x86/vdso: Make delta calculation overflow safe

x86/vdso: Make delta calculation overflow safe

Kernel timekeeping is designed to keep the change in cycles (since the last
timer interrupt) below max_cycles, which prevents multiplication overflow
when converting cycles to nanoseconds. However, if timer interrupts stop,
the calculation will eventually overflow.

Add protection against that. Select GENERIC_VDSO_OVERFLOW_PROTECT so that
max_cycles is made available in the VDSO data page. Check against
max_cycles, falling back to a slower higher precision calculation. Take
advantage of the opportunity to move masking and negative motion check
into the slow path.

The result is a calculation that has similar performance as before. Newer
machines showed performance benefit, whereas older Skylake-based hardware
such as Intel Kaby Lake was seen <1% worse.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240325064023.2997-9-adrian.hunter@intel.com