[BUG] Ryzen 7 5700X fatal MC5 MCE on linux-cachyos 7.2.3+ / LTS 6.18.50; vanilla Arch 7.2.4 clean

### Pre-flight checklist

- [x] I have searched existing issues and this is not a duplicate.
- [x] I have read the [Contributing Guidelines](https://github.com/CachyOS/linux-cachyos/blob/master/CONTRIBUTING.md).
- [x] I have verified the issue is reproducible with the latest available CachyOS kernel.
- [x] I have tried to reproduce the issue on Arch Linux's `linux` kernel.

### Upstream / vanilla kernel check

I tested with a vanilla/upstream kernel and the issue does NOT reproduce there (CachyOS-specific bug)

### Kernel variant

linux-cachyos (EEVDF, Clang)

### System information (cachyos-bugreport.sh)

```text
https://paste.cachyos.org/p/16896f3.log
```

### Manual system information (if cachyos-bugreport.sh is unavailable)

```text

```

### Bug description

After updating CachyOS, my Ryzen 7 5700X system began encountering a reproducible AMD Machine Check Exception (MCE) during boot.

I have isolated the regression by booting historical CachyOS kernels through Limine/Btrfs snapshots and comparing them against the vanilla Arch kernel.

Regression testing:

| Kernel | Result |
|---|---|
| `linux-cachyos 7.2.2-1` | GOOD - boots normally, no hardware MCE |
| `linux-cachyos 7.2.3-1` | BAD - MC5 hardware error occurs, but system still boots |
| `linux-cachyos 7.2.4-3` | BAD - System Fatal MC5 error and system does not boot normally |
| `linux-cachyos-lts 6.18.48-1` | GOOD - boots normally, no hardware MCE |
| `linux-cachyos-lts 6.18.50-3` | BAD - same MC5 error and does not boot normally |
| Arch `linux 7.2.4.arch1-2` | GOOD with respect to MCE - no hardware MCE and reaches systemd userspace |

The important comparison is that vanilla Arch `linux 7.2.4.arch1-2` does not produce the MCE on exactly the same hardware, BIOS configuration and microcode.

This suggests the issue may be related to a change shared by newer CachyOS kernel builds rather than an upstream Linux 7.2.4 regression.

The machine is currently running `linux-cachyos 7.2.2-1` successfully with no MCE.

### Steps to reproduce

1. Boot the machine using `linux-cachyos 7.2.2-1`.
- System boots normally.
- No `[Hardware Error]` MCE is present in the kernel journal.

2. Boot `linux-cachyos 7.2.3-1`.
- An AMD MC5 machine check with status `0xbea0000000000108` occurs.
- The system still reaches userspace.

3. Boot `linux-cachyos 7.2.4-3`.
- The same MC5 / System Fatal error occurs during boot.
- The system does not boot normally.

4. Boot `linux-cachyos-lts 6.18.48-1`.
- System boots normally.
- No hardware MCE occurs.

5. Boot `linux-cachyos-lts 6.18.50-3`.
- The same MC5 error occurs.
- The system does not boot normally.

6. Boot vanilla Arch `linux 7.2.4.arch1-2`.
- No MC5 hardware error occurs.
- Kernel successfully reaches systemd userspace.

7. Disable CPU/PBO and RAM performance tuning in BIOS and retest the affected CachyOS kernel.
- The same MCE remains reproducible.

### Expected behavior

The current CachyOS kernels should boot without triggering an AMD MC5 machine check, as occurs on the same machine with:

- `linux-cachyos 7.2.2-1`
- `linux-cachyos-lts 6.18.48-1`
- vanilla Arch `linux 7.2.4.arch1-2`

No `[Hardware Error]`, `System Fatal error`, or MC5 execution-unit error should be generated during normal boot.

### Actual behavior

Newer CachyOS kernels reproducibly trigger an AMD MC5 machine check during boot.

The first affected kernel I have identified is `linux-cachyos 7.2.3-1`. It reports the MCE but still manages to boot.

`linux-cachyos 7.2.4-3` produces the same error but no longer boots normally.

The regression also appears in the LTS kernel:

- `linux-cachyos-lts 6.18.48-1` is clean.
- `linux-cachyos-lts 6.18.50-3` produces the same MCE and does not boot normally.

