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 2F75F391E55 for ; Mon, 22 Jun 2026 08:42:10 +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=1782117732; cv=none; b=iy63jqHvZWk6dRtZtSWRIM/p7tlbrZWcE8OYxAUhNmXmxkq4n7kstigS9753UkNc3HnPBL7bBEUtLYlIw12wkKEH9pKrUSsK0Fu/qbsqTbz08uofc4sWHbAiPSz67xBw86UY2DWbDnAgQvJke5cLH9CGr5mWeODL73pZzslqVcs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782117732; c=relaxed/simple; bh=Pkl/PUDIZt4KS3ZbuQvTjfZsisKXSh2xJdoVCBlGGVk=; h=Message-ID:Date:MIME-Version:Subject:To:References:From: In-Reply-To:Content-Type; b=uxEyRQuVor7y1/aoKkWZQx7kKv6AGDeYhIn3pgqN8v32fM25S7vUvojlBdR1LjvBMIvJlQcpvL5WHubWFzH6W53J5z8tZV2ecGw8+nqCx4DjELODWNssPDc7o80oqtjiavZDmVKw7+X2bz5J3Hbi2D5LC801MvUkydgiYjoZ2fU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ZDgrapma; 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="ZDgrapma" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 934861F000E9; Mon, 22 Jun 2026 08:42:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782117730; bh=xh0MWjYizRFTYa+kD7h4zkk1P+Ps1TPM2ylHsDUWOwM=; h=Date:Subject:To:References:From:In-Reply-To; b=ZDgrapmaKr3AjHERXYgXs9ALFWKz9TrovNzH5e4o8tZWG+QDNRQRFO/H7pZ1tAfba uaqzg/3sekjHYPCoyCK15DSCjAgwhQyo3eO8dAulssUAYOFGwSdXygzA4TbPVydljP guinBnB45dJrRxacooxYULClTskzPiEqEqMl8SqNDQBmln/fL8PjKs+RiUmoTIwMjw VZ/dI0bpRmUKxWeN0Q/mEkHAI2bJ/yzeeS0stYzDIrWcimfgITUs7YFjWJRYM7tYsN BGoCHkUsUWfAIeoYKfSHR6Ejva3ETY3ktUiNOSJeJSFcNJfKk9/15NRbU8rt7Gx8AX RYJCtQCSEY1MQ== Message-ID: Date: Mon, 22 Jun 2026 10:42:06 +0200 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 v1] kernel: add a simple timer based software watchpoint To: Feng Tang , Andrew Morton , Petr Mladek , Steven Rostedt , paulmck@kernel.org, Douglas Anderson , Thomas Gleixner , Peter Zijlstra , Vlastimil Babka , linux-kernel@vger.kernel.org References: <20260622081430.37557-1-feng.tang@linux.alibaba.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/22/26 10:14, Feng Tang wrote: > During debugging some bios/hardware related nasty memory corruption > issues, we found using periodic timer to monitor specific dram/mmio > physical address is very useful for debugging, which acts like > a basic software watchpoint. > > For those bugs, who (and when) change(corrupt) those dram or mmio > register is hard to trace, and sometimes even hardware jtag debugger > can't help (say the physical address watchpoint doesn't work). > > The biggest shortcoming of this method is it can never capture the > exact point like a hardware watchpoint. The idea is trying to > approach the point by adjusting the timer interval, hoping the caught > context have enough debug info (which did help us in solving quite > some bios/hardware bugs) > > The working flow is simple: after suspected address is identified, > start periodic timer polling it to catch if its value is changed to > target 'magic' value, then halt the cpu (better limit to have only > one cpu online), or panic, or print out system information, so that > the error environment is frozen for further check , or let > kexec/kdump to record the vmcore, etc. > > One real use case was: > " > On an arm64 platform, some BIOS/HW config caused OS boot easy to > stall in systemd init phase, then the reproducing was simplified it > by making it boot to console with a function-reduced rootfs, and > always triggering 'segmentation fault' when running 'less' command. > > By using GDB, some static array of 'less' is found corrupted before > being written, and one byte in its memory is always '0x33'.At this > stage the static array is in bss segment first, and backed by kernel > zero page after first read, so it was an obvious memory corruption. > > HW engineers tried to capture HW traces after the issue happened, but > could not find valuable hints, as the corruption could happen long > before 'less' is run, and the trace/context of that time was gone. > > As physical address of kernel zero page was known, and offset of the > corrupted byte was fixed, the address was A. But HW debugger failed > to breakpoint the point that address A was written with '0x33'. Then > this method was used to monitor 'writing 0x33 to A' with 30ms > interval, and halted the system by 'while (1);' (the system was made > to a UP by using 'nr_cpus=1' cmdline parameter) once hit, then HW > people collected the HW trace they need and root caused it to be a > bad config. > " > > The culprits of memory corruption issues we met are mainly: > * broken devices (like ethernet card) > * BIOS runtime service > * silicon bugs > * kernel itself > > As kernel already have many useful debug featues like slub_debug, > kasan, kfence, kmemleak etc.., this method could be more fit for the > upper three types. > > All the settings are module parameters: > > watch_interval_ms: SW watchpoint check interval in ms > paddr_dram_to_watch: Physical dram address to monitor. > target_dram_val: Expected value at the dram address that triggers the watchpoint. > paddr_mmio_to_watch: Physical mmio address to monitor. Must be 32-bit aligned. > target_mmio_val: Expected value at the mmio address that triggers the watchpoint. > panic_on_hit: Trigger kernel panic when watchpoint condition hits. > hang_on_hit: halt the CPU (wait for HW debugger) > > This provides the basic function, and there are more todoes: > * add a ftrace mode to do function level monitoring with ftrace hook, > which is more accurate timing wise, as suggested by Steven Rostedt > * merge the dram/mmio interface to auto detect it's dram or mmio > * support runtime changing the address > * move the starting point earlier in boot phase > * monitor a whole memory region > * currently is monitoring 'changing to a value', add support > for 'changing from a value' That really looks more like the kind of thing you would want to carry as a OOT hack for your special debugging needs :) -- Cheers, David