Messages in this thread | | | Date | Tue, 29 Nov 2005 17:04:21 +0100 | From | Mikael Pettersson <> | Subject | Re: Enabling RDPMC in user space by default |
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Andi Kleen writes: > Hallo, > > I'm considering to enable CR4.PCE by default on x86-64/i386. Currently it's 0 > which means RDPMC doesn't work. On x86-64 PMC 0 is always programmed > to be a cycle counter, so it would be useful to be able to access > this for measuring instructions. That's especially useful because RDTSC > does not necessarily count cycles in the current P state (already > the case on Intel CPUs and AMD's future direction seems to also > to decouple it from cycles) Drawback is that it stops during idle, but > that shouldn't be a big issue for normal measuring. It's not useful > as a real timer anyways. > > On Pentium 4 it also has the advantage that unlike RDTSC it's not > serializing so should be much faster. > > The kernel change would be to always set CR4.PCE to allow RDPMC > in ring 3. > > It would be actually a good idea to disable RDTSC in ring 3 too > (because user space usually doesn't have enough information to make > good use of it and gets it wrong), but I fear that will break > too many applications right now.
PMC0 stops being a cycle counter as soon as any real driver (not the NMI watchdog) takes over the hardware, such as oprofile, perfmon2, or perfctr. So user-space cannot rely on the semantics of PMC0. I have no objection to globally enabling CR4.PCE.
Disabling user-space RDTSC (setting CR4.TSD) seems evil and pointless. At least some users of it (the perfctr library and I hope eventually also perfmon2) do use it in an SMP-safe manner (through special user/kernel protocols).
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