Re: [PATCH] oprofile add Pentium Mobile support

[Will Cohen <wcohen@redhat.com>]

Mikael Pettersson wrote:
> Philippe Elie writes:
>  > From: Will Cohen <wcohen@redhat.com>
>  > 
>  > 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.

> 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.


-Will