From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 E064445628F; Wed, 12 Aug 2026 13:46:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786542392; cv=none; b=XRqjAFk/rq9p3YUqAs9y/VXKubIjOJGgl/54y0LoIqOJQ9qE3n6GTb7Gyj04A7IfMQ5+7gZZfpi4GkMjGlrcz7cnZWMEToHrLvp1Mf8yCOJyiXIt8pRWwHcMZTDwUYhtq7D9Rnu+BknO7v8GfQs3li0Vr5iKrnBm44tmB3PcE/Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786542392; c=relaxed/simple; bh=d6CFgm9ThDI74C5UpE0AaMmYwSV538IKMaCXrd5hQp4=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=kGrHHoHoTFlESGCP8uf4kcngVOETbPIzx4F9+RHylJ+oNNr20Pwf/ac0wxHVVMz0AVbxNVUPvQfn5Or0e0oDq3PA0UujTUQLn6VL1S2czRRGRvYskW5F6slszeW+R2sq6Z5zKrooXUVp2Hq+zTXE3K2DefN0yEhLRQDt6U0/w7w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=j/XwHSDD; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="j/XwHSDD" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CF67A1F00A3D; Wed, 12 Aug 2026 13:46:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786542390; bh=3sp0BNJKgir4wziOWxyB3nibDSrXXlF/m0GwaxwJDoc=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=j/XwHSDDlSeIfFJkwG+03RnS1FIDWCvalaj6TTMs9rirdJk8Xj+nPHaGkq+b+fLoh ygXhpCrNkZdM7uGIQHeW88B1speKaGqBlG01SE8kh8cJb+uDyzgiAVcljlW506ZpiY nHOxFIaPczrGkmhFBxtISWT9g10oUFpZKiUSQNbXFUQmYOzxw33it0XXiO/0yqrbRB MDPT3LCZ6X43BmuAF5gmtzyzNAgRpyULCZ1xUPw+xybHn8mZFDkdXa4psyDC9bMwby eTvlLhpK2k+EBw4SrbaOnUc2AEHeZcY1xVuI9pK+bLo3wv/2XcZi9HG2DCv0V5l0lc wFGqJ1/M2YreQ== Date: Wed, 12 Aug 2026 14:46:25 +0100 From: "Lorenzo Stoakes (ARM)" To: Marc Zyngier , Oliver Upton , Fuad Tabba , Joey Gouly , Steffen Eiden , Suzuki K Poulose , Zenghui Yu , Catalin Marinas , Will Deacon , Jintack Lim , Christoffer Dall , Christoffer Dall Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH 1/2] KVM: arm64: Fix spurious warning for benign stage 2 teardown race Message-ID: References: <20260812-kvm-arm-nested-virt-fix-v1-0-4ad883f1b6a5@kernel.org> <20260812-kvm-arm-nested-virt-fix-v1-1-4ad883f1b6a5@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260812-kvm-arm-nested-virt-fix-v1-1-4ad883f1b6a5@kernel.org> The splats: ------------[ cut here ]------------ WARNING: arch/arm64/kvm/mmu.c:336 at __unmap_stage2_range+0x64/0x80, CPU#5: kcompactd0/66 ... Tainted: [S]=CPU_OUT_OF_SPEC Hardware name: Apple Inc. Mac14,7/J493, BIOS 2026.04 04/01/2026 pstate: 61401009 (nZCv daif +PAN -UAO -TCO +DIT +SSBS BTYPE=--) pc : __unmap_stage2_range (arch/arm64/kvm/mmu.c:335 (discriminator 3)) lr : __unmap_stage2_range (arch/arm64/kvm/mmu.c:336 (discriminator 5)) sp : ffff8000856e7450 x29: ffff8000856e7450 x28: ffff0000404b2000 x27: 0000000000000030 x26: ffffffffffffffe0 x25: 0000000000000001 x24: ffff00000eaa5920 x23: ffff0000404b2000 x22: 0000000000000000 x21: 0000000000000040 x20: 0000000000000001 x19: 0000000000000001 x18: 0000000000000006 x17: 0000000000000000 x16: 0000000000000002 x15: