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. Am Dienstag, 22. November 2016, 14:16:22 BRST schrieb Dave Young: > Hi Michael > > On 11/22/16 at 05:01pm, Michael Ellerman wrote: > > In general I dislike the level of complexity of the kexec-tools > > purgatory, and in particular I'm not comfortable with things like: > > > > diff --git a/arch/powerpc/purgatory/sha256.c > > b/arch/powerpc/purgatory/sha256.c new file mode 100644 > > index 000000000000..6abee1877d56 > > --- /dev/null > > +++ b/arch/powerpc/purgatory/sha256.c > > @@ -0,0 +1,6 @@ > > +#include "../boot/string.h" > > + > > +/* Avoid including x86's boot/string.h in sha256.c. */ > > +#define BOOT_STRING_H > > + > > +#include "../../x86/purgatory/sha256.c" > > Agreed, include x86 code in powerpc looks bad > > > I think the best way to get this over the line would be to take the > > kexec-lite purgatory implementation and use that to begin with. I know > > it doesn't have all the features of the kexec-tools version, but it > > should work, and we can look at adding the extra features later. > > Instead of adding other implementation, moving the purgatory sha256 code > out of x86 sounds better so that we can reuse them cleanly.. Do you have a suggestion of where that code can live so that it can be shared between purgatories for different arches? Do we need a purgatory with generic and arch-specific code like in kexec- tools? -- Thiago Jung Bauermann IBM Linux Technology Center
