svn commit: r358247 - in head/sys: conf riscv/include

[John Baldwin]

On 2/22/20 5:23 AM, Kristof Provost wrote:
> Author: kp
> Date: Sat Feb 22 13:23:27 2020
> New Revision: 358247
> URL: https://svnweb.freebsd.org/changeset/base/358247
> 
> Log:
>   riscv: Set MACHINE_ARCH correctly
>   
>   MACHINE_ARCH sets the hw.machine_arch sysctl in the kernel. In userspace
>   it sets MACHINE_ARCH in bmake, which bsd.cpu.mk uses to configure the
>   target ABI for ports.
>   
>   For riscv64sf builds (i.e. soft-float) that needs to be riscv64sf, but
>   the sysctl didn't reflect that. It is static.
>   
>   Set the define from the riscv makefile so that we correctly reflect our
>   actual build (i.e. riscv64 or riscv64sf), depending on what TARGET_ARCH
>   we were built with.
>   
>   That still doesn't satisfy userspace builds (e.g. bmake), so check if
>   we're building with a software-floating point toolchain there. That
>   check doesn't work in the kernel, because it never uses floating point.
>   
>   Reviewed by:	philip (previous version), mhorne
>   Sponsored by:	Axiado
>   Differential Revision:	https://reviews.freebsd.org/D23741

I actually think this is kind of busted as the kernel is always the same,
and we have the same problem on MIPS which also already does this wrong.
I think instead we should treat soft-float as an alternate ABI (though
very light weight as it doesn't need a separate syscall handler, etc.) and
make the sysctl handler smart enough to return the right value based on the
ELF header of the binary.

I saw the later followup commit to add GENERICSF and I think that is possibly
even farther down the wrong path.  The kernel is _always_ soft-float in terms
of the ABI it is built with.  However, kernel with FPE support can run binaries
with either ABI.  It's true that on RISC-V there is not a nice way to detect
if the FPU is present (I haven't looked at OpenSBI, but the SBI used with bbl
doesn't provide misa to S-mode to know if it is present or not).  You could
just try to execute an FPU instruction during boot to see if you get an
illegal instruction fault (subject to your SBI choosing to emulate that).

-- 
John Baldwin