From 0b4a8b40866a1ed44bacbccc788ce3001eab8b20 Mon Sep 17 00:00:00 2001 From: Andreas Sandberg Date: Tue, 11 Jun 2013 09:24:38 +0200 Subject: [PATCH] x86: Fix bug when copying TSC on CPU handover The TSC value stored in MISCREG_TSC is actually just an offset from the current CPU cycle to the actual TSC value. Writes with side-effects to the TSC subtract the current cycle count before storing the new value, while reads add the current cycle count. When switching CPUs, the current value is copied without side-effects. This works as long as the source and the destination CPUs have the same clock frequencies. The TSC will jump, sometimes backwards, if they have different clock frequencies. Most OSes assume the TSC to be monotonic and break when this happens. This changeset makes sure that the TSC is copied with side-effects to ensure that the offset is updated to match the new CPU. --- src/arch/x86/utility.cc | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/src/arch/x86/utility.cc b/src/arch/x86/utility.cc index ae6c37be6..e3d25fcdf 100644 --- a/src/arch/x86/utility.cc +++ b/src/arch/x86/utility.cc @@ -209,6 +209,10 @@ copyMiscRegs(ThreadContext *src, ThreadContext *dest) dest->setMiscRegNoEffect(i, src->readMiscRegNoEffect(i)); } + // The TSC has to be updated with side-effects if the CPUs in a + // CPU switch have different frequencies. + dest->setMiscReg(MISCREG_TSC, src->readMiscReg(MISCREG_TSC)); + dest->getITBPtr()->flushAll(); dest->getDTBPtr()->flushAll(); } -- 2.30.2