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Fri, 04 Sep 2026 07:51:28 +0000 Date: Fri, 04 Sep 2026 08:54:02 +0100 Message-ID: <874ig55u5h.wl-maz@kernel.org> From: Marc Zyngier To: Shuai Xue Cc: Wei-Lin Chang , Wang Han , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, oupton@kernel.org, tabba@google.com, joey.gouly@arm.com, seiden@linux.ibm.com, suzuki.poulose@arm.com, catalin.marinas@arm.com, will@kernel.org, ljs@kernel.org, itaru.kitayama@fujitsu.com Subject: Re: [PATCH v5 0/6] KVM: arm64: nv: Implement nested stage-2 reverse map (new data structure) In-Reply-To: <46342b48-550e-42c1-9d9f-e800c72269c2@linux.alibaba.com> References: <20260810205038.118843-1-weilin.chang@arm.com> <20260902163500.1841671-1-wanghan@linux.alibaba.com> <877bl26aqg.wl-maz@kernel.org> <46342b48-550e-42c1-9d9f-e800c72269c2@linux.alibaba.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/30.1 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: xueshuai@linux.alibaba.com, weilin.chang@arm.com, wanghan@linux.alibaba.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, oupton@kernel.org, tabba@google.com, joey.gouly@arm.com, seiden@linux.ibm.com, suzuki.poulose@arm.com, catalin.marinas@arm.com, will@kernel.org, ljs@kernel.org, itaru.kitayama@fujitsu.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Fri, 04 Sep 2026 08:01:24 +0100, Shuai Xue wrote: > > > > On 9/3/26 9:28 PM, Wei-Lin Chang wrote: > > On Thu, Sep 03, 2026 at 08:43:35AM +0100, Marc Zyngier wrote: > >> On Wed, 02 Sep 2026 17:35:00 +0100, > >> Wang Han wrote: > >>> > >>> Hi Wei-Lin, > >>> > >>> I tested this series on a Yitian 710 system with an ARM Neoverse-N2 CPU > >>> (128 CPUs, 2 NUMA nodes). > >>> > >>> Test environment > >>> ---------------- > >>> > >>> L0 kernel: Linux v7.2-rc6 > >>> L1 guest: Ubuntu 26.04 LTS, kernel 7.0.0-27-generic (aarch64) > >>> QEMU: 10.2.3 > >>> > >>> L0 NUMA balancing was enabled (`/proc/sys/kernel/numa_balancing=1`). > >>> The host was booted with `kvm_arm.mode=nested`. > >>> > >>> This series fixes a functional hang that is exposed when NUMA balancing is > >>> enabled. The previous nested stage-2 unmap path is too slow for this > >>> workload, making the performance problem user-visible: NUMA balancing can > >>> leave the L1 guest unable to make progress and eventually hang during boot. > >>> > >>> The L1 was started with 8 vCPUs and 32 GiB of RAM using: > >>> > >>> qemu-system-aarch64 -smp 8 -m 32G \ > >>> -machine virt,accel=kvm,gic-version=3,virtualization=on \ > >>> -cpu host -nographic -enable-kvm \ > >>> -drive if=pflash,format=raw,readonly=on,file=pflash0_bak.img \ > >>> -drive if=pflash,format=raw,file=pflash1_bak.img \ > >>> -drive file=./ubuntu-vm.qcow2,format=qcow2,if=virtio,cache=none,aio=native \ > >>> -nic user,model=virtio-net-pci,hostfwd=tcp::11234-:22 \ > >>> -serial mon:stdio > >>> > >> > >> Puzzling. If you are only running an L1 in VHE mode, there is no > >> shadow S2, and therefore nothing to unmap. For shadow S2s to be built > >> and affect the MMU notifiers, you need to run an L2. > > > > I was thinking the same at first, but realized even with L1 in VHE mode > > there is a small period of time where L1 runs in its EL1 during boot, so > > one nested MMU will become valid for each vCPU. That causes > > kvm_nested_s2_unmap() to iterate through the entire IPA space 8 times > > (-smp 8). > > > > What I am curious about is whether one single notifier unmap is enough > > to hang L1, or were there multiple notifier unmaps. > > > > QEMU with -machine virt uses 40 IPA bits only, unmapping that takes: > > 1024 (4KB pages, unmapping 1GB per iteration) > > 32768 (16KB pages, unmapping 32MB per iteration) > > 2048 (64KB pages, unmapping 512MB per iteration) > > iterations for each page size. There aren't many mappings in each > > iteration too. Does this really take that long on real hardware (even if > > this must be done 8 times)? > > > > Thanks, > > Wei-Lin Chang > > > >> > >> So what are your actual test conditions? > >> > >> M. > >> > > Hi, Wei-Lin and Marc, > > I was able to reproduce this issue and capture ftrace evidence that confirms > the root cause. Below is the analysis, trace log, and timing data. [...] > Each set_migration_pte line is a single-page NUMA migration. Yet each > migration triggers one full kvm_nested_s2_unmap() that takes 877 ms. And why is it taking so long? It should be *empty* after the first iteration. [...] > ## Conclusion > > The root cause is confirmed: kvm_nested_s2_unmap() performs a full IPA space > unmap in the MMU notifier path instead of unmapping only the affected > GPA/CPAI range. The interval-tree-based precise range unmap approach is the > right fix. No. This just indicates that this is papering over a bigger problem, and your AI is jumping to conclusions. > Please consider applying the patch that replaces the full unmap with > kvm_nested_unmap_cipa_range() to avoid scanning the entire nested stage-2 > page table on every NUMA migration. Not until we get to the bottom of this issue. M. -- Jazz isn't dead. It just smells funny.