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([2401:4900:1c44:36f7:ca41:f096:f228:c06a]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-74237a0ffc2sm345715b3a.102.2025.05.08.11.40.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 08 May 2025 11:41:00 -0700 (PDT) Message-ID: <278be55b-49e8-41ae-a7d4-c0eec6f1d90c@gmail.com> Date: Fri, 9 May 2025 00:10:57 +0530 Precedence: bulk X-Mailing-List: linux-openrisc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Queries regarding ftrace's implementation for OpenRISC-linux To: Stafford Horne Cc: Linux OpenRISC References: Content-Language: en-US From: Sahil Siddiq Autocrypt: addr=icegambit91@gmail.com; keydata= xsDNBGcgaYEBDADpKUSKbchLCMdCuZGkuF50/7BiraKc8Ch+mk4T+2+E2/6qXAkalvCkFoqx 3/sa35rconZAFzB/r19e7i3UajIQjATvENrGxqe/IFqcJxo2Jr1HQBwCrsmlQoUCilSC6nDi ejcEIAFytJORDkCcZwLXPjdf5/4pbqVAW5823LB5j5F0TqHAnGY1RhS2V1eBPdRqjAA3xecT zTmLHlkqAXgM2DOot1KbycedZSieCwEykTXMaLC0/3Gyo2Cp1WTWOIyD0hsXpLyFioV4FaX2 Lm+z45Zc4PoNXeC6+l4PdDxixs+saAbadknP+9omwlb+PkMd3esq2wkowTwTJVJK8FCCNTo5 2OArA/ddxcyXY25JHN7vzGooFNW6Bb9YV+lbX6y95ytE3KcAmid73tQrcjlebIpgNAvOMyyZ BgQJY0HSu3DGNZuKtbNM3iTl82TFj7MVgkEffgF83N6XyBqDztIz2lN47/q5wyRi3jda9NDt geI+Nv145HjulO7bI3NT048AEQEAAc0kU2FoaWwgU2lkZGlxIDxpY2VnYW1iaXQ5MUBnbWFp bC5jb20+wsENBBMBCAA3FiEERtYfQYWFu+uAZjYrrzGlXdb6f1cFAmcgaYEFCQWjmoACGwME CwkIBwUVCAkKCwUWAgMBAAAKCRCvMaVd1vp/V/nnC/9KnNIr4a3JW3E/snxv1+XIyUmHBDLn PKBmLDYxO9RJe1xKo/sNmLEno4c8G1F/y12TLV086cpBYGKkE8mPMBABqxuiPG8srwoKc2HW bvoC2Zfeu/WeQ0YqeI9ZEwRhsDGQZ7vc8PnKnEUaPZn6iWW4GeX7dXWeGNrK0wU2B04l2d+M FIKaoPHk8w5Ff++QNcn0YRkm//nYlukHUrMxhNcuc18jaLLftOh7BH/4EbKtTN75KAFePQBi I2CbuC41fchTt12QrPB3yz1GKfudsEMLFHBNeComJNnuolPOq0YSyuKdRO8Jubn5ZqWQeTwj XbG7wTonDc8xe46irOhz36VcjsjSY+PYhVZSeDWeDUZgpaJkBjQDDodIN2eoMwVEyUByos9H mKrqrpBMmylOspAZzqjb5FtOqM0BCxQINdKKiMwRelSb6pHYCrbS0XzpwDUEpp7RWCbHgg+6 Ot72kQCEFxj2LzX9VxF24GGQy9inlUfN51IV04klSibtBuuz/NbOwM0EZyBpgQEMAJelVX4k CtCxD4Ji3FQ8LZs22z7VoUvqIb7Gj2lNvhPeijlqqBkSMIgnSCLxlH4ahqKnEV58IrfVriV0 92zb94Az2nl0r+bZYfvev1qCcVIYxk+pYYcRl5qPXX8XGalrkcBBWmkgTSwzNK9rV4850iVI hsJNel49qen9JwiFYMSKa2MYgdYSbeuuwXwUp0ZHeVFc5RnPK2wxws1xcnsdb9hRXs2UeTEE 0klG3HuXqJ96DzKrCieKHLjs330h+16gDWAFZSEoT7Mh3HFGI2dscVuBstQNgnwUMnsJv8jx c005CfLCjCBnJEhMd2/QFuLwCZv4IdoghKwYw18e61UbX2bFovo9dduD527pD4sFqi7U7ofv aO3yf+ulL6jiKypGvnbiBP3KY3aKxx6pHHH3aDc9eOqCUgrtS3+xt1du4+qxrYqEnrywFoJy 5zqSzbnTTjFpdTbY5SS52fIOktLlAKzEg6V9hkg2r08hC3/L4NVj6I4tsGZlqb2neRlHFmCr bQARAQABwsD8BBgBCAAmFiEERtYfQYWFu+uAZjYrrzGlXdb6f1cFAmcgaYIFCQWjmoACGwwA CgkQrzGlXdb6f1fDIgwAmpB7eL3XNSx3F+gbmksOPMqCU5rEswRedjEt6tBzFTXhdNFfhZTb vCddUNePZnzddgxAnDBcTqI1jx6Go6Hkti/mxJqXSczMYBsImD/lEm47axsADvpnNaEM+tmu m/cMKfpILUpy2Ey7CKXUA1vpzYeUD29EQWi0fxM0arplrVt/uzUdFRFQRn2hCqeDLBLONX1F Adq+re6M0dhKl4a2+erzZRIXh3vIGiDmpJEGrajrhqEnMXFp6toSiMGian94m8H3NT6rB64E JmdHgyjXADFbn2G5Mb6Pwa8KnnK1kYcZ+Pwu9LfMXfgI01Sh/k01hjUVmnpYep4nHUfwXA8r kn6WekD80DYbAfKyFAXQCO/nclZ82RNmJbDRi3AeMFrxKi6KgdGCp1Izhj9USaMOVqcuV2p0 