Author: kib Date: Thu Apr 2 20:14:51 2015 New Revision: 281003 URL: https://svnweb.freebsd.org/changeset/base/281003
Log: Speed up symbol lookup for the amd64 kernel modules. Amd64 uses relocatable object files as the modules format. It is good WRT not having unneeded overhead for PIC code, in particular, due to absence of useless GOT and PLT. But the cost is that the module linking process cannot use hash to speed up the symbol lookup, and that each reference to the symbol requiring a relocation, instead of single-place relocation in GOT. Cache the successfull symbol lookup results in the module symbol table, using the newly allocated SHN_FBSD_CACHED value from SHN_LOOS-HIOS range as an indicator. The SHN_FBSD_CACHED together with the non-existent definition of the found symbol are reverted after successfull relocations, which is done under kld_sx lock, so it should not be visible to other consumers of the symbol table. Submitted by: Conrad Meyer Differential Revision: https://reviews.freebsd.org/D1718 MFC after: 3 weeks Modified: head/sys/kern/link_elf_obj.c head/sys/sys/elf_common.h Modified: head/sys/kern/link_elf_obj.c ============================================================================== --- head/sys/kern/link_elf_obj.c Thu Apr 2 20:07:05 2015 (r281002) +++ head/sys/kern/link_elf_obj.c Thu Apr 2 20:14:51 2015 (r281003) @@ -173,6 +173,7 @@ static struct linker_class link_elf_clas }; static int relocate_file(elf_file_t ef); +static void elf_obj_cleanup_globals_cache(elf_file_t); static void link_elf_error(const char *filename, const char *s) @@ -1075,6 +1076,13 @@ relocate_file(elf_file_t ef) } } + /* + * Only clean SHN_FBSD_CACHED for successfull return. If we + * modified symbol table for the object but found an + * unresolved symbol, there is no reason to roll back. + */ + elf_obj_cleanup_globals_cache(ef); + return 0; } @@ -1223,6 +1231,21 @@ link_elf_each_function_nameval(linker_fi return (0); } +static void +elf_obj_cleanup_globals_cache(elf_file_t ef) +{ + Elf_Sym *sym; + Elf_Size i; + + for (i = 0; i < ef->ddbsymcnt; i++) { + sym = ef->ddbsymtab + i; + if (sym->st_shndx == SHN_FBSD_CACHED) { + sym->st_shndx = SHN_UNDEF; + sym->st_value = 0; + } + } +} + /* * Symbol lookup function that can be used when the symbol index is known (ie * in relocations). It uses the symbol index instead of doing a fully fledged @@ -1234,7 +1257,7 @@ static Elf_Addr elf_obj_lookup(linker_file_t lf, Elf_Size symidx, int deps) { elf_file_t ef = (elf_file_t)lf; - const Elf_Sym *sym; + Elf_Sym *sym; const char *symbol; Elf_Addr ret; @@ -1262,7 +1285,22 @@ elf_obj_lookup(linker_file_t lf, Elf_Siz if (*symbol == 0) return (0); ret = ((Elf_Addr)linker_file_lookup_symbol(lf, symbol, deps)); - return ret; + + /* + * Cache global lookups during module relocation. The failure + * case is particularly expensive for callers, who must scan + * through the entire globals table doing strcmp(). Cache to + * avoid doing such work repeatedly. + * + * After relocation is complete, undefined globals will be + * restored to SHN_UNDEF in elf_obj_cleanup_globals_cache(), + * above. + */ + if (ret != 0) { + sym->st_shndx = SHN_FBSD_CACHED; + sym->st_value = ret; + } + return (ret); case STB_WEAK: printf("link_elf_obj: Weak symbols not supported\n"); Modified: head/sys/sys/elf_common.h ============================================================================== --- head/sys/sys/elf_common.h Thu Apr 2 20:07:05 2015 (r281002) +++ head/sys/sys/elf_common.h Thu Apr 2 20:14:51 2015 (r281003) @@ -363,6 +363,9 @@ typedef struct { #define SHN_LOPROC 0xff00 /* First processor-specific. */ #define SHN_HIPROC 0xff1f /* Last processor-specific. */ #define SHN_LOOS 0xff20 /* First operating system-specific. */ +#define SHN_FBSD_CACHED SHN_LOOS /* Transient, for sys/kern/link_elf_obj + linker only: Cached global in local + symtab. */ #define SHN_HIOS 0xff3f /* Last operating system-specific. */ #define SHN_ABS 0xfff1 /* Absolute values. */ #define SHN_COMMON 0xfff2 /* Common data. */ _______________________________________________ [email protected] mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "[email protected]"
