x86/asm/tsc: Use rdtsc_ordered() in read_tsc() instead of get_cycles()

There are two logical changes here.  First, this removes a check
for cpu_has_tsc.  That check is unnecessary, as we don't
register the TSC as a clocksource on systems that have no TSC.

Second, it adds a barrier, thus preventing observable
non-monotonicity.

I suspect that the missing barrier was never a problem in
practice because system calls themselves were heavy enough
barriers to prevent user code from observing time warps due to
speculation. (Without the corresponding barrier in the vDSO,
however, non-monotonicity is easy to detect.)

Signed-off-by: Andy Lutomirski <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Huang Rui <[email protected]>
Cc: John Stultz <[email protected]>
Cc: Len Brown <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: kvm ML <[email protected]>
Link: http://lkml.kernel.org/r/c6ff621a053127a65b70f175443578db7a0711be.1434501121.git.luto@kernel.org
Signed-off-by: Ingo Molnar <[email protected]>

0 files changed, 0 insertions, 0 deletions