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

[Matt Thomas <matt%3am-software.com@localhost>]

On Mar 3, 2014, at 12:54 AM, minux <minux.ma%gmail.com@localhost> wrote:

> 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?

Alas no.

In ARM1176JZF-S processors when Flush-to-Zero and Default_NaN and 
Round-to-Nearest are all selected in FPSCR, the coprocessor does not require 
support code. Otherwise floating point support code is required.

That's how I choose the default.  If someone wants to write and donate that 
code then I can change it. 


>>> 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.

Most cortex don't support VFP exceptions :(