Re: [PATCH] xen/riscv: Fix build with GCC 10

[Julien Grall <julien@xxxxxxx>]

Hi,

On 16/03/2023 07:21, Jan Beulich wrote:
> On 15.03.2023 19:51, Andrew Cooper wrote:
> >    riscv64-linux-gnu-gcc -MMD -MP -MF arch/riscv/.early_printk.o.d -DBUILD_ID 
> > -fno-strict-aliasing -std=gnu99 -Wall -Wstrict-prototypes 
> > -Wdeclaration-after-statement -Wno-unused-but-set-variable 
> > -Wno-unused-local-typedefs   -O1 -fno-omit-frame-pointer -nostdinc -fno-builtin 
> > -fno-common -Werror -Wredundant-decls -Wno-pointer-arith -Wvla -pipe -D__XEN__ 
> > -include ./include/xen/config.h -Wa,--strip-local-absolute -g -mabi=lp64  
> > -I./include -I./arch/riscv/include -march=rv64gc -mstrict-align -mcmodel=medany 
> >   -c arch/riscv/early_printk.c -o arch/riscv/early_printk.o
> >    arch/riscv/early_printk.c:18:2: error: #error "early_*() can be called from 
> > head.S with MMU-off"
> >       18 | #error "early_*() can be called from head.S with MMU-off"
> >          |  ^~~~~
> >
> >    $ riscv64-linux-gnu-gcc --version
> >    riscv64-linux-gnu-gcc (Debian 10.2.1-6) 10.2.1 20210110
> >
> > The binary is otherwise correct, so remove the incorrect check.
> >
> > Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> I'm with Julien here - this needs further explaining: The compiler (even 8.2)
> clearly provides this definition with the given set of command line options,
> as supported by trying it out om godbolt. So there must be more to this -
> could be a bad patch in Debian's build, could be some odd interaction of
> command line options which for whatever reason only triggers with certain
> builds, or about anything else.
I have tried to build RISC-v on my Debian Bulleyes machine today. The 
build failed with the same reason.
The Linux kernel (which has the exact same check) could be built. So I 
decided to dig why this happens.
I got a below code compiled when both -mcmodel=medany and -fno-pie are 
passed to the GCC command line:
#ifndef __riscv_cmodel_medany
#error "medany not enabled"
#endif

I am guessing that's because GCC on Debian has PIE enabled by default.
Now, Xen is meant to be built with -fno-pie, but this is not on the 
command line. So does any flags from EMBEDDED_EXTRA_CFLAGS.
Skimming through xen-devel, there is already a patch from Oleksii to add 
the cflags (see [1]). So with that in place, this patch becomes 
unnecessary to build Xen RISC-v on Debian.
Cheers,

[1] 
https://lore.kernel.org/all/2785518800dce64fafb3096480a5ae4c4e026bcb.1678970065.git.oleksii.kurochko@xxxxxxxxx/
--
Julien Grall