[OpenWrt-Devel] [PATCH] kernel: MIPS: math-emu Write-protect delay slot emulation pages

[Rosen Penev]

On Mon, Dec 24, 2018 at 2:16 PM Kevin 'ldir' Darbyshire-Bryant
<ldir at darbyshire-bryant.me.uk> wrote:
>
>
>
> > On 24 Dec 2018, at 03:53, Yousong Zhou <yszhou4tech at gmail.com> wrote:
> >
> >>
> > Hi, Kevin
> >
> > I just took another look at the build system.  On OpenWrt, we have a
> > pending patch 304-mips_disable_fpu.patch that makes fpu emulation for
> > mips an optional feature.  We have most mips targets have that feature
> > disabled.   The absence should be fine as we tell toolchain to use
> > soft-float explicitly for the whole build.
> >
> > So another approach would be enhancing the disable-fpu patch to also
> > not mapping the dsemu page at all.
> <snip>
>
> ha - you can’t exploit a page that doesn’t exist :-)
All of this work reminds me of this old pull request:
https://github.com/openwrt/openwrt/pull/629/commits
>
> Just pushed the backport to openwrt and I have your ‘don’t allocate the page’ patch as an additional commit in my staging tree here https://git.openwrt.org/?p=openwrt/staging/ldir.git;a=summary
>
> It’s all running on my box ok so far
>
> cat /proc/self/maps
> 00400000-0047a000 r-xp 00000000 1f:03 1825       /bin/busybox
> 00489000-0048a000 r-xp 00079000 1f:03 1825       /bin/busybox
> 0048a000-0048b000 rwxp 0007a000 1f:03 1825       /bin/busybox
> 77e9e000-77ec3000 r-xp 00000000 1f:03 2298       /lib/libgcc_s.so.1
> 77ec3000-77ec4000 rwxp 00015000 1f:03 2298       /lib/libgcc_s.so.1
> 77ec4000-77f57000 r-xp 00000000 1f:03 2474       /lib/libc.so
> 77f66000-77f68000 rwxp 00092000 1f:03 2474       /lib/libc.so
> 77f68000-77f6a000 rwxp 00000000 00:00 0
> 7f744000-7f765000 rw-p 00000000 00:00 0          [stack]
> 7ff88000-7ff89000 r--p 00000000 00:00 0          [vvar]
> 7ff89000-7ff8a000 r-xp 00000000 00:00 0          [vdso]
I recommend trying out something that uses floating point
instructions. Say, mp3 decoding or whatnot.

I also wonder if it would be worth experimenting with enabling
floating point on several targets.
>
> Nothing 7xxxxxxx related has a static address :-)
>
>
> Cheers,
>
> Kevin D-B
>
> 012C ACB2 28C6 C53E 9775  9123 B3A2 389B 9DE2 334A
>
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