Re: CVS commit: src/sys/arch/x86/x86

[Maxime Villard <max%m00nbsd.net@localhost>]

Le 22/06/2018 à 03:40, matthew green a écrit :
> "Maxime Villard" writes:
> > Module Name:	src
> > Committed By:	maxv
> > Date:		Tue Jun 19 09:25:13 UTC 2018
> >
> > Modified Files:
> > 	src/sys/arch/x86/x86: fpu.c
> >
> > Log Message:
> > When using EagerFPU, create the fpu state in execve at IPL_HIGH.
>
> why splhigh instead of kpreempt_disable()?

Nick Hudson asked me the same question yesterday, here's the relevant part
of my answer (quoting his mail, I guess he doesn't mind).

The fpu code already runs at splhigh, and this disables preemption. So I
used splhigh too.




-------- Message transféré --------
Sujet : Re: Fwd: CVS commit: src/sys/arch/x86/x86
Date : Thu, 21 Jun 2018 07:23:33 +0100
De : Nick Hudson <nick.hudson%gmx.co.uk@localhost>
Pour : Maxime Villard <max%m00nbsd.net@localhost>

On 21/06/2018 07:21, Maxime Villard wrote:
> Le 21/06/2018 à 08:11, Nick Hudson a écrit :
> > On 21/06/2018 06:42, Maxime Villard wrote:
> > > Le 21/06/2018 à 07:34, Maxime Villard a écrit :
> > > > Le 21/06/2018 à 07:21, Nick Hudson a écrit :
> > > > > Surely, kpreempt_{disable,enable}() is what you really mean?
> > > > >
> > > > > Nick
> > > >
> > > > No, I mean splhigh. splhigh does prevent preemption, is that 
> > > > incorrect?
> >
> > If you want to disable preemption then kpreempt_disable() is what you 
> > want.
> >
> > If you want to disable interrupts (and as a consequence pre-emption) 
> > then splhigh is what you want.
> >
> > I firmly believe you want the former.
> >
> > Why disable interupts unnecessarily? This is "bad practice" and 
> > shouldn't be left for others to cargo cult copy.
>
> There is a KASSERT in fpusave_cpu(), which I didn't introduce. We want
> IPL_HIGH, unconditionally.
>
> So no, I don't want the former, the FPU code wants the latter. Then we 
> can say that maybe splhigh is not needed after all in fpusave_cpu; I won't 
> risk introducing random races.
>
> In the commit I said "disable preemption" because I had in mind the 
> problem where a context switch occurs as a result of preemption.

Up to you... really don't know why FPU code needs interrupts disabled.