[Bug 218662] bhyve exposes CPU feature SDBG to guests, causing guest panic on OpenBSD 6.1

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=218662

            Bug ID: 218662
           Summary: bhyve exposes CPU feature SDBG to guests, causing
                    guest panic on OpenBSD 6.1
           Product: Base System
           Version: 10.3-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: todd.mortimer at gmail.com

When attempting to create an OpenBSD 6.1 bhyve guest on 10.3-RELEASE-p18, the
guest panics early in the boot process when attempting to disable the Silicon
Debug feature identified by CPU feature bit SDBG. This feature bit is passed to
guests if present on the host CPU. 

In my case, my CPU is this:

CPU: Intel(R) Core(TM) i7-4790S CPU @ 3.20GHz (3192.68-MHz K8-class CPU)
  Origin="GenuineIntel"  Id=0x306c3  Family=0x6  Model=0x3c  Stepping=3
 
Features=0xbfebfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CLFLUSH,DTS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,PBE>
 
Features2=0x7ffafbff<SSE3,PCLMULQDQ,DTES64,MON,DS_CPL,VMX,SMX,EST,TM2,SSSE3,SDBG,FMA,CX16,xTPR,PDCM,PCID,SSE4.1,SSE4.2,x2APIC,MOVBE,POPCNT,TSCDLT,AESNI,XSAVE,OSXSAVE,AVX,F16C,RDRAND>
  AMD Features=0x2c100800<SYSCALL,NX,Page1GB,RDTSCP,LM>
  AMD Features2=0x21<LAHF,ABM>
  Structured Extended
Features=0x27ab<FSGSBASE,TSCADJ,BMI1,AVX2,SMEP,BMI2,ERMS,INVPCID,NFPUSG>
  XSAVE Features=0x1<XSAVEOPT>
  VT-x: PAT,HLT,MTF,PAUSE,EPT,UG,VPID
  TSC: P-state invariant, performance statistics


When OpenBSD 6.1 boots under byhve, it detects the SDBG feature and attempts to
disable it, resulting in a protection fault / panic.

This was discussed on the OpenBSD misc list:

http://marc.info/?l=openbsd-misc&m=149136052120277&w=4

And a patch for HardenedBSD is here:

https://github.com/HardenedBSD/hardenedBSD/commit/cc91b57f4d1dabddfbf8b1e7655

As a workaround, passing -w to byhve allows the guest to boot, but the CPU
feature should be masked out by the host.

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