Am Mittwoch, 23. November 2016, 09:32:58 BRST schrieb Dave Young: > On 11/22/16 at 11:44am, Thiago Jung Bauermann wrote: > > Am Dienstag, 22. November 2016, 17:01:10 BRST schrieb Michael Ellerman: > > > Thiago Jung Bauermann <bauerman at linux.vnet.ibm.com> writes: > > > > Am Sonntag, 20. November 2016, 10:45:46 BRST schrieb Dave Young: > > > >> On 11/10/16 at 01:27am, Thiago Jung Bauermann wrote: > > > >> > powerpc's purgatory.ro has 12 relocation types when built as > > > >> > a relocatable object. To implement support for them requires > > > >> > arch_kexec_apply_relocations_add to duplicate a lot of code with > > > >> > module_64.c:apply_relocate_add. > > > >> > > > > >> > When built as a Position Independent Executable there are only 4 > > > >> > relocation types in purgatory.ro, so it becomes practical for the > > > >> > powerpc > > > >> > implementation of kexec_file to have its own relocation > > > >> > implementation. > > > >> > > > > >> > Also, the purgatory is an executable and not an intermediary output > > > >> > from > > > >> > the compiler so it makes sense conceptually that it is easier to > > > >> > build > > > >> > it as a PIE than as a partially linked object. > > > >> > > > > >> > Apart from the greatly reduced number of relocations, there are two > > > >> > differences between a relocatable object and a PIE: > > > >> > > > > >> > 1. __kexec_load_purgatory needs to use the program headers rather > > > >> > than > > > >> > the > > > >> > > > > >> > section headers to figure out how to load the binary. > > > >> > > > > >> > 2. Symbol values are absolute addresses instead of relative to the > > > >> > > > > >> > start of the section. > > > >> > > > > >> > This patch adds the support needed in generic code for the > > > >> > differences > > > >> > above and allows powerpc to load and relocate a position > > > >> > independent > > > >> > purgatory. > > > >> > > > >> [snip] > > > >> > > > >> The kexec-tools machine_apply_elf_rel is pretty simple for ppc64, it > > > >> is > > > >> not that complex. So could you look into simplify your kexec_file > > > >> implementation? > > > > > > > > I can try, but there is one fundamental issue here: powerpc > > > > position-dependent code relies more on relocations than x86 > > > > position-dependent code does, so there's a limit to how simple it can > > > > be > > > > made without switching to position- independent code. And it will > > > > always > > > > be more involved than it is on x86. > > > > > > I think we need to go back to the drawing board on this one. > > > > > > My hope was that building purgatory as PIE would reduce the amount of > > > complexity, but instead it's just added more. Sorry for sending you in > > > that direction. > > > > It added complexity because in my series powerpc was using a PIE purgatory > > but x86 kept using a partially-linked object (because of the problem I > > mentioned I had when trying out a PIE x86 purgatory), so generic code > > needed two purgatory loaders. > > > > I'll see if I can make the PIE x86 purgatory to work so that generic code > > can have only one loader implementation. Then it will indeed be simpler. > Do we really need the PIE purgatory, after moving generic code out of > x86, there will be no much benefit, no? It still makes a big difference on powerpc, even after moving out the generic code. I just got the PIE purgatory working on x86 and it also simplifies the code there, so it's a win for both architectures. I'll clean up the code and post tomorrow so that you can see what you think. > Anyway, the first step should be > making the purgatory code more generic so that it can be easier for > other arches to support kexec_file in the future. I'll try putting sha256.c in lib/purgatory/ as you suggested. -- Thiago Jung Bauermann IBM Linux Technology Center
