Cannot compile UML in Ubuntu 20.04.5 anymore

[Roberto Sassu]

On Sat, 2023-03-04 at 12:35 +0800, David Gow wrote:
> On Fri, 3 Mar 2023 at 16:37, Roberto Sassu
> <roberto.sassu at huaweicloud.com> wrote:
> > Hi
> > 
> > it looks like one recent commit causes the kernel build to break.
> > 
> >   GEN     scripts/gdb/linux/constants.py
> > In file included from /usr/include/stdlib.h:1013,
> >                  from arch/um/os-Linux/helper.c:6:
> > /usr/include/x86_64-linux-gnu/bits/stdlib-float.h: In function ‘atof’:
> > /usr/include/x86_64-linux-gnu/bits/stdlib-float.h:26:1: error: SSE register return with SSE disabled
> >    26 | {
> >       | ^
> >   CC      init/noinitramfs.o
> > 
> > It works again by reverting commit 884981867947 ("rust: arch/um:
> > Disable FP/SIMD instruction to match x86").
> > 
> > $ cat /proc/cpuinfo
> > 
> > [...]
> > 
> > processor       : 31
> > vendor_id       : AuthenticAMD
> > cpu family      : 23
> > model           : 49
> > model name      : AMD Ryzen Threadripper PRO 3955WX 16-Cores
> > 
> > Thanks
> > 
> > Roberto
> > 
> 
> Yup, this is definitely caused by 884981867947 ("rust: arch/um:
> Disable FP/SIMD instruction to match x86").
> 
> It looks like GCC fixed this upstream in GCC 11:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99652
> 
> Equally though, not having this change breaks older versions of LLVM,
> so I think it's best to not totally revert it.
> 
> Something like this should work:
> ---
> diff --git a/arch/x86/Makefile.um b/arch/x86/Makefile.um
> index d2e95d1d4db7..1aa64846e539 100644
> --- a/arch/x86/Makefile.um
> +++ b/arch/x86/Makefile.um
> @@ -3,9 +3,14 @@ core-y += arch/x86/crypto/
> 
> #
> # Disable SSE and other FP/SIMD instructions to match normal x86
> +# This is required to work around issues in older LLVM versions, but breaks
> +# GCC versions < 11. See:
> +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99652
> #
> +ifeq ($(CONFIG_CC_IS_CLANG),y)
> KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
> KBUILD_RUSTFLAGS +=
> -Ctarget-feature=-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-avx,-avx2
> +endif
> 
> ifeq ($(CONFIG_X86_32),y)
> START := 0x8048000
> ---
> 
> Though the more correct solution would be to just check
> gcc-min-version || clang. That probably requires a bit more
> makefile-fu to do cleanly, though.

Thanks, works for me.

Roberto