Re: Enabling RDPMC in user space by default

[Mikael Pettersson]

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).

/Mikael
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