OpenBSD needs DE_CFG MSR on AMD bhyve
When attempting to boot openbsd 6.7 under bhyve on an AMD (Rome) machine, it attempts to unconditionally write to the DE_CFG (0xc0011029) MSR. When this is done from the installer, the system simply panics on the #GP and halts. For an already-installed VM, it'll drop into the debugger on that instruction:
kernel: protection fault trap, code=0 Stopped at identifycpu+0xa5c: wrmsr ... ddb> show registers rdi 0xffffffff81f12035 cpu_vendor+0xd rsi 0xffffffff81c5dacb cmd0646_9_tim_udma+0x15c35 rbp 0xffffffff822618c0 end+0x2618c0 rbx 0 rdx 0 rcx 0xc0011029 rax 0x3 r8 0xffffffff82261800 end+0x261800 r9 0xffffffff82261667 end+0x261667 r10 0x633b42a234a4dd88 r11 0x2ae3b7a13024bb27 r12 0x1 r13 0xffffffff81ef7030 cpu_cd r14 0xffffffff81ef5ff0 cpu_info_full_primary+0x1ff0 r15 0xa69c1f70 rip 0xffffffff812906cc identifycpu+0xa5c cs 0x8 rflags 0x10206 __ALIGN_SIZE+0xf206 rsp 0xffffffff82261850 end+0x261850 ss 0x10 identifycpu+0xa5c: wrmsr
When the MSR is emulated as a no-op, the VM is able to boot and run without any issues.
Updated by Patrick Mooney about 3 years ago
Scraping some public AMD docs for how they've defined certain bits in this MSR for different families.
0 - Workaround for errata 721 on family 10h
1 - Make LFENCE serializing - "All AMD family 10h/12h/14h/15h/16h/17h processors support this MSR. LFENCE support is indicated by CPUID function1 EDX bit 26, SSE2. AMD family 0Fh/11h processors support LFENCE as serializing always but do not support this MSR. AMD plans support for this MSR and access to this bit for all future processors."
23 - ClflushSerialize. Read-write. 1=CLFLUSH is a serializing instruction. 0=CLFLUSH is not a serializing instruction.
2:7 - ConvertPrefetchToNop. Read-write. Reset: 0. 1=The specified PREFETCH instruction is converted into a NOP. Each of these bits apply to a different instruction as follows: ...
10 - ResyncPredSingleDispDis. Read-write. Reset: 0. BIOS: IF (PROC>=OR_C0) THEN 0 ELSE 1 ENDIF1. 1=Disables the single dispatch of loads that hit in the resync predictor.
13 - Workaround for errata 1021
Updated by Electric Monk about 3 years ago
- Status changed from In Progress to Closed
- % Done changed from 0 to 100
commit 425a83377a83720d1e9907ec3641a499a5eed18f Author: Patrick Mooney <firstname.lastname@example.org> Date: 2020-08-01T18:53:27.000Z 12998 OpenBSD needs DE_CFG MSR on AMD bhyve Reviewed by: Andy Fiddaman <email@example.com> Reviewed by: Mike Zeller <firstname.lastname@example.org> Reviewed by: Robert Mustacchi <email@example.com> Approved by: Dan McDonald <firstname.lastname@example.org>