vdso： virtual dynamic shared object

目的是解决用户态频繁调用内核接口性能较低，用它来提升访问性能，目前支持接口如下：

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test@linux:~/work/kernel/baseos/mykernel> objdump -T  ./arch/arm64/kernel/vdso/vdso.so

./arch/arm64/kernel/vdso/vdso.so:     file format elf64-littleaarch64

DYNAMIC SYMBOL TABLE:
0000000000000000 g    DO *ABS*  0000000000000000  LINUX_2.6.39 LINUX_2.6.39
00000000000005c8 g    DF .text  0000000000000068  LINUX_2.6.39 __kernel_clock_getres
0000000000000634 g    D  .text  0000000000000008  LINUX_2.6.39 __kernel_rt_sigreturn
0000000000000490 g    DF .text  000000000000012c  LINUX_2.6.39 __kernel_gettimeofday
0000000000000308 g    DF .text  0000000000000180  LINUX_2.6.39 __kernel_clock_gettime

所有用户态进程都会依赖它，名称如下：

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vDSO names
       The name of the vDSO varies across architectures.  It will often show up in things like glibc's ldd(1) output.  The exact name should not matter to any code, so do not hardcode it.

       user ABI   vDSO name
       ─────────────────────────────
       aarch64    linux-vdso.so.1
       arm        linux-vdso.so.1
       ia64       linux-gate.so.1
       ppc/32     linux-vdso32.so.1
       ppc/64     linux-vdso64.so.1
       s390       linux-vdso32.so.1
       s390x      linux-vdso64.so.1
       sh         linux-gate.so.1
       i386       linux-gate.so.1
       x86_64     linux-vdso.so.1
       x86/x32    linux-vdso.so.1