On Wed, Nov 8, 2017 at 6:55 AM, Greentime Hu [off-list ref] wrote:

+#ifndef __ASM_VDSO_DATAPAGE_H
+#define __ASM_VDSO_DATAPAGE_H
+
+#ifdef __KERNEL__
+
+#ifndef __ASSEMBLY__
+
+struct vdso_data {
+       bool cycle_count_down;  /* timer cyclye counter is decrease with time */
+       u32 cycle_count_offset; /* offset of timer cycle counter register */
+       u32 seq_count;          /* sequence count - odd during updates */
+       u32 xtime_coarse_sec;   /* coarse time */
+       u32 xtime_coarse_nsec;
+
+       u32 wtm_clock_sec;      /* wall to monotonic offset */
+       u32 wtm_clock_nsec;
+       u32 xtime_clock_sec;    /* CLOCK_REALTIME - seconds */
+       u32 cs_mult;            /* clocksource multiplier */
+       u32 cs_shift;           /* Cycle to nanosecond divisor (power of two) */
+
+       u64 cs_cycle_last;      /* last cycle value */
+       u64 cs_mask;            /* clocksource mask */
+
+       u64 xtime_clock_nsec;   /* CLOCK_REALTIME sub-ns base */
+       u32 tz_minuteswest;     /* timezone info for gettimeofday(2) */
+       u32 tz_dsttime;
+};

I need some insight from Deepa and Palmer here: to prepare for 64-bit
time_t in the
future, would it make sense to define the vdso to use 64-bit seconds numbers
consistently, and provide vdso symbols that return 64-bit times, having the
glibc convert that to normal timespec values, or should we leave it for now?

For the normal syscalls I think we are better off keeping things consistent
between architectures, but the vdso is architecture specific by definition, so
we may as well use 64-bit times there now (same for risc-v, which still
has time to modify this before the 4.15 release and glibc merge).

+/*
+ * This controls what symbols we export from the DSO.
+ */
+VERSION
+{
+       LINUX_2.6 {
+       global:
+               __kernel_rt_sigreturn;
+               __vdso_gettimeofday;
+               __vdso_clock_getres;
+               __vdso_clock_gettime;
+       local: *;
+       };
+}

I still struggle to understand how symbol versioning is supposed to work
in a vdso (as opposed to a library you compile against), but I think this should
use the version from the kernel that you plan to merge into, i.e. LINUX_4
or LINUX_4_16.

       Arnd