REA zdv4gnUNagRpdAaDgyvrjZnGiVlp4UQ7tkct5dWHzmGBP/itQYmuy1xclLUqnXxrnhXZZ11urnt EKw== X-Google-Smtp-Source: AGHT+IGTqFQO1W9uWgNfx86uFJoJR25380gE+MX0EH8aoPTaRHgwGtgxURTOkAOuuR3wa2vaOk4BcpKiUBQ= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a05:690c:660c:b0:62f:f535:f2c with SMTP id 00721157ae682-62ff53512f5mr5078687b3.2.1718205771477; Wed, 12 Jun 2024 08:22:51 -0700 (PDT) Date: Wed, 12 Jun 2024 08:22:49 -0700 In-Reply-To: <20240419112432.GB2972@willie-the-truck> Mime-Version: 1.0 References: <20240405115815.3226315-1-pbonzini@redhat.com> <20240405115815.3226315-2-pbonzini@redhat.com> <20240412104408.GA27645@willie-the-truck> <86jzl2sovz.wl-maz@kernel.org> <86h6g5si0m.wl-maz@kernel.org> <20240418141932.GA1855@willie-the-truck> <20240419112432.GB2972@willie-the-truck> Message-ID: Subject: Re: [PATCH 1/4] KVM: delete .change_pte MMU notifier callback From: Sean Christopherson To: Will Deacon Cc: Marc Zyngier , Paolo Bonzini , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Oliver Upton , Tianrui Zhao , Bibo Mao , Thomas Bogendoerfer , Nicholas Piggin , Anup Patel , Atish Patra , Andrew Morton , David Hildenbrand , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org Content-Type: text/plain; charset="us-ascii" X-Rspamd-Queue-Id: BE993180004 X-Rspam-User: X-Rspamd-Server: rspam12 X-Stat-Signature: 1fcj6qgtx11inujf7okuex77hf736suo X-HE-Tag: 1718205772-167199 X-HE-Meta: U2FsdGVkX1+CTBR/5lxCxuE0ITYlcDa/4XDVW3t+FmAhGUedUDCUD+b/JVA4ylZpguy0aMQsQTH8DBf40/UBMmAesXa4TcpPpecRM1oHl4FoU3Qv66om1Xk4q4269/YhSPdT30YIbcvbaRzd63oCWivA2TWDTpF9HT6a/dr8qHhP9kPex0PM4zOZrf3BKTfaJVW0ZrEdUbOfi6y4quKn0/pE2Iy7pLfFNr/ieAD51+b1G2KBGBq2HfLg/IZwRqH5bH+317okKYUna6Pax8hbPUaVUEMMEKLBYkWPEffRZPM5BVDTtOk33LAZJ2l8tufReUiXi7SqEfqXA4hcNUvQpd2SXcGabstHze1MHohJKSYhf1EBlb6KOy6zb+xh8RkEM0ExmbuZXUXtjok2+MOZ+Nnf9BUFqzhBLyPfLn34TBK0E4BkaXlwarCQ0K3leH6GG9uqqJTfdgUGvE4TNmc7mIHp/4VSmvDzflkz7qdCFXx5VIIJJUwZUQw0iLZjFY+DYLpeZBk1Qi1QMw0TVZ+JnO5TCMKjYpAwtGFG9YdvwKzeqiVUOJvef8vmg67m5meeh152oSDv3ueeoVYWWjzTBmQags+QJJMsOtBeB5MnAxAmIi/23W35H3A0J0Zud/tsvxUXoghF8vRPV+m4d+Fon19eC/9yEOFyh1qEiwMwmB8BdTBiw/vAzEhXxkvDec3McK5jgmirkjfMrJxj+10olUezEEfnRgj+RufEuzFVZozK9/8ayTkEdMdSo9RBZ3jkX0gR0ZsOobnYqk+AerZSZ/DLj/C0MxEZ2kSdRK16DFKOyViUEB3FXAYRSoOuoMQq9G1FOlnX+ffkLvTiJgpyMBFRiLl8XSP2dC/GmnqFcZAroxubwHA/X4fxLFm2753YWIcvEoWhmghalg2XgMvFqeUojDvwSpSTe5P75fPbH+wVo3sKXyrioax63K0Bvtw6osvaksVtZ0j8W6R9s8N r/jzq4iP 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Apr 19, 2024, Will Deacon wrote: > On Thu, Apr 18, 2024 at 12:53:26PM -0700, Sean Christopherson wrote: > > On Thu, Apr 18, 2024, Will Deacon wrote: > > > > I assume the idea would be to let arch code do single-page invalidations of > > > > stage-2 entries for each gfn? > > > > > > Right, as it's the only code which knows which ptes actually ended up > > > being aged. > > > > > > > Unless I'm having a brain fart, x86 can't make use of that functionality. Intel > > > > doesn't provide any way to do targeted invalidation of stage-2 mappings. AMD > > > > provides an instruction