[Bug 259878] [hyper-v] Kernel hangs at boot after printing Hyper-V features

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=259878

--- Comment #4 from Thomas Eberhardt <sneakywumpus@gmail.com> ---
(In reply to Mark Johnston from comment #3)
Ok. I built a full debug stable/13 GENERIC kernel and learned how to add a
serial console to a Hyper-V VM to capture the boot log.

boot log:
GDB: no debug ports present
KDB: debugger backends: ddb
KDB: current backend: ddb
---<<BOOT>>---
Copyright (c) 1992-2021 The FreeBSD Project.
Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 1992, 1993, 1994
        The Regents of the University of California. All rights reserved.
FreeBSD is a registered trademark of The FreeBSD Foundation.
FreeBSD 13.0-STABLE #0 stable/13-n248070-7d95b0f32832-dirty: Tue Nov 16
16:18:10 CET 2021
    root@jones.ocp.lan:/usr/obj/usr/src/amd64.amd64/sys/GENERIC amd64
FreeBSD clang version 12.0.1 (git@github.com:llvm/llvm-project.git
llvmorg-12.0.1-0-gfed41342a82f)
WARNING: WITNESS option enabled, expect reduced performance.
subsystem ffffff
   parse_acpi_tables(0)... SRAT: Ignoring memory at addr 0x188000000
SRAT: Ignoring memory at addr 0x1000000000
SRAT: Ignoring memory at addr 0x10000000000
SRAT: Ignoring memory at addr 0x20000000000
[lots of output deleted]
   sd_mkdir_show_add(0)... done.
   sd_mkdir_list_show_add(0)... done.
   sd_allocdirect_show_add(0)... done.
   sd_allocindir_show_add(0)... done.
   ffs_show_add(0)... done.
   witness_show_add(0)... done.
   badstacks_show_add(0)... done.
   vpath_show_add(0)... done.
subsystem 2100000
   cpu_startup(0)...

and there it hangs. The only thing dirty in this built is the GENERIC kernel
config with the added debug options. I also tried booting the VM with only 1
CPU instead of 6, but it hangs at the same place.


CPU-ID output from a build with my patch:

CPU: Intel(R) Core(TM) i7-9700T CPU @ 2.00GHz (1992.01-MHz K8-class CPU)
  Origin="GenuineIntel"  Id=0x906ed  Family=0x6  Model=0x9e  Stepping=13
 
Features=0x1f83fbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,MMX,FXSR,SSE,SSE2,SS,HTT>
 
Features2=0xfeda3203<SSE3,PCLMULQDQ,SSSE3,FMA,CX16,PCID,SSE4.1,SSE4.2,MOVBE,POPCNT,AESNI,XSAVE,OSXSAVE,AVX,F16C,RDRAND,HV>
  AMD Features=0x2c100800<SYSCALL,NX,Page1GB,RDTSCP,LM>
  AMD Features2=0x121<LAHF,ABM,Prefetch>
  Structured Extended
Features=0x9c2fb9<FSGSBASE,BMI1,HLE,AVX2,SMEP,BMI2,ERMS,INVPCID,RTM,NFPUSG,RDSEED,ADX,SMAP,CLFLUSHOPT>
  Structured Extended
Features3=0xbc000400<MD_CLEAR,IBPB,STIBP,L1DFL,ARCH_CAP,SSBD>
  XSAVE Features=0xf<XSAVEOPT,XSAVEC,XINUSE,XSAVES>
  IA32_ARCH_CAPS=0xa7<RDCL_NO,IBRS_ALL,RSBA,MDS_NO,TSX_CTRL>

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