[Bug 207068] hwpmc wrap around/sign extension

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=207068

--- Comment #7 from Konstantin Belousov <kib at FreeBSD.org> ---
(In reply to joss.upton from comment #6)
I believe that the behaviour of reading MSR 0xc1 is architectural ?  Anyway, on
sandybridge:

sandy% sudo cpucontrol -m 0xc1=0x80000000 /dev/cpuctl0
sandy% sudo cpucontrol -m 0xc1 /dev/cpuctl0
MSR 0xc1: 0x0000ffff 0x80000000

SB does have full-width PMC write capability:
sandy% sudo cpucontrol -m 0x345 /dev/cpuctl0
MSR 0x345: 0x00000000 0x000031c3
bit 13 (FW_WRITE) is set

But my concern is that I cannot reproduce the issue with the following script:
for x in $(jot 8); do pmcstat -P CPU_CLK_UNHALTED_CORE perl ./loop.pl
2>/dev/null &; done

Unpatched kernel must exhibit the problem because it does not write to PMC
through aliases and all writes are clipped.  I want to be able to reproduce
this before commit, at least I need to validate the change and to understand
that it is complete (I agree that this is the right approach, at least).

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