I've also started to upload some of these crash files (like the 0x88.0.txt
file) to Lumo for analysis. The AI info is interesting but it tends to have
errors on invalid bhyve command line flags or sometimes even what code it
thinks 0x88 belongs to. I'll post some of it below. This was its response to
the 0x88.0.txt data. If we look at what it says for the 0x88 code, it doesn't
match what I see in the freebsd code (#define VMCB_EXIT_ICEBP
0x88):
https://github.com/freebsd/freebsd-src/blob/4c3a868d13c053ef173268cdfe1365978a282178/sys/amd64/vmm/amd/vmcb.h#L94
is the AI wrong about this?
---- Lumo Response:
I can see you’ve uploaded a text dump named 0x88.0.txt that looks like a
snapshot of a bhyve/AMD‑SVM virtual‑machine (VM) state and statistics. Here’s a
concise walkthrough of what the file contains and what it tells us about the
VM’s execution.
1. High‑level picture
Item Value / Meaning
VM exit reason 0x88 – this is the exit code for VM_EXIT_REP_INSB (a repeated
string‑input operation, i.e., rep insb/insw/insd).
Current exception vector 6 – a #UD (invalid opcode) or #GP depending on
context; the dump shows the VM is presently handling an exception.
Number of CPUs 16 vCPUs (IDs 0‑15) are active and suspended; the host has a
16‑core topology.
Total VM exits 3 259 037 – a fairly large number, indicating heavy interaction
between guest and hypervisor.
Instruction‑emulation exits 1 465 913 – most of the exits are for
instructions that bhyve had to emulate (e.g., I/O, privileged ops).
Pause‑instruction exits 1 138 635 – the guest frequently executed pause, which
bhyve intercepts for power‑saving.
HLT exits 75 539 – the guest entered the halt state many times (often
seen in idle loops).
In/out exits 492 316 – the guest performed port‑I/O operations that required
hypervisor handling.
CPUID exits 3 735 – the guest queried CPU features.
MSR reads/writes 1 783 reads, 95 writes – typical for a modern OS that
accesses model‑specific registers.
Nested page‑fault exits 610 – indicates the guest’s second‑level address
translation (NPT/EPT) missed some pages.
Memory layout Two huge sysmem regions (≈33 GB and ≈29 GB) plus a 16 KB boot
ROM region. Most of the RAM is “wired” (pinned) for the VM.
RTC time Mon Sep 22 10:35:00 2025 – matches today’s date, confirming the
snapshot is recent.
2. What the 0x88 exit really means
Opcode involved: rep insb/insw/insd (string input from an I/O port).
Why bhyve exits: AMD‑SVM (and Intel VT‑x) cannot directly execute this
instruction in guest mode; they force a VM exit so the hypervisor can either
emulate the I/O or inject a #DB (debug) exception.
Typical causes:
Guest reading from a legacy serial port, keyboard controller, or other
device that uses bulk input.
Anti‑debugging tricks where malware repeatedly executes rep insb to
detect a hypervisor.
Impact: Each such exit costs a few microseconds of latency. A high
frequency can degrade performance, especially if the guest is in a tight loop
doing bulk reads.
3. Other notable fields
Field Interpretation
cr_intercept = 0xfee2fee2 The hypervisor is set to intercept many
control‑register accesses (CR0, CR3, CR4, etc.).
exc_intercept = 0x00040000 Exception interception is enabled for the #PF
(page‑fault) vector.
inst1_intercept = 0xddc4000f Intercepts a specific instruction pattern
(likely hlt and pause).
exception_bitmap = 0x40000 Only the page‑fault exception (bit 18) is being
intercepted.
io_bitmap = 0x2b053b000 Defines which I/O ports cause exits; the presence of
many in/out exits suggests a fairly wide bitmap.
msr_bitmap = 0x2b0539000 Controls which Model‑Specific Register accesses
trigger exits.
capability flags The VM supports hlt_exit, pause_exit,
unrestricted_guest, ipi_exit; lacks mtrap_exit, enable_invpcid, etc.
guest_pat = 0x0007010600070106 PAT (Page Attribute Table) settings – standard
write‑combining / write‑through configuration.
asid = 0x07fd Address‑Space Identifier used for NPT/EPT – helps reduce TLB
flushes.
rflags = 0x40193 Typical user‑mode flag state (IF=1, ZF=0, etc.).
rip = 0xffffd001b2ce151d Current instruction pointer; the address falls
within the first sysmem region, suggesting the guest is executing normal code
when the dump was taken.
rsp = 0xffffe3018da8e7b8 Stack pointer – also in the wired sysmem area.
4. Performance clues
High pause‑exit count (>1 M) + high hlt‑exit count (~75 k) indicate the
guest spends a lot of time in idle loops. That’s typical for a server VM that
sleeps when idle.
Large number of instruction‑emulation exits (~1.46 M) suggests the guest is
doing a fair amount of privileged I/O (e.g., disk, network, or device drivers)
that the hypervisor must emulate.
Relatively low NMI and ExtINT counts (0) imply the VM isn’t receiving
asynchronous hardware interrupts, which aligns with a mostly self‑contained
workload.
5. What you might want to investigate next
Why so many rep insb exits?
Look at the guest’s I/O port map (e.g., serial console, legacy floppy).
If the guest is repeatedly reading from a port that isn’t needed, you could
disable that device in the bhyve config.
If the exits are caused by malicious anti‑debug code, consider adding a
pass‑through for that port or using a more permissive I/O bitmap.
Tune the I/O bitmap
If you know which ports the guest actually needs, narrow the bitmap to
reduce unnecessary exits.
Check pause‑exit handling
Modern kernels already use pause for spin‑waits; the high count is
normal, but you can enable the pause_exit capability (already set) to let the
hypervisor handle them efficiently.
Memory wiring
Almost all RAM is marked “wired/iommu”. If you don’t need that much
pinned memory, you could relax the wiring to allow the host to reclaim pages
under pressure.
Look at the instruction pointer (rip)
Mapping 0xffffd001b2ce151d back to the guest’s binary (via objdump/gdb
on the guest image) could reveal exactly which instruction caused the 0x88
exit. That can confirm whether it’s a legitimate driver read or something
suspicious.
TL;DR
Your dump shows a bhyve VM that is currently handling a VM_EXIT_REP_INSB (0x88)
– a bulk input operation from an I/O port. The VM has been quite busy: over
three million exits, many of which are for instruction emulation, pause, and
I/O. Memory is largely wired, and the guest is idle‑heavy (lots of hlt/pause).
If you want to improve performance or diagnose a possible anti‑debugging
pattern, focus on the ports generating the rep insb exits and consider
tightening the I/O bitmap or providing a proper device backend.
