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Fri, 17 Feb 2023 08:00:09 -0800 (PST) Date: Fri, 17 Feb 2023 08:00:08 -0800 In-Reply-To: Mime-Version: 1.0 References: <20230217041230.2417228-1-yuzhao@google.com> <20230217041230.2417228-4-yuzhao@google.com> Message-ID: Subject: Re: [PATCH mm-unstable v1 3/5] kvm/arm64: add kvm_arch_test_clear_young() From: Sean Christopherson To: Oliver Upton Content-Type: text/plain; charset="us-ascii" X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-mm@google.com, Yu Zhao , kvm@vger.kernel.org, Jonathan Corbet , Michael Larabel , x86@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, kvmarm@lists.linux.dev, Paolo Bonzini , Andrew Morton , linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Fri, Feb 17, 2023, Oliver Upton wrote: > Hi Yu, > > scripts/get_maintainers.pl is your friend for getting the right set of > emails for a series :) Don't know about others, but generally I would > prefer to be Cc'ed on an entire series (to gather context) than just an > individual patch. +1 > > On Thu, Feb 16, 2023 at 09:12:28PM -0700, Yu Zhao wrote: > > This patch adds kvm_arch_test_clear_young() for the vast majority of > > VMs that are not pKVM and run on hardware that sets the accessed bit > > in KVM page tables. At least for the x86 changes, please read Documentation/process/maintainer-tip.rst and rewrite the changelogs. > > It relies on two techniques, RCU and cmpxchg, to safely test and clear > > the accessed bit without taking the MMU lock. The former protects KVM > > page tables from being freed while the latter clears the accessed bit > > atomically against both the hardware and other software page table > > walkers. > > > > Signed-off-by: Yu Zhao > > --- > > arch/arm64/include/asm/kvm_host.h | 7 +++ > > arch/arm64/include/asm/kvm_pgtable.h | 8 +++ > > arch/arm64/include/asm/stage2_pgtable.h | 43 ++++++++++++++ > > arch/arm64/kvm/arm.c | 1 + > > arch/arm64/kvm/hyp/pgtable.c | 51 ++-------------- > > arch/arm64/kvm/mmu.c | 77 ++++++++++++++++++++++++- > > 6 files changed, 141 insertions(+), 46 deletions(-) > > > > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h > > index 35a159d131b5..572bcd321586 100644 > > --- a/arch/arm64/include/asm/kvm_host.h > > +++ b/arch/arm64/include/asm/kvm_host.h > > @@ -1031,4 +1031,11 @@ static inline void kvm_hyp_reserve(void) { } > > void kvm_arm_vcpu_power_off(struct kvm_vcpu *vcpu); > > bool kvm_arm_vcpu_stopped(struct kvm_vcpu *vcpu); > > > > +/* see the comments on the generic kvm_arch_has_test_clear_young() */ Please eliminate all of these "see the comments on blah", in every case they do nothing more than redirect the reader to something they're likely already aware of. > > +#define kvm_arch_has_test_clear_young kvm_arch_has_test_clear_young > > +static inline bool kvm_arch_has_test_clear_young(void) > > +{ > > + return IS_ENABLED(CONFIG_KVM) && cpu_has_hw_af() && !is_protected_kvm_enabled(); > > +} ... > Also, I'm at a loss for why we'd need to test if CONFIG_KVM is enabled. > My expectation is that we should provide an implementation that returns > false if !CONFIG_KVM, avoiding the need to repeat that bit in every > single implementation of the function. mm/vmscan.c uses kvm_arch_has_test_clear_young(). I have opinions on that, but I'll hold off on expressing them until there's actual justification presented somewhere. Yu, this series and each patch needs a big pile of "why". I get that the goal is to optimize memory oversubscribe, but there needs to be justification for why this is KVM only, why nested VMs and !A/D hardware are out of scope, why yet another mmu_notifier hook is being added, etc.