Re: [RFC PATCH 32/42] docker: Add gentoo-mipsr5900el-cross image

[Philippe Mathieu-Daudé]

Hi Maciej,

On 3/12/21 6:05 PM, Maciej W. Rozycki wrote:
> On Fri, 12 Mar 2021, Philippe Mathieu-Daudé wrote:
> 
>>>>> Is there any way we can do this with a distro that isn't Gentoo
>>>>> so that we can get a container build that is fast enough to be
>>>>> useful for CI ?
>>
>> Using the Debian cross image I get:
>>
>> /home/phil/source/qemu/tests/docker/docker.py --engine auto cc --cc
>> mips64el-linux-gnuabi64-gcc -i qemu/debian-mips64el-cross -s
>> /home/phil/source/qemu --   -Wall -Werror -O0 -g -fno-strict-aliasing
>> -mabi=n32 -march=r5900
>> /home/phil/source/qemu/tests/tcg/mips/test-r5900-dmult.c -o
>> test-r5900-dmult  -static
>> cc1: error: unsupported combination: -march=r5900 -mhard-float
>> -mdouble-float
>>
>> No clue what is setting '-mhard-float -mdouble-float' yet.
> 
>  The R5900 has an FPU that only supports the single floating-point format.  
> It's also not an IEEE 754 format.  The Linux kernel ABI does support the 
> double and also the single floating-point format, both compliant with IEEE 
> 754.
> 
>  In the absence of a suitable FPU emulation code included with the kernel 
> will handle the missing instructions (you can use the `nofpu' kernel 
> parameter to force that in the presence of an FPU too).  Beware however 
> that a recent change to the Linux kernel made FPU emulation code optional 
> to suit some deeply embedded applications known never to use FPU machine 
> instructions.
> 
>  NB the presence of emulation is always required for MIPS ISA compliance 
> if FPU machine instructions are ever to be used in a given application, 
> because operations are allowed to trap regardless and rely on emulation.
> 
>  I don't know what you are trying to achieve,

The previous maintainer let the QEMU MIPS codebase with the R5900 code
unreachable. I'm trying to see if I can get a closure on Fredrik work
before removing it, because there is no point in maintaining unreachable
code.

QEMU uses Docker images of distributions to cross-compile its tests.

Currently all Linux cross-tests are built using Debian based images.
Daniel asked me to see if I can use our current Debian based image
to build the r5900 tests, instead of adding yet another one (based
on Gentoo).

> but your two options to 
> choose from are:
> 
> 1. Build for the soft-float ABI (`-msoft-float') where any FP calculations 
>    are compiled such as to be made by the CPU using integer arithmetic.

With the Debian toolchain I get:

/usr/include/stdio.h:27:10: fatal error: bits/libc-header-start.h: No
such file or directory
 #include <bits/libc-header-start.h>
          ^~~~~~~~~~~~~~~~~~~~~~~~~~

> 2. Build for a generic MIPS ISA, for the R5900/n32 that would be MIPS III
>    (`-march=mips3'), and rely on the kernel FPU emulation.

Shouldn't -march=r5900 imply -march=mips3?

>  Note that some 
>    integer MIPS III operations are missing too from the R5900 and have to 
>    be emulated by the kernel for MIPS/Linux n32 psABI compliance (an 
>    implementation can be pinched from an old libgcc version that was still 
>    under GNU GPLv2 or another algorithm reused, e.g. my `__div64_32' piece
>    easily adapted).

Regards,

Phil.