The consistent MCE signature is:

MC5_STATUS: `0xbea0000000000108`
IPID: `0x000500b000000000`
Syndrome: `0x000000004d000000`

The logical CPU reporting the error and the error address can vary between boots.

Vanilla Arch `linux 7.2.4.arch1-2` does not produce this MCE on the same system.

### Logs / stack traces

```text
### Affected CachyOS kernel

One affected boot produced:

[Hardware Error]: System Fatal error.
[Hardware Error]: CPU:0 (19:21:2) MC5_STATUS[-|UE|MiscV|AddrV|PCC|TCC|SyndV|-|-|-]: 0xbea0000000000108
[Hardware Error]: Error Addr: 0x01ffffffc0a0c8ea
[Hardware Error]: IPID: 0x000500b000000000, Syndrome: 0x000000004d000000
[Hardware Error]: Execution Unit Ext. Error Code: 0
[Hardware Error]: cache level: RESV, tx: GEN, mem-tx: GEN

Another affected boot (`7.2.3-1`) produced the same signature on CPU 11:

[Hardware Error]: System Fatal error.
[Hardware Error]: CPU:11 (19:21:2) MC5_STATUS[-|UE|MiscV|AddrV|PCC|TCC|SyndV|-|-|-]: 0xbea0000000000108
[Hardware Error]: Error Addr: 0x01ffffff953a8dca
[Hardware Error]: IPID: 0x000500b000000000, Syndrome: 0x000000004d000000
[Hardware Error]: Execution Unit Ext. Error Code: 0
[Hardware Error]: cache level: RESV, tx: GEN, mem-tx: GEN

### Known-good CachyOS 7.2.2

`linux-cachyos 7.2.2-1`:

MCE: In-kernel MCE decoding enabled.
EDAC MC: Ver: 3.0.0

No `[Hardware Error]` entries are present.

### Known-good CachyOS LTS

`linux-cachyos-lts 6.18.48-1`:

MCE: In-kernel MCE decoding enabled.
EDAC MC: Ver: 3.0.0
microcode: Current revision: 0x0a201213

No `[Hardware Error]` entries are present.

### Vanilla Arch comparison

Arch `linux 7.2.4.arch1-2`:

MCE: In-kernel MCE decoding enabled.
EDAC MC: Ver: 3.0.0

There is no `[Hardware Error]`, `MC5_STATUS`, `Execution Unit Ext. Error`, or `System Fatal error`.

The vanilla kernel reaches systemd userspace.
```

### Additional system information

### CPU

```text
AMD Ryzen 7 5700X 8-Core Processor
Family: 0x19
Model: 0x21
Stepping: 0x2
```

### Microcode

```text
Current revision: 0x0a201213
amd-ucode: 1:20260810-2
```

### Firmware

```text
linux-firmware: 1:20260810-2
```

Neither `amd-ucode` nor `linux-firmware` was updated as part of the update that changed `linux-cachyos 7.2.3-1` to `7.2.4-3`.

The same microcode revision is therefore being used by both known-good and affected kernels.

### amd_pstate

The kernel reports:

```text
amd_pstate: the _CPC object is not present in SBIOS or ACPI disabled
```

This is also present when running known-good kernels and predates the regression.

### BIOS / tuning testing

CPU/PBO and RAM performance tuning were disabled during troubleshooting.

The affected CachyOS kernels continued to produce exactly the same MCE, so the issue remains reproducible with those settings returned to stock/automatic values.

### Relevant package history

```text
2026-09-08 linux-cachyos 7.2.2-1 -> 7.2.3-1
2026-09-12 linux-cachyos 7.2.3-1 -> 7.2.4-3
2026-09-12 linux-cachyos-lts 6.18.48-1 -> 6.18.50-3
```

### Vanilla Arch kernel comparison

Arch `linux 7.2.4.arch1-2` was also tested on the same machine.

It does **not** generate the MC5 hardware error. Its MCE/EDAC output contains only:

```text
MCE: In-kernel MCE decoding enabled.
EDAC MC: Ver: 3.0.0
```

There is no `[Hardware Error]`, `MC5_STATUS`, `Execution Unit Ext. Error`, or `System Fatal error`.

The vanilla Arch kernel successfully reaches systemd userspace.

It currently enters emergency mode later because `/boot` fails to mount and several modules expected by the CachyOS installation are unavailable. This appears unrelated to the MCE and occurs after successful kernel initialization.

