arch/arm64/kernel/vdso/datapage.h

   1 /*
   2  * Userspace implementations of __get_datapage
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License version 2 as
   6  * published by the Free Software Foundation.
   7  *
   8  * This program is distributed in the hope that it will be useful,
   9  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  10  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  11  * GNU General Public License for more details.
  12  *
  13  * You should have received a copy of the GNU General Public License
  14  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  15  */
  16
  17 #ifndef __VDSO_DATAPAGE_H
  18 #define __VDSO_DATAPAGE_H
  19
  20 #include <linux/bitops.h>
  21 #include <linux/types.h>
  22 #include <asm/vdso_datapage.h>
  23
  24 /*
  25  * We use the hidden visibility to prevent the compiler from generating a GOT
  26  * relocation. Not only is going through a GOT useless (the entry couldn't and
  27  * mustn't be overridden by another library), it does not even work: the linker
  28  * cannot generate an absolute address to the data page.
  29  *
  30  * With the hidden visibility, the compiler simply generates a PC-relative
  31  * relocation (R_ARM_REL32), and this is what we need.
  32  */
  33 extern const struct vdso_data _vdso_data __attribute__((visibility("hidden")));
  34
  35 static inline const struct vdso_data *__get_datapage(void)
  36 {
  37         const struct vdso_data *ret;
  38         /*
  39          * This simply puts &_vdso_data into ret. The reason why we don't use
  40          * `ret = &_vdso_data` is that the compiler tends to optimise this in a
  41          * very suboptimal way: instead of keeping &_vdso_data in a register,
  42          * it goes through a relocation almost every time _vdso_data must be
  43          * accessed (even in subfunctions). This is both time and space
  44          * consuming: each relocation uses a word in the code section, and it
  45          * has to be loaded at runtime.
  46          *
  47          * This trick hides the assignment from the compiler. Since it cannot
  48          * track where the pointer comes from, it will only use one relocation
  49          * where __get_datapage() is called, and then keep the result in a
  50          * register.
  51          */
  52         asm("" : "=r"(ret) : "0"(&_vdso_data));
  53         return ret;
  54 }
  55
  56 /* We can only guarantee 56 bits of precision. */
  57 #define ARCH_CLOCK_FIXED_MASK GENMASK_ULL(55, 0)
  58
  59 #endif /* __VDSO_DATAPAGE_H */