linux-kernel - Re: [PATCH] MIPS: VDSO: Always select -msoft-float

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Message-ID: <20161104165047.GA29628@roeck-us.net>
Date:   Fri, 4 Nov 2016 09:50:47 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     "Maciej W. Rozycki" <macro@...tec.com>
Cc:     Matthew Fortune <Matthew.Fortune@...tec.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        "linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        James Hogan <James.Hogan@...tec.com>
Subject: Re: [PATCH] MIPS: VDSO: Always select -msoft-float

On Fri, Nov 04, 2016 at 04:09:37PM +0000, Maciej W. Rozycki wrote:
> On Fri, 4 Nov 2016, Guenter Roeck wrote:
> 
> > > As above, unless absolutely critical to have floating point code then
> > > the vDSO should just avoid all FP related issues and build soft-float.
> > 
> > FWIW, my logic was quite simple: The rest of the kernel builds with
> > -msoft-float, thus vDSO should do the same. Of course, I don't know the
> > entire context, so there may well be a reason to handle it differently
> > than the rest of the kernel.
> 
>  VDSO is not a part of the kernel, it's user mode code, made visible in 
> the user virtual memory, and implicitly loaded along the rest of the DSOs 
> on program startup by the dynamic loader (ld.so).  It has to be PIC for 
> that reason, too, causing all the hassle we had with making it such that 
> it does not need a GOT.
> 
> > Anyway, isn't the kernel supposed to not use floating point operations
> > in the first place ? Is this different for vDSO ?
> 
>  This code is executed in the user mode so while floating-point code may 
> not be needed there, not at least right now, there's actually nothing 
> which should stop us from from adding some should such a need arise.
> 
Just for my understanding - so the code is compiled with the kernel and part
of the kernel source but executed in user mode ?

If you ever add real floating point code, doesn't that also mean that you'll
have to implement the necessary linker helper functions or wrappers (such
as the wrappers needed for 64-bit integer divide operations in 32 bit code) ?

Thanks,
Guenter