From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-111.freemail.mail.aliyun.com (out30-111.freemail.mail.aliyun.com [115.124.30.111]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 370163CF02D for ; Sun, 6 Sep 2026 13:02:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.111 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788699736; cv=none; b=iS2TC++0bL4nH1JvD+HT6VzgVlU44vM3g7ZVPHdEAsDiq34QpcsQcbdIzujVl9cdF5sYKNNVKKKxYCx19HGGilnC9I+dqeYtkFuVG7uYltmgrNj+15iv3iUXlRIfq+YF3ULA6LMW4H/IeKw4vu9STYW0FAGVfZY6K9PrwTmtOXo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788699736; c=relaxed/simple; bh=SL7KSLOVnOiXvOm2ScZarE9e8mOLSuHEMTpnQa02NWE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Y3EcCd3FGQv+LY4BlCzjeRsJ1vwsDKKNoaEkwIXvfa1K9BSIJnFacu41p3YIIt+1vUZEdfh83rrLPfKyvG2R/4quSrb8IGjquVyMD14v5aYzlhhj9wtVs0wd1CmjgO0sPreeajr4jZUcH35Cok4rXYboREKHFzAM/A/0RRoOgUo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=H7HH1kbR; arc=none smtp.client-ip=115.124.30.111 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="H7HH1kbR" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1788699724; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=nWlIJmD0HPwJ35ujSThXX3BByP+6GX+If6pUuoC+n4M=; b=H7HH1kbR1DJxvSZoMDqq5uKwSpt76UnNlfda2s/Rr6hm96Axg+WFWZMWOUZwyKc6NqFfrks1wTpX5mwzf/+bWvpUpWhPKbRvNJjol0TvOxkq3GSHXyrc/4MqQ64ZyC1S1G6LmmdzOeyDp4bQzj+hvGqTrAIPG/nxr6Y3xWST+/s= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R801e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam011083073210;MF=xueshuai@linux.alibaba.com;NM=1;PH=DS;RN=15;SR=0;TI=SMTPD_---0XAN1JSC_1788699722; Received: from 30.100.134.114(mailfrom:xueshuai@linux.alibaba.com fp:SMTPD_---0XAN1JSC_1788699722 cluster:ay36) by smtp.aliyun-inc.com; Sun, 06 Sep 2026 21:02:03 +0800 Message-ID: <1cca51a6-4315-43da-8400-837ee2196d15@linux.alibaba.com> Date: Sun, 6 Sep 2026 21:02:01 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 0/6] KVM: arm64: nv: Implement nested stage-2 reverse map (new data structure) To: Marc Zyngier 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 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> <874ig55u5h.wl-maz@kernel.org> <87o6ea4q67.wl-maz@kernel.org> From: Shuai Xue In-Reply-To: <87o6ea4q67.wl-maz@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 9/6/26 6:42 PM, Marc Zyngier wrote: > On Sat, 05 Sep 2026 16:35:01 +0100, > Shuai Xue wrote: >> >> >> >> On 9/4/26 3:54 PM, Marc Zyngier wrote: >>> 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. >> >> Good question. >> >> After dive into the details trace, let to try to answer the question. >> >> Yes, it is empty -- and that is exactly the point: the 877ms is paid *for* >> an empty table. >> >> The cost is not in the walk and not in clearing PTEs; >> **it is 262,144 broadcast TLB invalidations**, one at the end of every >> 1GB chunk, each ~3.3us. Since v6.6 the cost of an unmap is >> proportional to the size of the IPA range, not to the number of >> mappings; an empty table pays in full. >> >>> >>> [...] >>> >>>> ## 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. >> >> Sorry for the jumping up. >> >> The culprit is 7657ea920c54 ("KVM: arm64: Use TLBI range-based >> instructions for unmap", v6.6). kvm_pgtable_stage2_unmap() ends >> *every* call with an unconditional kvm_tlb_flush_vmid_range(), whether >> or not the walk cleared a single PTE: >> >> ret = kvm_pgtable_walk(pgt, addr, size, &walker); >> if (stage2_unmap_defer_tlb_flush(pgt)) >> /* Perform the deferred TLB invalidations */ >> kvm_tlb_flush_vmid_range(pgt->mmu, addr, size); >> >> stage2_apply_range() calls it once per 1GB chunk, and the nested MMU >> covers the guest PARange -- 48 bits here, so 262,144 calls per >> kvm_nested_s2_unmap(). Each broadcast is one IPAS2E1IS (range) plus one VMALLE1IS >> plus two DSB(ish), ~3.2-3.4us without ftrace: >> >> 877ms / 262,144 chunks = 3.35us per chunk >> >> which is exactly the per-broadcast cost. > > Right. That's pretty compelling, thanks for digging into this. Your > proposed approach (counting the invalidated regions) is interesting, > but I don't think it is the correct one. > > The real issue here is that we treat a full S2 unmap as if it was a > set of ranges. This is what needs fixing, because we can invalidate > the whole thing with exactly *ONE* TLBI. Aha, I see your point. Nice insight. > > This is even more important once you run an L2, as L1 will also > perform its own TLB invalidation, and we want to avoid having trapping > pointlessly. This also propagates in the way we handle TLB emulation. Quite agree. > > [...] > >> The candidate fix is below. Table entries >> (KVM_PGTABLE_WALK_TABLE_POST) are unaffected: stage2_unmap_put_pte() >> keeps issuing the immediate __kvm_tlb_flush_vmid_ipa() for them, and >> their child leaves are counted as leaves within the same walk, so any >> call that clears something still flushes a superset of what it >> cleared. This restores the pre-v6.6 semantics -- cost proportional to >> what is mapped. With it, an empty nested unmap costs ~110ms >> instrumented (the pure walk); getting to "exactly zero" would >> additionally require kvm_nested_s2_unmap() to skip nested MMUs that >> are valid but empty. > > That's an interesting remark. I guess we could add some extra tracking > for that, but let's see what we can do about the above first. > > I've hacked something together and pushed the result at [1] > (compile-tested only). I'd appreciate it if you could put it to the > test with your setup. > > Thanks, > > M. > > [1] https://web.git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/unmap-vmall Sure, I will test your proposed approach and update latter. Thanks. Shuai