to do broadcast invalidations, but it takes a virtual > > > > address, i.e. a stage-1 address. I can't tell if it's a host virtual address or > > > > a guest virtual address, but it's a moot point because KVM doen't have the guest > > > > virtual address, and if it's a host virtual address, there would need to be valid > > > > mappings in the host page tables for it to work, which KVM can't guarantee. > > > > > > Ah, so it sounds like it would need to be an arch opt-in then. > > > > Even if x86 (or some other arch code) could use the precise tracking, I think it > > would make sense to have the behavior be arch specific. Adding infrastructure > > to get information from arch code, only to turn around and give it back to arch > > code would be odd. > > Sorry, yes, that's what I had in mind. Basically, a way for the arch code > to say "I've handled the TLBI, don't worry about it." > > > Unless arm64 can't do the invalidation immediately after aging the stage-2 PTE, > > the best/easiest solution would be to let arm64 opt out of the common TLB flush > > when a SPTE is made young. > > > > With the range-based flushing bundled in, this? > > > > --- > > include/linux/kvm_host.h | 2 ++ > > virt/kvm/kvm_main.c | 40 +++++++++++++++++++++++++--------------- > > 2 files changed, 27 insertions(+), 15 deletions(-) > > > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > > index afbc99264ffa..8fe5f5e16919 100644 > > --- a/include/linux/kvm_host.h > > +++ b/include/linux/kvm_host.h > > @@ -2010,6 +2010,8 @@ extern const struct kvm_stats_header kvm_vcpu_stats_header; > > extern const struct _kvm_stats_desc kvm_vcpu_stats_desc[]; > > > > #ifdef CONFIG_KVM_GENERIC_MMU_NOTIFIER > > +int kvm_arch_flush_tlb_if_young(void); > > + > > static inline int mmu_invalidate_retry(struct kvm *kvm, unsigned long mmu_seq) > > { > > if (unlikely(kvm->mmu_invalidate_in_progress)) > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > > index 38b498669ef9..5ebef8ef239c 100644 > > --- a/virt/kvm/kvm_main.c > > +++ b/virt/kvm/kvm_main.c > > @@ -595,6 +595,11 @@ static void kvm_null_fn(void) > > } > > #define IS_KVM_NULL_FN(fn) ((fn) == (void *)kvm_null_fn) > > > > +int __weak kvm_arch_flush_tlb_if_young(void) > > +{ > > + return true; > > +} > > I tend to find __weak functions a little ugly, but I think the gist of the > diff looks good to me. Thanks for putting it together! Circling back to this, I don't think we should pursue this specific tweak, at least not without hard data for a concrete use case. The clear_flush_young() hook is the only callback that overloads the return value, e.g. for invalidate_range_start(), arch code can simply return false if the flush has already been performed. And clear_flush_young() _always_ operates on a single page, i.e. the range will only ever cover a single page in the primary MMU. It's obviously possible that KVM's MMU has mapped a transparent hugepage using multiple smaller pages, but that should be relatively uncommon, and probably not worth optimizing for.