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Thu, 11 Dec 2025 04:56:05 -0800 (PST) Received: from bogeyman ([51.15.199.25]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-47a89dac074sm33178245e9.8.2025.12.11.04.56.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 11 Dec 2025 04:56:04 -0800 (PST) From: Philippe Gerum X-Google-Original-From: Philippe Gerum To: Florian Bezdeka Cc: xenomai@lists.linux.dev, Jan Kiszka Subject: Re: [PATCH RFC Dovetail 6.1 0/3] arm64: Enable in-kernel FPU / FPSIMD usage In-Reply-To: <0b02043cfe5b2032611180d514dcb2eff690d844.camel@siemens.com> (Florian Bezdeka's message of "Wed, 10 Dec 2025 13:14:59 +0100") References: <20251119-flo-6-1-arm64-fpsimd-test-enablement-v1-0-8653c0b573fd@siemens.com> <875xaq88tf.fsf@xenomai.org> <0b02043cfe5b2032611180d514dcb2eff690d844.camel@siemens.com> User-Agent: mu4e 1.12.12; emacs 30.2 Date: Thu, 11 Dec 2025 13:56:02 +0100 Message-ID: <87h5txuqhp.fsf@xenomai.org> Precedence: bulk X-Mailing-List: xenomai@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Florian Bezdeka writes: > On Mon, 2025-12-01 at 17:29 +0100, Philippe Gerum wrote: >> Florian Bezdeka writes: >> >> > Hi all, >> > >> > these are the necessary backports for dovetail 6.1 that would allow us to >> > enable the in-kernel FPU usage of switchtest. This is currently not >> > possible as dovetail < 6.12 does not allow / implement the necessary fpsimd >> > context switching. >> > >> > I have backported the necessary patches down to 5.10. That would allow >> > us to unconditionally enable the tests for the xenomai3-next branch. I >> > will send out the 5.15 and 5.10 series once we agreed that this version >> > (targeting 6.1) is correct. >> > >> > Please note that there is a small problem with this series. Patch 1 >> > removes the check/flag that was used by the dovetail enablement code to >> > detect in-band FPU usage. The new flag is implemented differently in >> > patch 2. Patch 2 also fixes the usage of that flag. I had no better >> > idea. Otherwise I would have to delete? the in-kernel fpu preemption >> > code (temporary). >> >> TIF_KERNEL_FPSTATE is not covering a preemption of a softirq handler >> using the NEON unit by an oob thread when PREEMPT_RT is disabled, kernel >> or user for that matter. i.e. It looks like this aspect was overlooked >> during the busy -> TIF_KERNEL_FPSTATE transition for Dovetail, so a >> broader fix may be required. We may have to use a marker which covers >> any type of preempted context. >> > > I agree, but this is something that we should address in addition / on > top, no? > Yep, would make backporting easier. > This issue seems to affect all versions of dovetail. If we do the > backports first we can address the issue on the same feature set. That > should simplify the back-porting again. Did I miss something? > Agreed. -- Philippe.