From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 028EBD0E6CF for ; Tue, 25 Nov 2025 12:23:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Type:MIME-Version: Message-ID:Date:References:In-Reply-To:Subject:Cc:To:From:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=Q6u/e9Bl+58OV6dGb303lDWzf7gS76A7HV+spLpj29o=; b=mY9BjEkvMN/oo3hq6nju3ecgdg OljvN/60BFhzV+zw3zAGgoq7yk/lrul9rp1Fx5u4jx51zQuG4nMl4RSLenFlgb/O3aIIID9dXAkB1 K7v3icBHxYi0n/SPDm6i1inI7OaByBCd5lXhgr5y3U5smucsWg8/DBaNUl0QpkjqdFkXtZdmnkQor R2SLoWlLxGEa0I9VZHM8aToVwwCuY16c7CcrrFRGjiSovpKx3LFqlSGJKXbUnRVR0dSIWmXUlsfCt 2lK73xCU5UjBX1FPrSYho9xPO7PJ5lLOVwFVsmPJ7NRGHsSYNJl3S9dP0kd8Z0m+NbjCnIlf4y5a0 X9SKEYkw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vNs4i-0000000DHJI-2wB8; Tue, 25 Nov 2025 12:23:16 +0000 Received: from sea.source.kernel.org ([2600:3c0a:e001:78e:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1vNs4f-0000000DHIh-3WFU; Tue, 25 Nov 2025 12:23:15 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 587AB43CD0; Tue, 25 Nov 2025 12:23:12 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id CA125C4CEF1; Tue, 25 Nov 2025 12:23:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764073392; bh=dgBsZgJWsujT1G5ZZyKp5poEm84jGPGY7WbAbiL0MR0=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=PrkIGiiHSiipdIgzxPoyKCJ4Fy3jRwcIwcp5kblp2SuzY9IJRC9td8hSxj7WBX+OK tHsOa1jxjoE8ceReNCftcWRT98GvaRM0Ia8paupqWSuu06gNtntWVK2V4LQWr0WsV7 VGIT5x3vE1vHyOipQTu/CVZ/WVCeMe+WT6R2hFP8Jj95v9ajS+QegQGNA9Diro6RHD FIV26stHU8SOFSWUTETfYamJZmJqnmIg2kx494WXDxRs+HUC0o5o9j6B2MVoEucVx2 vyKGLTcBVIH5jsgc2lFyq6s8Xt3D454mrw3aOT/8J72JonvIU8bLUsk2B2hRd/b7TU THa3yTkV/FFbA== From: Pratyush Yadav To: "H. Peter Anvin" Cc: Usama Arif , Changyuan Lyu , akpm@linux-foundation.org, linux-kernel@vger.kernel.org, Mike Rapoport , anthony.yznaga@oracle.com, arnd@arndb.de, ashish.kalra@amd.com, benh@kernel.crashing.org, bp@alien8.de, catalin.marinas@arm.com, corbet@lwn.net, dave.hansen@linux.intel.com, devicetree@vger.kernel.org, dwmw2@infradead.org, ebiederm@xmission.com, graf@amazon.com, jgowans@amazon.com, kexec@lists.infradead.org, krzk@kernel.org, linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, luto@kernel.org, mark.rutland@arm.com, mingo@redhat.com, pasha.tatashin@soleen.com, pbonzini@redhat.com, peterz@infradead.org, robh@kernel.org, rostedt@goodmis.org, saravanak@google.com, skinsburskii@linux.microsoft.com, tglx@linutronix.de, thomas.lendacky@amd.com, will@kernel.org, x86@kernel.org, Breno Leitao , thevlad@meta.com Subject: Re: [PATCH v8 12/17] x86/e820: temporarily enable KHO scratch for memory below 1M In-Reply-To: <22BDBF5C-C831-4BBC-A854-20CA77234084@zytor.com> (H. Peter Anvin's message of "Mon, 24 Nov 2025 16:56:34 -0800") References: <20250509074635.3187114-1-changyuanl@google.com> <20250509074635.3187114-13-changyuanl@google.com> <22BDBF5C-C831-4BBC-A854-20CA77234084@zytor.com> Date: Tue, 25 Nov 2025 13:23:05 +0100 Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20251125_042313_937562_E3735045 X-CRM114-Status: GOOD ( 21.72 ) 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 Mon, Nov 24 2025, H. Peter Anvin wrote: > On November 24, 2025 11:24:58 AM PST, Usama Arif wrote: >> >> >>On 09/05/2025 08:46, Changyuan Lyu wrote: >>> From: Alexander Graf >>> >>> KHO kernels are special and use only scratch memory for memblock >>> allocations, but memory below 1M is