### Potentially related issue

CachyOS issue #681 contains the same general MCE signature:

```text
Bank 5: bea0000000000108
SYND: 4d000000
IPID: 500b000000000
```

However, that issue has a different trigger (`modprobe -r amdgpu`) and reports the behaviour across multiple distributions.

In this case, the error occurs during boot with the affected CachyOS kernels and does **not** occur with vanilla Arch `linux 7.2.4.arch1-2`.

For that reason, I am reporting this separately rather than assuming it is a duplicate.

### Additional testing available

I can test additional kernel versions, CachyOS kernel variants, configuration changes, or specific patched builds if required.

Full inxi -Fxxxz output

```text
System:
Kernel: 7.2.2-1-cachyos arch: x86_64 bits: 64 compiler: clang v: 22.1.8
clocksource: tsc
Desktop: KDE Plasma v: 6.7.4 tk: Qt v: N/A wm: kwin_wayland vt: 1 dm: N/A
Distro: CachyOS base: Arch Linux
Machine:
Type: Desktop Mobo: Gigabyte model: B550 AORUS ELITE V2 v: x.x
serial: uuid: Firmware: UEFI
vendor: American Megatrends LLC. v: FId date: 08/18/2026
CPU:
Info: 8-core model: AMD Ryzen 7 5700X bits: 64 type: MT MCP smt: enabled
arch: Zen 3+ rev: 2 cache: L1: 512 KiB L2: 4 MiB L3: 32 MiB
Speed (MHz): avg: 3525 high: 3629 min/max: N/A cores: 1: 3591 2: 3625
3: 3599 4: 3597 5: 3591 6: 3629 7: 3620 8: 3394 9: 3394 10: 3587 11: 3394
12: 3394 13: 3592 14: 3394 15: 3394 16: 3620 bogomips: 108595
Flags-basic: avx avx2 ht lm nx pae sse sse2 sse3 sse4_1 sse4_2 sse4a
ssse3 svm
Graphics:
Device-1: Advanced Micro Devices [AMD/ATI] Navi 48 [Radeon RX 9070/9070
XT/9070 GRE] vendor: Sapphire driver: amdgpu v: kernel arch: RDNA-4 pcie:
speed: 32 GT/s lanes: 16 ports: active: DP-1,DP-2,HDMI-A-1
empty: HDMI-A-2,Writeback-1 bus-ID: 07:00.0 chip-ID: 1002:7550
class-ID: 0300
Display: wayland server: Xwayland v: 24.1.13 compositor: kwin_wayland
driver: gpu: amdgpu d-rect: 11520x2160 display-ID: 0
Monitor-1: DP-1 pos: center model: Gigabyte AORUS FO48U serial:
res: mode: 3840x2160 hz: 120 scale: 130% (1.3) to: 2954x1662 dpi: 140
size: 697x392mm (27.44x15.43") diag: 800mm (31.5") modes: max: 3840x2160
min: 640x480
Monitor-2: DP-2 pos: right model: Dell P2725QE serial: res:
mode: 3840x2160 hz: 100 scale: 170% (1.7) to: 2259x1271 dpi: 163
size: 597x336mm (23.5x13.23") diag: 685mm (27") modes: max: 3840x2160
min: 720x400
Monitor-3: HDMI-A-1 pos: primary,left model: 32MCV4KVAHDR serial:
res: mode: 3840x2160 hz: 60 scale: 170% (1.7) to: 2259x1271 dpi: 164
size: 596x335mm (23.46x13.19") diag: 684mm (26.9") modes: max: 3840x2160
min: 640x480
API: EGL v: 1.5 hw: drv: amd radeonsi platforms: device: 0 drv: radeonsi
device: 1 drv: swrast gbm: drv: radeonsi surfaceless: drv: radeonsi wayland:
drv: radeonsi x11: drv: radeonsi
API: OpenGL v: 4.6 vendor: amd mesa v: 26.2.2-arch3.2 glx-v: 1.4
direct-render: yes renderer: AMD Radeon RX 9070 XT (radeonsi gfx1201 ACO
