On Tue, 2016-08-09 at 19:09 -0700, Luis R. Rodriguez wrote: > On Aug 9, 2016 6:50 PM, "Mark Salter" <msal...@redhat.com> wrote: > > > > On Tue, 2016-08-09 at 20:40 +0200, Luis R. Rodriguez wrote: > > > On Tue, Aug 09, 2016 at 01:04:00PM -0400, Mark Salter wrote: > > > > > > > > On Tue, 2016-08-09 at 06:37 -0700, Guenter Roeck wrote: > > > > > > > > > > On 08/09/2016 01:11 AM, Luis R. Rodriguez wrote: > > > > > > > > > > > > > > > > > > Mark, Aurelien, > > > > > > > > > > > > I've run into a linker (ld) issue caused by the linker table work > > > > > > I've > > > > > > been working on [0]. I looked into this and for the life of me, I > > > > > > cannot comprehend what the problem is, so was hoping you folks might > > > > > > be able to chime in. > > > > > > > > > > > For reference, the error is > > > > > > > > > > c6x-elf-ld: drivers/built-in.o: SB-relative relocation but > > > > > __c6xabi_DSBT_BASE not defined > > > > > c6x-elf-ld: drivers/built-in.o: SB-relative relocation but > > > > > __c6xabi_DSBT_BASE not defined > > > > DSBT is a reference to the no-MMU userspace ABI used by c6x. The kernel > > > > shouldn't > > > > be referencing DSBT base. The -mno-dsbt gcc flag should prevent it. > > > I see -mno-dsbt on arch/c6x/Makefile already -- however at link time this > > > is > > > an issue if linker tables are used it seems. Do you have any other > > > recommendation? > > > > > > I will note that it would seem that even i386 and x86-64 > > > compiler/binutils seem > > > to have relocation issues on older compiler/binutils, for instance: > > > > I see the problem with gcc 6 as well. > > > > So there appears to be some toolchain issues at play here. We build the > > kernel with two > > c6x-specific options: -mno-dsbt and -msdata=none. I already mentioned dsbt. > > The sdata > > option may be one of: > > > > -msdata=default > > Put small global and static data in the .neardata section, which is > > pointed to by > > register B14. Put small uninitialized global and static data in the > > .bss section, > > which is adjacent to the .neardata section. Put small read-only data > > into the > > .rodata section. The corresponding sections used for large pieces of > > data are > > .fardata, .far and .const. > > > > -msdata=all > > Put all data, not just small objects, into the sections reserved for > > small data, > > and use addressing relative to the B14 register to access them. > > > > -msdata=none > > Make no use of the sections reserved for small data, and use absolute > > addresses > > to access all data. Put all initialized global and static data in the > > .fardata > > section, and all uninitialized data in the .far section. Put all > > constant data > > into the .const section. > > > > > > Both small data and DSBT make use of base register + 15-bit offset to > > access data > > and thus the SB-relative reloc in the above error message. > > > > I think that gcc sees the .rodata section from DEFINE_LINKTABLE_RO() for > > builtin_fw > > and thinks it needs an SB-relative reloc. When the linker sees that reloc, > > it thinks > > it needs the dsbt base register and thus the error. Interestingly, weak > > data is > > never put in the small data section so if gcc sees that data is weak, it > > doesn't > > check the section name to see if it is a small data section. So SB-relative > > only > > gets used for builtin_fw__end, but not the weak builtin_fw even though they > > both > > are in the .rodata section. > > > > I suspect gcc should avoid being fooled by .rodata if -msdata=none is used. > > Regardless, I think this could all be avoided if the RO tables used .const > > instead of .rodata for c6x. > Thanks for the thorough analysis, would you be OK for c6x to use .const for > all read only linker tables or section ranges ? > I had not added #ifndef around the core-sections.h main ELF definitons but > could add one as its needed. In this case perhals that is needed and fine by > you > for SECTION_RODATA. > We can also override any of the core section setter helpers for archs but in > this case based on what you say it seems this is needed. Unless of course just > -msdata=none is fine and that's not yet used and you prefer that. > Luis
We're already using -msdata=none for kernel builds. From the gcc docs, one would think all const data goes into .const with -msdata=none, but the kernel forces a lot of weak const kallsyms data ,rodata so c6x vmlinux.lds still needs to have a .rodata section. I think we need to use .const for the c6x read-only linker tables and keep .rodata for RO_DATA_SECTION in vmlinux.lds.h.