ignored by kernel after early boot >>> and cannot be naturally marked as scratch. >>> >>> To allow allocation of the real-mode trampoline and a few (if any) other >>> very early allocations from below 1M forcibly mark the memory below 1M >>> as scratch. >>> >>> After real mode trampoline is allocated, clear that scratch marking. >>> >>> Signed-off-by: Alexander Graf >>> Co-developed-by: Mike Rapoport (Microsoft) >>> Signed-off-by: Mike Rapoport (Microsoft) >>> Co-developed-by: Changyuan Lyu >>> Signed-off-by: Changyuan Lyu >>> Acked-by: Dave Hansen >>> --- >>> arch/x86/kernel/e820.c | 18 ++++++++++++++++++ >>> arch/x86/realmode/init.c | 2 ++ >>> 2 files changed, 20 insertions(+) >>> >>> diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c >>> index 9920122018a0b..c3acbd26408ba 100644 >>> --- a/arch/x86/kernel/e820.c >>> +++ b/arch/x86/kernel/e820.c >>> @@ -1299,6 +1299,24 @@ void __init e820__memblock_setup(void) >>> memblock_add(entry->addr, entry->size); >>> } >>> >>> + /* >>> + * At this point memblock is only allowed to allocate from memory >>> + * below 1M (aka ISA_END_ADDRESS) up until direct map is completely set >>> + * up in init_mem_mapping(). >>> + * >>> + * KHO kernels are special and use only scratch memory for memblock >>> + * allocations, but memory below 1M is ignored by kernel after early >>> + * boot and cannot be naturally marked as scratch. >>> + * >>> + * To allow allocation of the real-mode trampoline and a few (if any) >>> + * other very early allocations from below 1M forcibly mark the memory >>> + * below 1M as scratch. >>> + * >>> + * After real mode trampoline is allocated, we clear that scratch >>> + * marking. >>> + */ >>> + memblock_mark_kho_scratch(0, SZ_1M); >>> + >>> /* >>> * 32-bit systems are limited to 4BG of memory even with HIGHMEM and >>> * to even less without it. >>> diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c >>> index f9bc444a3064d..9b9f4534086d2 100644 >>> --- a/arch/x86/realmode/init.c >>> +++ b/arch/x86/realmode/init.c >>> @@ -65,6 +65,8 @@ void __init reserve_real_mode(void) >>> * setup_arch(). >>> */ >>> memblock_reserve(0, SZ_1M); >>> + >>> + memblock_clear_kho_scratch(0, SZ_1M); >>> } >>> >>> static void __init sme_sev_setup_real_mode(struct trampoline_header *th) >> >>Hello! >> >>I am working with Breno who reported that we are seeing the below warning at boot >>when rolling out 6.16 in Meta fleet. It is difficult to reproduce on a single host >>manually but we are seeing this several times a day inside the fleet. >> >> 20:16:33 ------------[ cut here ]------------ >> 20:16:33 WARNING: CPU: 0 PID: 0 at mm/memblock.c:668 memblock_add_range+0x316/0x330 >> 20:16:33 Modules linked in: >> 20:16:33 CPU: 0 UID: 0 PID: 0 Comm: swapper Tainted: G S 6.16.1-0_fbk0_0_gc0739ee5037a #1 NONE >> 20:16:33 Tainted: [S]=CPU_OUT_OF_SPEC >> 20:16:33 RIP: 0010:memblock_add_range+0x316/0x330 >> 20:16:33 Code: ff ff ff 89 5c 24 08 41 ff c5 44 89 6c 24 10 48 63 74 24 08 48 63 54 24 10 e8 26 0c 00 00 e9 41 ff ff ff 0f 0b e9 af fd ff ff <0f> 0b e9 b7 fd ff ff 0f 0b 0f 0b cc cc cc cc cc cc cc cc cc cc cc >> 20:16:33 RSP: 0000:ffffffff83403dd8 EFLAGS: 00010083 ORIG_RAX: 0000000000000000 >> 20:16:33 RAX: ffffffff8476ff90 RBX: 0000000000001c00 RCX: 0000000000000002 >> 20:16:33 RDX: 00000000ffffffff RSI: 0000000000000000 RDI: ffffffff83bad4d8 >> 20:16:33 RBP: 000000000009f000 R08: 