Re: [PATCH RFC Dovetail 6.1 0/3] arm64: Enable in-kernel FPU / FPSIMD usage

[Philippe Gerum <philippe.gerum@gmail.com>]

Florian Bezdeka <florian.bezdeka@siemens.com> writes:

> On Mon, 2025-12-01 at 17:29 +0100, Philippe Gerum wrote:
>> Florian Bezdeka <florian.bezdeka@siemens.com> 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.