alistair23-linux/lib/vdso/Kconfig
Thomas Gleixner 660fd04f93 lib/vdso: Prepare for time namespace support
To support time namespaces in the vdso with a minimal impact on regular non
time namespace affected tasks, the namespace handling needs to be hidden in
a slow path.

The most obvious place is vdso_seq_begin(). If a task belongs to a time
namespace then the VVAR page which contains the system wide vdso data is
replaced with a namespace specific page which has the same layout as the
VVAR page. That page has vdso_data->seq set to 1 to enforce the slow path
and vdso_data->clock_mode set to VCLOCK_TIMENS to enforce the time
namespace handling path.

The extra check in the case that vdso_data->seq is odd, e.g. a concurrent
update of the vdso data is in progress, is not really affecting regular
tasks which are not part of a time namespace as the task is spin waiting
for the update to finish and vdso_data->seq to become even again.

If a time namespace task hits that code path, it invokes the corresponding
time getter function which retrieves the real VVAR page, reads host time
and then adds the offset for the requested clock which is stored in the
special VVAR page.

If VDSO time namespace support is disabled the whole magic is compiled out.

Initial testing shows that the disabled case is almost identical to the
host case which does not take the slow timens path. With the special timens
page installed the performance hit is constant time and in the range of
5-7%.

For the vdso functions which are not using the sequence count an
unconditional check for vdso_data->clock_mode is added which switches to
the real vdso when the clock_mode is VCLOCK_TIMENS.

[avagin: Make do_hres_timens() work with raw clocks too: choose vdso_data
 pointer by CS_RAW offset.]

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20191112012724.250792-21-dima@arista.com
2020-01-14 12:20:57 +01:00

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# SPDX-License-Identifier: GPL-2.0
config HAVE_GENERIC_VDSO
bool
if HAVE_GENERIC_VDSO
config GENERIC_GETTIMEOFDAY
bool
help
This is a generic implementation of gettimeofday vdso.
Each architecture that enables this feature has to
provide the fallback implementation.
config GENERIC_VDSO_32
bool
depends on GENERIC_GETTIMEOFDAY && !64BIT
help
This config option helps to avoid possible performance issues
in 32 bit only architectures.
config GENERIC_COMPAT_VDSO
bool
help
This config option enables the compat VDSO layer.
config GENERIC_VDSO_TIME_NS
bool
help
Selected by architectures which support time namespaces in the
VDSO
endif