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    SubjectRe: [PATCH 03/12] powerpc/module: Create a special stub for ftrace_caller()
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    On 25/02/16 01:28, Michael Ellerman wrote:
    > In order to support the new -mprofile-kernel ABI, we need to be able to
    > call from the module back to ftrace_caller() (in the kernel) without
    > using the module's r2. That is because the function in this module which
    > is calling ftrace_caller() may not have setup r2, if it doesn't
    > otherwise need it (ie. it accesses no globals).
    >
    > To make that work we add a new stub which is used for calling
    > ftrace_caller(), which uses the kernel toc instead of the module toc.
    >
    > Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    > ---
    > arch/powerpc/kernel/module_64.c | 48 ++++++++++++++++++++++++++++++++++++++++-
    > 1 file changed, 47 insertions(+), 1 deletion(-)
    >
    > diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
    > index 9629966e614b..e711d40a3b8f 100644
    > --- a/arch/powerpc/kernel/module_64.c
    > +++ b/arch/powerpc/kernel/module_64.c
    > @@ -671,10 +671,56 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
    > }
    >
    > #ifdef CONFIG_DYNAMIC_FTRACE
    > +
    > +#define PACATOC offsetof(struct paca_struct, kernel_toc)
    > +
    > +static unsigned long create_ftrace_stub(const Elf64_Shdr *sechdrs, struct module *me)
    > +{
    > + struct ppc64_stub_entry *entry;
    > + unsigned int i, num_stubs;
    How about some comments on r2
    r2 is still pointing to the module's toc, will be saved by ftrace_caller and restored by the instruction following bl ftrace_caller (after patching _mcount/nop)

    > + static u32 stub_insns[] = {
    > + 0xe98d0000 | PACATOC, /* ld r12,PACATOC(r13) */
    > + 0x3d8c0000, /* addis r12,r12,<high> */
    > + 0x398c0000, /* addi r12,r12,<low> */
    > + 0x7d8903a6, /* mtctr r12 */
    > + 0x4e800420, /* bctr */
    > + };
    > + long reladdr;
    > +
    > + num_stubs = sechdrs[me->arch.stubs_section].sh_size / sizeof(*entry);
    > +
    > + /* Find the next available stub entry */
    > + entry = (void *)sechdrs[me->arch.stubs_section].sh_addr;
    > + for (i = 0; i < num_stubs && stub_func_addr(entry->funcdata); i++, entry++);
    > +
    > + if (i >= num_stubs) {
    > + pr_err("%s: Unable to find a free slot for ftrace stub.\n", me->name);
    > + return 0;
    > + }
    > +
    > + memcpy(entry->jump, stub_insns, sizeof(stub_insns));
    > +
    > + /* Stub uses address relative to kernel_toc */
    > + reladdr = (unsigned long)ftrace_caller - get_paca()->kernel_toc;
    > + if (reladdr > 0x7FFFFFFF || reladdr < -(0x80000000L)) {
    > + pr_err("%s: Address of ftrace_caller out of range of kernel_toc.\n", me->name);
    > + return 0;
    > + }
    > +
    > + entry->jump[1] |= PPC_HA(reladdr);
    > + entry->jump[2] |= PPC_LO(reladdr);
    > +
    > + /* Eventhough we don't use funcdata in the stub, it's needed elsewhere. */
    > + entry->funcdata = func_desc((unsigned long)ftrace_caller);
    > + entry->magic = STUB_MAGIC;
    > +
    > + return (unsigned long)entry;
    > +}
    > +
    > int module_finalize_ftrace(struct module *mod, const Elf_Shdr *sechdrs)
    > {
    > mod->arch.toc = my_r2(sechdrs, mod);
    > - mod->arch.tramp = stub_for_addr(sechdrs, (unsigned long)ftrace_caller, mod);
    > + mod->arch.tramp = create_ftrace_stub(sechdrs, mod);
    >
    > if (!mod->arch.tramp)
    > return -ENOENT;
    Reviewed-by: Balbir Singh <bsingharora@gmail.com>

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