Commit eee6946e authored Jun 25, 2015 by Andy Lutomirski Committed by Ingo Molnar Jul 06, 2015

x86/asm/tsc/sync: Use rdtsc_ordered() in check_tsc_warp() and drop extra barriers



Using get_cycles was unnecessary: check_tsc_warp() is not called
on TSC-less systems. Replace rdtsc_barrier(); get_cycles() with
rdtsc_ordered().

While we're at it, make the somewhat more dangerous change of
removing barrier_before_rdtsc after RDTSC in the TSC warp check
code. This should be okay, though -- the vDSO TSC code doesn't
have that barrier, so, if removing the barrier from the warp
check would cause us to detect a warp that we otherwise wouldn't
detect, then we have a genuine bug.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kvm ML <kvm@vger.kernel.org>
Link: http://lkml.kernel.org/r/387c4c3a75f875bcde6cd68cee013273a744f364.1434501121.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 03b9730b

arch/x86/kernel/tsc_sync.c

+6 −8

Original line number	Diff line number	Diff line
		@@ -39,16 +39,15 @@ static cycles_t max_warp;
		static int nr_warps;

		/*
		* TSC-warp measurement loop running on both CPUs:
		* TSC-warp measurement loop running on both CPUs. This is not called
		* if there is no TSC.
		*/
		static void check_tsc_warp(unsigned int timeout)
		{
		cycles_t start, now, prev, end;
		int i;

		rdtsc_barrier();
		start = get_cycles();
		rdtsc_barrier();
		start = rdtsc_ordered();
		/*
		* The measurement runs for 'timeout' msecs:
		*/
		@@ -63,9 +62,7 @@ static void check_tsc_warp(unsigned int timeout)
		*/
		arch_spin_lock(&sync_lock);
		prev = last_tsc;
		rdtsc_barrier();
		now = get_cycles();
		rdtsc_barrier();
		now = rdtsc_ordered();
		last_tsc = now;
		arch_spin_unlock(&sync_lock);

		@@ -126,7 +123,7 @@ void check_tsc_sync_source(int cpu)

		/*
		* No need to check if we already know that the TSC is not
		* synchronized:
		* synchronized or if we have no TSC.
		*/
		if (unsynchronized_tsc())
		return;
		@@ -190,6 +187,7 @@ void check_tsc_sync_target(void)
		{
		int cpus = 2;

		/* Also aborts if there is no TSC. */
		if (unsynchronized_tsc() \|\| tsc_clocksource_reliable)
		return;