ffff800083adbed0 x14: ffff800083adbed0 x13: 0000000000000008 x12: 0000b00000000000 x11: 0000000000000001 x10: 0000006000000000 x9 : ffff8000817334fc x8 : 0000000000200000 x7 : ffff0000c59ea000 x6 : 0000000000000800 x5 : 0000000038000000 x4 : 0000000000000000 x3 : ffff0000404b2000 x2 : 0000000000000001 x1 : 0000000038000000 x0 : 00000000ffffffea Call trace: __unmap_stage2_range (arch/arm64/kvm/mmu.c:335 (discriminator 3)) (P) kvm_stage2_unmap_range (arch/arm64/kvm/mmu.c:346) kvm_nested_s2_unmap (arch/arm64/kvm/nested.c:1168 (discriminator 14)) kvm_unmap_gfn_range (arch/arm64/kvm/mmu.c:2416) kvm_mmu_notifier_invalidate_range_start (arch/arm64/kvm/../../../virt/kvm/kvm_main.c:718 (discriminator 32768) arch/arm64/kvm/../../../virt/kvm/kvm_main.c:633 (discriminator 32768) arch/arm64/kvm/../../../virt/kvm/kvm_main.c:765 (discriminator 32768)) mn_hlist_invalidate_range_start (mm/mmu_notifier.c:525) __mmu_notifier_invalidate_range_start (mm/mmu_notifier.c:580) try_to_migrate_one (./include/linux/mmu_notifier.h:478 ./include/linux/mmu_notifier.h:471 mm/rmap.c:2459) rmap_walk_anon (mm/rmap.c:3001) rmap_walk (mm/rmap.c:3106 mm/rmap.c:3101) try_to_migrate (mm/rmap.c:2774) migrate_folio_unmap (mm/migrate.c:1330 (discriminator 3)) migrate_pages_batch (mm/migrate.c:1909) migrate_pages_sync (mm/migrate.c:2026) migrate_pages (mm/migrate.c:2135) compact_zone (mm/compaction.c:2663) compact_node (mm/compaction.c:2932) kcompactd (mm/compaction.c:3230) kthread (kernel/kthread.c:436) ret_from_fork (arch/arm64/kernel/entry.S:858) ---[ end trace 0000000000000000 ]--- ------------[ cut here ]------------ WARNING: arch/arm64/kvm/mmu.c:336 at __unmap_stage2_range+0x64/0x80, CPU#5: kcompactd0/66 ... Tainted: [S]=CPU_OUT_OF_SPEC, [W]=WARN Hardware name: Apple Inc. Mac14,7/J493, BIOS 2026.04 04/01/2026 pstate: 61401009 (nZCv daif +PAN -UAO -TCO +DIT +SSBS BTYPE=--) pc : __unmap_stage2_range (arch/arm64/kvm/mmu.c:335 (discriminator 3)) lr : __unmap_stage2_range (arch/arm64/kvm/mmu.c:336 (discriminator 5)) sp : ffff8000856e7450 x29: ffff8000856e7450 x28: ffff0000404b2000 x27: 0000000000000030 x26: ffffffffffffffe0 x25: 0000000000000001 x24: ffff00000eaa5920 x23: ffff0000404b2000 x22: 0000000000000001 x21: 0000000000000040 x20: 0000000000000001 x19: 0000000000000001 x18: 0000000000000006 x17: 0000000000000000 x16: 0000000000000002 x15: ffff800083adbed0 x14: ffff800083adbed0 x13: 0000000000000008 x12: 0000b00000000000 x11: 0000000000000001 x10: 0000006000000000 x9 : ffff8000817334fc x8 : 0000000000200000 x7 : ffff0000c59ea078 x6 : 0000000000000800 x5 : 0000000002000000 x4 : 0000000000000000 x3 : ffff0000404b2000 x2 : 0000000000000001 x1 : 0000000002000000 x0 : 00000000ffffffea Call trace: __unmap_stage2_range (arch/arm64/kvm/mmu.c:335 (discriminator 3)) (P) kvm_stage2_unmap_range (arch/arm64/kvm/mmu.c:346) kvm_nested_s2_unmap (arch/arm64/kvm/nested.c:1168 (discriminator 14)) kvm_unmap_gfn_range (arch/arm64/kvm/mmu.c:2416) kvm_mmu_notifier_invalidate_range_start (arch/arm64/kvm/../../../virt/kvm/kvm_main.c:718 (discriminator 32768) arch/arm64/kvm/../../../virt/kvm/kvm_main.c:633 (discriminator 32768) arch/arm64/kvm/../../../virt/kvm/kvm_main.c:765 (discriminator 32768)) mn_hlist_invalidate_range_start (mm/mmu_notifier.c:525) __mmu_notifier_invalidate_range_start (mm/mmu_notifier.c:580) try_to_migrate_one (./include/linux/mmu_notifier.h:478 ./include/linux/mmu_notifier.h:471 mm/rmap.c:2459) rmap_walk_anon (mm/rmap.c:3001) rmap_walk (mm/rmap.c:3106 mm/rmap.c:3101) try_to_migrate (mm/rmap.c:2774) migrate_folio_unmap (mm/migrate.c:1330 (discriminator 3)) migrate_pages_batch (mm/migrate.c:1909) migrate_pages_sync (mm/migrate.c:2026) migrate_pages (mm/migrate.c:2135) compact_zone (mm/compaction.c:2663) compact_node (mm/compaction.c:2932) kcompactd (mm/compaction.c:3230) kthread (kernel/kthread.c:436) ret_from_fork (arch/arm64/kernel/entry.S:858) ---[ end trace 0000000000000000 ]--- ------------[ cut here ]------------ WARNING: arch/arm64/kvm/mmu.c:336 at __unmap_stage2_range+0x64/0x80, CPU#5: kcompactd0/66 ... Tainted: [S]=CPU_OUT_OF_SPEC, [W]=WARN Hardware name: Apple Inc. Mac14,7/J493, BIOS 2026.04 04/01/2026 pstate: 61401009 (nZCv daif +PAN -UAO -TCO +DIT +SSBS BTYPE=--) pc : __unmap_stage2_range (arch/arm64/kvm/mmu.c:335 (discriminator 3)) lr : __unmap_stage2_range (arch/arm64/kvm/mmu.c:336 (discriminator 5)) sp : ffff8000856e7450 x29: ffff8000856e7450 x28: ffff0000404b2000 x27: 0000000000000030 x26: ffffffffffffffe0 x25: 0000000000000001 x24: ffff00000eaa5920 x23: ffff0000404b2000 x22: 0000000000000002 x21: 0000000000000040 x20: 0000000000000001 x19: 0000000000000001 x18: 0000000000000006 x17: 0000000000000000 x16: 0000000000000002 x15: ffff800083adbed0 x14: ffff800083adbed0 x13: 0000000000000008 x12: 0000b00000000000 x11: 0000000000000001 x10: 0000006000000000 x9 : ffff8000817334fc x8 : 0000000000200000 x7 : ffff0000c59ea0f0 x6 : 0000000000000800 x5 : 0000000002000000 x4 : 0000000000000000 x3 : ffff0000404b2000 x2 : 0000000000000001 x1 : 0000000002000000 x0 : 00000000ffffffea Call trace: __unmap_stage2_range (arch/arm64/kvm/mmu.c:335 (discriminator 3)) (P) kvm_stage2_unmap_range (arch/arm64/kvm/mmu.c:346) kvm_nested_s2_unmap (arch/arm64/kvm/nested.c:1168 (discriminator 14)) kvm_unmap_gfn_range (arch/arm64/kvm/mmu.c:2416) kvm_mmu_notifier_invalidate_range_start (arch/arm64/kvm/../../../virt/kvm/kvm_main.c:718 (discriminator 32768) arch/arm64/kvm/../../../virt/kvm/kvm_main.c:633 (discriminator 32768) arch/arm64/kvm/../../../virt/kvm/kvm_main.c:765 (discriminator 32768)) mn_hlist_invalidate_range_start (mm/mmu_notifier.c:525) __mmu_notifier_invalidate_range_start (mm/mmu_notifier.c:580) try_to_migrate_one (./include/linux/mmu_notifier.h:478 ./include/linux/mmu_notifier.h:471 mm/rmap.c:2459) rmap_walk_anon (mm/rmap.c:3001) rmap_walk (mm/rmap.c:3106 mm/rmap.c:3101) try_to_migrate (mm/rmap.c:2774) migrate_folio_unmap (mm/migrate.c:1330 (discriminator 3)) migrate_pages_batch (mm/migrate.c:1909) migrate_pages_sync (mm/migrate.c:2026) migrate_pages (mm/migrate.c:2135) compact_zone (mm/compaction.c:2663) compact_node (mm/compaction.c:2932) kcompactd (mm/compaction.c:3230) kthread (kernel/kthread.c:436) ret_from_fork (arch/arm64/kernel/entry.S:858) ---[ end trace 0000000000000000 ]--- -- Also I wrote something extra but dropped from the already-far-too-long commit msg but the registers match the diagnosis, basically: -- The observed assembly from the splats is consistent with this analysis: Examining the assembly for stage2_apply_range() loop and pgt check, conveniently x1 contains the current IPA being examined (addr) and is not clobbered on return: mov x1, x19 addr = next ldr x0, [x26, #16] struct kvm_pgtable *pgt = mmu->pgt; cbnz x0, ... if (!pgt)... ldp x19, x20, [sp, #16] } mov w0, #0xffffffea } return -EINVAL ret } Splat 1 has x1 of 0x38000000 which tells us the splat occurred 27 * 32 MiB or 864 MiB into the nested guest physical address space (27 iterations). This is consistent with an an interleaving stage 2 tear down occurring part-way through this operation. There being 2 other splats is consistent with there being a total of 3 nested mmus, as expected, and that the mmu is invalidated before the warning can occur. The remaining 2 have x1 equal to 0x2000000 or exactly 32 MiB, which is what you would expect with the stage2_apply_range() and kvm_arch_flush_shadow_all() running in lockstep across mmus: kvm_unmap_gfn_range() | kvm_arch_flush_shadow_all() ---------------------------------|---------------------------- < kvm->mmu_lock held > | stage2_apply_range() | .. 26 successful iterations .. | < contend kvm->mmu_lock > < releases kvm->mmu lock > | < acquires kvm->mmu_lock > | mmu->pgt = NULL | < releases kvm->mmu_lock > Then 3 occurrences of: < reacquires kvm->mmu_lock > | return to __unmap_stage2_range() | WARN_ON(-EINVAL); | mmu = &kvm->arch.nested_mmus[1]; | stage2_apply_range() | .. 1 successful iteration .. | < releases kvm->mmu lock > | < contend kvm->mmu_lock > | < acquires kvm->mmu_lock > | mmu->pgt = NULL | < releases kvm->mmu_lock > One single iteration sets addr = next = 0x2000000 which is reflected in the splats. Finally, in kvm_nested_s2_unmap() the disassembly indicates that w22 maintains the i counter, which happens not to be clobbered over the operation either: mov w22, #0x0 for (i = 0; add w22, w22, #0x1 ; i++) sxtw x2, w22 } cmp x2, x1 } i < nested_mmus_size The splats have w22 equal to 0, 1 and 2 respectively which further corroborates this analysis. -- Cheers, Lorenzo