DRM 3.64 7.2.2-1-cachyos) device-ID: 1002:7550 display-ID: :0.0
API: Vulkan v: 1.4.357 layers: 9 surfaces: N/A device: 0
type: discrete-gpu hw: amd driver: mesa radv device-ID: 1002:7550
Info: Tools: api: clinfo, eglinfo, glxinfo, vulkaninfo
de: kscreen-console,kscreen-doctor wl: wayland-info
x11: xdpyinfo, xprop, xrandr
Audio:
Device-1: Advanced Micro Devices [AMD/ATI] Navi 48 HDMI/DP Audio
driver: snd_hda_intel v: kernel pcie: speed: 32 GT/s lanes: 16
bus-ID: 07:00.1 chip-ID: 1002:ab40 class-ID: 0403
Device-2: Advanced Micro Devices [AMD] Starship/Matisse HD Audio
vendor: Gigabyte driver: snd_hda_intel v: kernel pcie: speed: 16 GT/s
lanes: 16 bus-ID: 09:00.4 chip-ID: 1022:1487 class-ID: 0403
Device-3: SteelSeries ApS Arctis Nova Pro Wireless
driver: hid-generic,snd-usb-audio,usbhid type: USB rev: 2.0 speed: 12 Mb/s
lanes: 1 bus-ID: 3-3:3 chip-ID: 1038:12e5 class-ID: 0300
API: ALSA v: k7.2.2-1-cachyos status: kernel-api
Server-1: sndiod v: N/A status: off
Server-2: JACK v: 1.9.22 status: off
Server-3: PipeWire v: 1.6.8 status: active with: 1: pipewire-pulse
status: active 2: wireplumber status: active 3: pipewire-alsa type: plugin
Network:
Device-1: Realtek RTL8125 2.5GbE vendor: Gigabyte driver: r8169 v: kernel
pcie: speed: 5 GT/s lanes: 1 port: f000 bus-ID: 04:00.0 chip-ID: 10ec:8125
class-ID: 0200
IF: eno1 state: up speed: 2500 Mbps duplex: full mac:
IF-ID-1: docker0 state: down mac:
IF-ID-2: tailscale0 state: unknown speed: -1 duplex: full mac: N/A
Bluetooth:
Device-1: UGREEN BT6.0 Adapter driver: btusb v: 0.8 type: USB rev: 2.0
speed: 12 Mb/s lanes: 1 bus-ID: 3-1:2 chip-ID: 33fa:0012 class-ID: e001
Report: btmgmt ID: hci0 rfk-id: 0 state: up address: lmp-v: 14
class-ID: 7c0104
Drives:
Local Storage: total: 1.82 TiB used: 395.38 GiB (21.2%)
ID-1: /dev/nvme0n1 vendor: Crucial model: CT2000P3SSD8 size: 1.82 TiB
speed: 31.6 Gb/s lanes: 4 tech: SSD serial: fw-rev: P9CR30A
temp: 38.9 C scheme: GPT
Partition:
ID-1: /boot size: 3.99 GiB used: 902.1 MiB (22.1%) fs: vfat
dev: /dev/nvme0n1p1
ID-2: /home size: 1.82 TiB used: 394.5 GiB (21.2%) fs: btrfs
dev: /dev/nvme0n1p2
ID-3: /var/log size: 1.82 TiB used: 394.5 GiB (21.2%) fs: btrfs
dev: /dev/nvme0n1p2
ID-4: /var/tmp size: 1.82 TiB used: 394.5 GiB (21.2%) fs: btrfs
dev: /dev/nvme0n1p2
Swap:
ID-1: swap-1 type: zram size: 31.26 GiB used: 1.2 MiB (0.0%) priority: 100
dev: /dev/zram0
Sensors:
System Temperatures: cpu: 38.0 C mobo: 30.0 C gpu: amdgpu temp: 56.0 C
mem: 55.0 C
Fan Speeds (rpm): N/A gpu: amdgpu fan: 124
Info:
Memory: total: 32 GiB available: 31.26 GiB used: 4.86 GiB (15.5%)
Processes: 374 Power: uptime: 14m states: freeze,mem,disk suspend: deep
wakeups: 0 hibernate: platform Init: systemd v: 261 default: graphical
Packages: 1530 pm: pacman pkgs: 1522 pm: flatpak pkgs: 8 Compilers:
clang: 22.1.8 gcc: 16.2.1 Shell: fish v: 4.9.1 running-in: alacritty
inxi: 3.3.41
```