Rsoq+sFqWOKaHWnQHCM9RkynQVqrgUaSawEbGlCP1KIhVmjfjVsmsCaKkUb9T6VeO+ZNe+Pn rPgMe6IIvn24UuW2f6fIt0AaqOWq In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Hi Stafford, Thank you for your response. On 5/6/25 8:33 PM, Stafford Horne wrote: > On Sat, May 03, 2025 at 09:44:54PM +0530, Sahil Siddiq wrote: >> Hi, >> >> I have been looking into implementing ftrace for OpenRISC-linux. Based on what I have >> understood from the ftrace design document [1], mcount will have to be implemented in >> the kernel for ftrace to work. > > Thats right. > >> While this document is outdated, I did come across an article in the Linux Journal [2] >> that explains the use of fentry instead of mcount in the implementation of ftrace. It >> states that fentry is a powerful alternative to mcount. To make use of fentry, GCC's >> -pg flag needs to be used along with the -mfentry flag. Simply calling -pg results in >> mcount being used. >> >> Should it be possible for ftrace to make use of either option (fentry and mcount) when >> implementing it for OpenRISC? Or should it only use fentry (or mcount)? mcount still >> seems to be used in other architectures (I have only checked MIPS, RISC-V, LoongArch >> and ARM so far). > > I think we should stick to mcount for now. As I understand fentry is mainly and x86 > feature now. We should be able to get it done with mcount, as you say that is > what is what is used by other architectures like riscv. Going ahead with mcount would be relatively simpler especially since mcount is already supported in OpenRISC. I must point out, though, that I overlooked the following developments that have been made for other architectures: 1. Ftrace with fentry for s390. [1] 2. Optimizing function trace for RISC-V. [2] > Another point is that for ftrace to work it uses a kind live patching [1] to turn > the mcount calls to NOPs when the kernel boots up. Then when we turn on tracing > the code is patched to enable the mcount call. > > This live patching covers: > > - kprobes - adds single instruction break points into kernel code > - static keys - allows for static branches, i.e. the check for cache enabled can > be done once then code re-written to to make certain checks go away. We > could use this for cache availability checks. > - jump_label - low level support for code patching > > I think before going to far with ftrace we should look more into OpenRISC's > roadmap for all of these. Understood. As a first step, I think it should be possible to support some of the features in the debug subsystem - k(ret)probes, u(ret)probes, optprobes. Jump_labels and perf-related features (e.g. kprobes-event) can be built on top of them. What are your thoughts on this? Thanks, Sahil [1] https://patchwork.kernel.org/project/linux-kbuild/cover/cover.thread-aa7b8d.your-ad-here.call-01533557518-ext-9465@work.hours/ [2] https://lwn.net/ml/linux-kernel/20230511093234.3123181-1-suagrfillet@gmail.com/