On Wed, Aug 10, 2016 at 07:04:09PM -0400, Mark Salter wrote: > On Wed, 2016-08-10 at 23:30 +0200, Luis R. Rodriguez wrote: > > On Tue, Aug 09, 2016 at 11:04:07PM -0400, Mark Salter wrote: > > > > > > On Tue, 2016-08-09 at 19:09 -0700, Luis R. Rodriguez wrote: > > > > > > > > On Aug 9, 2016 6:50 PM, "Mark Salter" <msalter@xxxxxxxxxx> 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. > > OK thanks I've found a clean solution minimal solution to this as follows. This now > > builds fine. Is this a fine work around for now ? > > Almost. You also need: > > diff --git a/include/linux/tables.h b/include/linux/tables.h > index a39ab03..3fa8d4d 100644 > --- a/include/linux/tables.h > +++ b/include/linux/tables.h > @@ -325,7 +325,7 @@ > __attribute__((used, \ > weak, \ > __aligned__(LINUX_SECTION_ALIGNMENT(name)),\ > - section(SECTION_TBL(SECTION_RODATA, \ > + section(SECTION_TBL(SECTION_TBL_RO, \ > name, level)))) > > /** > > Otherwise, start and end RO table markers end up in different sections. I thought that was not needed as weak attributes already force it to go to .const ? Anyway I've added this as well. Thanks! Luis -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