0000000000000020 R09: 8000000000097101 >> 20:16:33 R10: ffffffffff2004b0 R11: 203a6d6f646e6172 R12: 000000000009ec00 >> 20:16:33 R13: 0000000000000002 R14: 0000000000100000 R15: 000000000009d000 >> 20:16:33 FS: 0000000000000000(0000) GS:0000000000000000(0000) knlGS:0000000000000000 >> 20:16:33 CR2: ffff888065413ff8 CR3: 00000000663b7000 CR4: 00000000000000b0 >> 20:16:33 Call Trace: >> 20:16:33 >> 20:16:33 ? __memblock_reserve+0x75/0x80 >> 20:16:33 ? setup_arch+0x30f/0xb10 >> 20:16:33 ? start_kernel+0x58/0x960 >> 20:16:33 ? x86_64_start_reservations+0x20/0x20 >> 20:16:33 ? x86_64_start_kernel+0x13d/0x140 >> 20:16:33 ? common_startup_64+0x13e/0x140 >> 20:16:33 >> 20:16:33 ---[ end trace 0000000000000000 ]--- >> >> >>Rolling out with memblock=debug is not really an option in a large scale fleet due to the >>time added to boot. But I did try on one of the hosts (without reproducing the issue) and I see: >> >>[ 0.000616] memory.cnt = 0x6 >>[ 0.000617] memory[0x0] [0x0000000000001000-0x000000000009bfff], 0x000000000009b000 bytes flags: 0x40 >>[ 0.000620] memory[0x1] [0x000000000009f000-0x000000000009ffff], 0x0000000000001000 bytes flags: 0x40 >>[ 0.000621] memory[0x2] [0x0000000000100000-0x000000005ed09fff], 0x000000005ec0a000 bytes flags: 0x0 >>... >> >>The 0x40 (MEMBLOCK_KHO_SCRATCH) is coming from memblock_mark_kho_scratch in e820__memblock_setup. I believe this >>should be under ifdef like the diff at the end? (Happy to send this as a patch for review if it makes sense). >>We have KEXEC_HANDOVER disabled in our defconfig, therefore MEMBLOCK_KHO_SCRATCH shouldnt be selected and >>we shouldnt have any MEMBLOCK_KHO_SCRATCH type regions in our memblock reservations. >> >>The other thing I did was insert a while(1) just before the warning and inspected the registers in qemu. >>R14 held the base register, and R15 held the size at that point. >>In the warning R14 is 0x100000 meaning that someone is reserving a region with a different flag to MEMBLOCK_NONE >>at the boundary of MEMBLOCK_KHO_SCRATCH. >> >>diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c >>index c3acbd26408ba..26e4062a0bd09 100644 >>--- a/arch/x86/kernel/e820.c >>+++ b/arch/x86/kernel/e820.c >>@@ -1299,6 +1299,7 @@ void __init e820__memblock_setup(void) >> memblock_add(entry->addr, entry->size); >> } >> >>+#ifdef CONFIG_MEMBLOCK_KHO_SCRATCH >> /* >> * At this point memblock is only allowed to allocate from memory >> * below 1M (aka ISA_END_ADDRESS) up until direct map is completely set >>@@ -1316,7 +1317,7 @@ void __init e820__memblock_setup(void) >> * marking. >> */ >> memblock_mark_kho_scratch(0, SZ_1M); >>- >>+#endif >> /* >> * 32-bit systems are limited to 4BG of memory even with HIGHMEM and >> * to even less without it. >>diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c >>index 88be32026768c..1cd80293a3e23 100644 >>--- a/arch/x86/realmode/init.c >>+++ b/arch/x86/realmode/init.c >>@@ -66,8 +66,9 @@ void __init reserve_real_mode(void) >> * setup_arch(). >> */ >> memblock_reserve(0, SZ_1M); >>- >>+#ifdef CONFIG_MEMBLOCK_KHO_SCRATCH >> memblock_clear_kho_scratch(0, SZ_1M); >>+#endif >> } >> >> static void __init sme_sev_setup_real_mode(struct trampoline_header *th) > > What does "scratch" mean in this exact context? (Sorry, don't have the code in front of me.) See https://docs.kernel.org/core-api/kho/concepts.html#scratch-regions -- Regards, Pratyush Yadav