On Mon, Jul 21, 2014 at 2:52 PM, Borislav Petkov <b...@alien8.de> wrote: > On Mon, Jul 21, 2014 at 02:41:50PM -0700, Andy Lutomirski wrote: >> How will this be compatible with the vdso? > > I've never thought about it yet. How compatible would you want it to be > and what do you expect from it?
I expect that users of __vdso_clock_gettime (e.g. glibc) will get the correct time :) They use vread_tsc, and they can't use preempt_disable, because they're in userspace. They also can't directly access per-cpu variables. Turning off vdso tsc support on these machines would be an option. > > Remember, this is only attempting to be a hardware workaround for a > smallish number of systems out there. Most of current machines should > have stable and synched TSCs. I actually own one of these systems. It's a Sandy Bridge Core-i7 Extreme or something like that. > >> Also, IIRC, rdtscp does not need rdtsc_barrier(), whereas rdtsc does. >> Getting this wrong will be a significant slowdown. > > This is too cryptic for me - get_cycles doesn't barrier around the TSC > now either. Again, we will most likely end up not using RDTSCP anyway. I wonder if that's a bug in get_cycles. The basic issue is that rdtsc is not ordered with respect to nearby loads, so it's fairly easy to see it behaving non-monotonically across CPUs. rdtscp is ordered, but it's a little slower. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
