Re: Why sysarch(ARM_VFP_FPSCR) only allows changing the rounding mode?

[minux <minux.ma%gmail.com@localhost>]

Hi matt,

On Mon, Mar 3, 2014 at 3:46 AM, Matt Thomas <matt%3am-software.com@localhost> 
wrote:
> On Mar 2, 2014, at 9:31 PM, minux <minux.ma%gmail.com@localhost> wrote:
> > I wonder why sysarch(ARM_VFP_FPSCR) only allows changing the rounding
> > mode, but not, for example, the flush to zero mode and the default NaN
> > mode?
> > (http://nxr.netbsd.org/source/xref/src/sys/arch/arm/arm32/sys_machdep.c#arm32_vfp_fpscr)
>
> Because I was playing it safe. :)
Makes sense.
> I just committed a change that should do the right thing if the VFP supports
> it.  For instance, on this cortex-a7 the default value for FPSCR is now 0
> since it supports NaN propogation and denormals.
Great. I haven't checked the source code, but is ARM1176JZF-S used on
Raspberry Pi
has initial FPSCR of 0?
> > Lifting the restriction is easy, for example, the attached patch allows the
> > DN and FZ bits to be changed via sysarch(ARM_VFP_FPSCR). I don't think there
> > is any security implication in allowing this.
> Not that easy.  Some VFP don't support NaNs, or non-nearest rounding modes,
> or denormals.  Allowing them to used could cause a panic.
I see. Thank you for the explanations.

I wonder if it makes the user's life easier to just allow user-space
writes to FPSCR
and handle the exceptions transparently. Seems easier to use than
adding a sysarch
call.

Best regards,
minux