Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/x86/linux-2.6-x86

* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/x86/linux-2.6-x86: (77 commits)
  x86: UV startup of slave cpus
  x86: integrate pci-dma.c
  x86: don't do dma if mask is NULL.
  x86: return conditional to mmu
  x86: remove kludge from x86_64
  x86: unify gfp masks
  x86: retry allocation if failed
  x86: don't try to allocate from DMA zone at first
  x86: use a fallback dev for i386
  x86: use numa allocation function in i386
  x86: remove virt_to_bus in pci-dma_64.c
  x86: adjust dma_free_coherent for i386
  x86: move bad_dma_address
  x86: isolate coherent mapping functions
  x86: move dma_coherent functions to pci-dma.c
  x86: merge iommu initialization parameters
  x86: merge dma_supported
  x86: move pci fixup to pci-dma.c
  x86: move x86_64-specific to common code.
  x86: move initialization functions to pci-dma.c
  ...

1 files changed, 20 insertions, 3 deletions

diff --git a/arch/x86/kernel/tsc_64.c b/arch/x86/kernel/tsc_64.c
index ceeba01e7f4..fcc16e58609 100644
--- a/arch/x86/kernel/tsc_64.c
+++ b/arch/x86/kernel/tsc_64.c
@@ -11,6 +11,7 @@
 #include <asm/hpet.h>
 #include <asm/timex.h>
 #include <asm/timer.h>
+#include <asm/vgtod.h>
 
 static int notsc __initdata = 0;
 
@@ -287,18 +288,34 @@ int __init notsc_setup(char *s)
 
 __setup("notsc", notsc_setup);
 
+static struct clocksource clocksource_tsc;
 
-/* clock source code: */
+/*
+ * We compare the TSC to the cycle_last value in the clocksource
+ * structure to avoid a nasty time-warp. This can be observed in a
+ * very small window right after one CPU updated cycle_last under
+ * xtime/vsyscall_gtod lock and the other CPU reads a TSC value which
+ * is smaller than the cycle_last reference value due to a TSC which
+ * is slighty behind. This delta is nowhere else observable, but in
+ * that case it results in a forward time jump in the range of hours
+ * due to the unsigned delta calculation of the time keeping core
+ * code, which is necessary to support wrapping clocksources like pm
+ * timer.
+ */
 static cycle_t read_tsc(void)
 {
 	cycle_t ret = (cycle_t)get_cycles();
-	return ret;
+
+	return ret >= clocksource_tsc.cycle_last ?
+		ret : clocksource_tsc.cycle_last;
 }
 
 static cycle_t __vsyscall_fn vread_tsc(void)
 {
 	cycle_t ret = (cycle_t)vget_cycles();
-	return ret;
+
+	return ret >= __vsyscall_gtod_data.clock.cycle_last ?
+		ret : __vsyscall_gtod_data.clock.cycle_last;
 }
 
 static struct clocksource clocksource_tsc = {