[Bug 268943] virtualized kernel panics on process exit running under pmcstat

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=268943

            Bug ID: 268943
           Summary: virtualized kernel panics on process exit running
                    under pmcstat
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: jfc@mit.edu

Running a recent CURRENT under bhyve, pmcstat reliably causes a panic when the
monitored process exits.

# pmcstat -p instructions ls > /dev/null 2> /tmp/2
panic: [amd,770] PMC0, CPU0 "K8-0" already stopped
cpuid = 0
time = 1673651198
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe00c326dc20
vpanic() at vpanic+0x151/frame 0xfffffe00c326dc70
panic() at panic+0x43/frame 0xfffffe00c326dcd0
amd_stop_pmc() at amd_stop_pmc+0x12e/frame 0xfffffe00c326dcf0
pmc_process_exit() at pmc_process_exit+0x26d/frame 0xfffffe00c326dd80
exit1() at exit1+0x36e/frame 0xfffffe00c326ddf0
sys_exit() at sys_exit+0xd/frame 0xfffffe00c326de00
amd64_syscall() at amd64_syscall+0x12e/frame 0xfffffe00c326df30
fast_syscall_common() at fast_syscall_common+0xf8/frame 0xfffffe00c326df30
--- syscall (1, FreeBSD ELF64, exit), rip = 0x2ee9ac03458a, rsp =
0x2ee9a9c13488, rbp = 0x2ee9a9c134a0 ---

System is
CPU: AMD EPYC 7402P 24-Core Processor                (2794.91-MHz K8-class CPU)
  Origin="AuthenticAMD"  Id=0x830f10  Family=0x17  Model=0x31  Stepping=0
and the VM has 8 cores.

This does not happen on the 13.1-STABLE host or on a desktop class Zen CPU
("AMD Ryzen 5 PRO 2400GE") running CURRENT.

I am splitting this off from bug 268897.  I found it trying to reproduce that
bug, but it looks different.

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