Re: [PATCH v4 4/4] selftests/kho: add LoongArch vmtest support

[George Guo <dongtai.guo@linux.dev>]

Hi Huacai,

On Sun, 9 Aug 2026 at 12:16, Huacai Chen <chenhuacai@kernel.org> wrote:
> You never answer me why in your test you should disable i8042 from V1
> until V4. In normal use we use a single kernel for both host and guest.

I owe you an apology: I did not actually understand your question. I
thought the commit message had already explained the reason (the
i8042_flush() panic), so I treated it as answered and never addressed
what you were really asking. Let me answer it directly now.

You are right that this is not just a selftest detail. i8042 is built on
every LoongArch kernel: arch/loongarch/Kconfig selects
ARCH_MIGHT_HAVE_PC_SERIO, and KEYBOARD_ATKBD and MOUSE_PS2 (both default
y) select SERIO_I8042 under it. So your "single kernel for host and
guest" point is correct -- a normal defconfig kernel has i8042 built in.

On the QEMU virtual machine there is no i8042, so PNP finds nothing and
i8042_pnp_init() falls back to probing the i8042 status and data
registers (I8042_STATUS_REG, I8042_DATA_REG) directly. On LoongArch those
registers are memory-mapped, and on the virtual machine they are not backed
by any device, so i8042_flush() takes a page fault and the kernel panics
before userspace. A normal LoongArch defconfig kernel booted as a QEMU
guest hits the same panic -- the "# CONFIG_SERIO_I8042 is not set" in
loongarch.conf is only a workaround to let the KHO selftest boot, not a
fix.

The real fix is separate from the KHO series: make i8042 bail out on
LoongArch when ACPI/PNP does not describe a controller, so the direct-port
fallback never runs. I have prepared that as a standalone patch -- the
relevant hunk is below -- and will send the full patch separately. The
selftest workaround can be dropped once it lands.

diff --git a/drivers/input/serio/i8042-acpipnpio.h b/drivers/input/serio/i8042-acpipnpio.h
--- a/drivers/input/serio/i8042-acpipnpio.h
+++ b/drivers/input/serio/i8042-acpipnpio.h
@@ -1633,8 +1633,14 @@ static int __init i8042_pnp_init(void)
 #else
		pr_info("PNP: No PS/2 controller found.\n");
 #if defined(__loongarch__)
-		if (acpi_disabled == 0)
-			return -ENODEV;
+		/*
+		 * LoongArch legacy I/O ports are memory-mapped and are not
+		 * safely probeable when no controller is present: a direct
+		 * probe faults on the QEMU virtual machine and on any FDT-based
+		 * boot.  Do not fall back to probing them directly; if ACPI/PNP
+		 * did not describe a controller, assume there is none.
+		 */
+		return -ENODEV;
 #else
                if (x86_platform.legacy.i8042 !=
                                X86_LEGACY_I8042_EXPECTED_PRESENT)

Thanks,
George