Messages in this thread | | | Date | Sat, 5 Nov 2005 17:07:20 +0100 | From | Andrea Arcangeli <> | Subject | Re: disable tsc with seccomp |
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On Sat, Nov 05, 2005 at 04:37:44PM +0100, Andi Kleen wrote: > It was useless, you can get exactly the same information by using RDPMC > on perfctr 0 which always runs the NMI watchdog and counts all cycles too.
nmi watchdog is off in my system, but it was used to be very slow. Anyway performance counters should be turned off too. They can be turned off on a per task basis right? Just switching another cr4 bit or what?
The fact turning off the tsc is not enough to remove all timing info, is sure not a good reason to remove that code IMHO, infact we should disable more stuff if there are other ways to gather that information.
The fast path cost is constant and not measurable, no matter how much stuff we disable in the slow path. So that's not a problem. We must disable everything that can be disabled and that can provide potential high precision timing info to userland. For example we could also flip the ptes to non-present over the hpet mapping. Zero-cost! The only _fixed_ cost is the one we already had before you backed out the feature. But note that hpet is a very very low prio compared to tsc, the precision is not high enough for it to matter, but certainly I would welcome a patch flipping the hpet ptes too. I only care about turning off timings sources that allows to count cycles. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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