diff options
| -rw-r--r-- | src/cpu/kvm/timer.cc | 35 |
1 files changed, 29 insertions, 6 deletions
diff --git a/src/cpu/kvm/timer.cc b/src/cpu/kvm/timer.cc index 03cdea6fb..6882063e4 100644 --- a/src/cpu/kvm/timer.cc +++ b/src/cpu/kvm/timer.cc @@ -37,6 +37,7 @@ * Authors: Andreas Sandberg */ +#include <algorithm> #include <csignal> #include <ctime> @@ -45,6 +46,14 @@ #include "cpu/kvm/timer.hh" #include "debug/KvmTimer.hh" +/** + * Minimum number of cycles that a host can spend in a KVM call (used + * to calculate the resolution of some timers). + * + * The value of this constant is a bit arbitrary, but in practice, we + * can't really do anything useful in less than ~1000 cycles. + */ +static const uint64_t MIN_HOST_CYCLES = 1000; PosixKvmTimer::PosixKvmTimer(int signo, clockid_t clockID, float hostFactor, Tick hostFreq) @@ -82,6 +91,8 @@ PosixKvmTimer::arm(Tick ticks) ts.it_value.tv_sec = hostNs(ticks) / 1000000000ULL; ts.it_value.tv_nsec = hostNs(ticks) % 1000000000ULL; + assert(ts.it_value.tv_nsec > 0 || ts.it_value.tv_sec > 0); + DPRINTF(KvmTimer, "Arming POSIX timer: %i ticks (%is%ins)\n", ticks, ts.it_value.tv_sec, ts.it_value.tv_nsec); @@ -109,9 +120,23 @@ PosixKvmTimer::calcResolution() if (clock_getres(clockID, &ts) == -1) panic("PosixKvmTimer: Failed to get timer resolution\n"); - Tick resolution(ticksFromHostNs(ts.tv_sec * 1000000000ULL + ts.tv_nsec)); - - return resolution; + const uint64_t res_ns(ts.tv_sec * 1000000000ULL + ts.tv_nsec); + // We preferrably want ticksFromHostNs() to calculate the the + // ceiling rather than truncating the value. However, there are + // other cases where truncating is fine, so we just add 1 here to + // make sure that the actual resolution is strictly less than what + // we return. We could get all kinds of nasty behavior if + // arm(resolution) is called and the resulting time is 0 (which + // could happen if we truncate the results and the resolution is + // 1ns). + const Tick resolution(ticksFromHostNs(res_ns) + 1); + // It might not make sense to enter into KVM for less than a + // certain number of host cycles. In some systems (e.g., Linux) + // the resolution of the timer we use is 1ns (a few cycles on most + // CPUs), which isn't very useful. + const Tick min_cycles(ticksFromHostCycles(MIN_HOST_CYCLES)); + + return std::max(resolution, min_cycles); } @@ -143,7 +168,5 @@ PerfKvmTimer::disarm() Tick PerfKvmTimer::calcResolution() { - // This is a bit arbitrary, but in practice, we can't really do - // anything useful in less than ~1000 anyway. - return ticksFromHostCycles(1000); + return ticksFromHostCycles(MIN_HOST_CYCLES); } |