[Bug 207068] hwpmc wrap around/sign extension

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

--- Comment #5 from joss.upton at yahoo.com ---
(In reply to Konstantin Belousov from comment #4)
I don't know if Sandy Bridge supports the aliases or not.  I do know that
vmware fusion on my Broadwell machine does NOT support the IA32_A_PMCn
registers and thus will panic.

Note you may also need other processes using the PMC to be
scheduled/unscheduled to reproduce... You have to catch a case where it writes
with EAX[31]=1 to IA32_PMCn.  This results in an automatic sign extension (as
described in the Intel arch manual, 18.2.2.3 Full-Width Writes to Performance
Counter Registers) and ignores EDX completely.

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