Re: [PATCH] oprofile add Pentium Mobile support

[Mikael Pettersson]

Will Cohen writes:
 > Mikael Pettersson wrote:
 > > Philippe Elie writes:
 > >  > From: Will Cohen <wcohen@xxxxxxxxxx>
 > >  > 
 > >  > Add oprofile support for Pentium Mobile (P6 core). Pentium Mobile needs
 > >  > to unmask LVPTC vector, since it doesn't hurt other P6 core based cpus
 > >  > we do it unconditionally for all these.
 > > 
 > > [Patch talking about the Pentium-M.]
 > > 
 > > I can find no support in Intel's documentation (IA32 Volume 3,
 > > 25366813.pdf) that Pentium-M:s need to unmask LVTPC.
 > > 
 > > How certain are you of this? Is this an undocumented hardware
 > > quirk? If it is documented, please indicate where.
 > 
 > I have tested it on a Pentium M machine. Without the unmask LVTPC the 
 > nmi handler collected precisely one interrupt. With the LVTPC unmask the 
 > OProfile data collection worked normally.
 > 
 > > It's my theory that P4 added the auto-masking to help PEBS
 > > buffer overflow situations, but since P-M doesn't have PEBS,
 > > they shouldn't have had to change this on P-M as well.
 > > OTOH, it's certainly possible they changed it by accident.
 > 
 > My theory is that the Pentium M uses same bus interface and local apic 
 > as the Pentium M. Thus, the Pentium M shares the Pentium 4 need to 
 > unmask LVTPC.

Sounds reasonable and is consistent with other P-M vs P6 changes.

 > > One way of testing this would be to run a P-M with
 > > nmi_watchdog=2. If the NMI counter keeps ticking, then
 > > LVTPC does not need unmasking.
 > 
 > Yes, I tested the nmi watchdog and it currently does not work on the 
 > Pentium M. It doesn't get the later interrupts and states it is stuck.

That confirms the "undocumented HW quirk" theory. Thanks. I'll fix
the (in-kernel) NMI watchdog and (out-of-kernel) perfctr drivers.

/Mikael
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