Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> writes: > On Tue, 2020-07-21 at 16:48 -0700, Palmer Dabbelt wrote: >> > Why ? Branch distance limits ? You can't use trampolines ? >> >> Nothing fundamental, it's just that we don't have a large code model in the C >> compiler. As a result all the global symbols are resolved as 32-bit >> PC-relative accesses. We could fix this with a fast large code model, but then >> the kernel would need to relax global symbol references in modules and we don't >> even do that for the simple code models we have now. FWIW, some of the >> proposed large code models are essentially just split-PLT/GOT and therefor >> don't require relaxation, but at that point we're essentially PIC until we >> have more that 2GiB of kernel text -- and even then, we keep all the >> performance issues. > > My memory might be out of date but I *think* we do it on powerpc > without going to a large code model, but just having the in-kernel > linker insert trampolines. We build modules with the large code model, and always have AFAIK: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/Makefile?commit=4fa640dc52302b5e62b01b05c755b055549633ae#n129 # -mcmodel=medium breaks modules because it uses 32bit offsets from # the TOC pointer to create pointers where possible. Pointers into the # percpu data area are created by this method. # # The kernel module loader relocates the percpu data section from the # original location (starting with 0xd...) to somewhere in the base # kernel percpu data space (starting with 0xc...). We need a full # 64bit relocation for this to work, hence -mcmodel=large. KBUILD_CFLAGS_MODULE += -mcmodel=large We also insert trampolines for branches, but IIUC that's a separate issue. cheers
