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[[!tag open_issue_glibc open_issue_gnumach open_issue_hurd]]

Evaluate whether or not usage of vDSOs (virtual dynamically linked shared
objects; [[!wikipedia vDSO]]) can be useful in a GNU Hurd system.

Explanation and example for the Linux kernel: [Creating a vDSO: the Colonel's
Other
Chicken](http://www.linuxjournal.com/content/creating-vdso-colonels-other-chicken),
Matt Davis, 2012-02-06.  The *Resources* given are also worth reading.
Basically, this is useful for exporting data from the kernel (generally, or
given a process context ([[Unix]]), or task/thread context, and so on).

On a GNU Hurd system, parts of the data that makes up a process context doesn't
actually live inside the kernel, but instead is directly held in glibc.  For
example `sysdeps/mach/hurd/getpid.c:__getpid` does a mere `return _hurd_pid`.
For this reason, vDSOs might not be as useful on GNU Hurd as they are with the
Linux kernel.  Or, put another way, as GNU Hurd system doesn't have many
[[system_call]]s, also there aren't many that could be replaced.

Generally only *real* [[system_call]]s should be candidates for implementation
with vDSO code, because otherwise that'd break the ([[RPC]]) system's inherent
[[/virtualization]] capabilities.

Having vDSO code might be useful for:

  * `mach_*_self`: `mach_host_self`, `mach_task_self`, `mach_thread_self`?

  * [[mapped-time_interface|microkernel/mach/gnumach/interface/device/time]]

    Every application can then use that via the regular
    `gettimeofday`/`clock_gettime` and similar calls instead of using the
    special [[hurd/libshouldbeinlibc]]'s `<maptime.h>` interface.

    Can implement [[`clock_gettime` stuff|clock_gettime]] more easily that way,
    for example for nanosecond precision?

    Now, the [[mapped-time_interface]] is virtualizable -- the question is
    whether there is a way so that we can make a compromise here?