tashaboot: arm64 bootloader

A small arm64 bootloader. No board code, no device tree porting, the
architecture manual is the whole story: exception vectors in the
fixed 16 slot layout (Table D1-7), ESR_ELx decoded by exception class
(D1-2172), EL entry and eret chains per the programmers model
(D1-2146), cache maintenance by set/way over the CLIDR_EL1 levels,
semihosting for console and file io per DUI 0203, and the A64 boot
protocol from Documentation/arch/arm64/booting.rst.

The loader boots a stock mainline Image end to end on the qemu virt
machine. Boot receipt with 7.3-rc3 (42MB Image):

  tashaboot 0.1
  loaded 43450368 bytes at 40200000, entry 40200000
  jumping
  [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x411fd070]
  [    0.000000] Linux version 7.3.0-rc3
  [    0.000000] Machine model: linux,dummy-virt
  [    0.000000] earlycon: pl11 MMIO32:0x0000000009000000
  ...
  ---[ end Kernel panic - not syncing: VFS: Unable to mount root fs ]---

The panic is the expected end state, no root filesystem is handed
over yet.

The boot chain, state per stage, start to payload:

+-----------+-----+--------------+----------------------------------+
| stage     | EL  | state        | work                             |
+-----------+-----+--------------+----------------------------------+
| firmware  | any | MMU maybe on | x0 = dtb, jump in               |
+-----------+-----+--------------+----------------------------------+
| tashaboot | 3-2 |              | SCR_EL3.NS = 1, eret to EL2     |
+-----------+-----+--------------+----------------------------------+
|           | 2   | virt scrub   | HCR/CNTHCTL/CPTR/HSTR, CNTFRQ,  |
|           |     |              | VBAR_EL2, MMU off, tlbi alle2   |
+-----------+-----+--------------+----------------------------------+
|           | 2   |              | load Image over semihosting,    |
|           |     |              | validate header, place per      |
|           |     |              | booting.rst                     |
+-----------+-----+--------------+----------------------------------+
|           | 2   | caches clean | flush dcache, inval icache,     |
|           |     |              | args ride x20/x21, regs last    |
+-----------+-----+--------------+----------------------------------+
| payload   | 2   | fresh start  | x0 = dtb, x1-x3 = 0, DAIF       |
|           |     |              | masked, br to image entry       |
+-----------+-----+--------------+----------------------------------+

Two handoff bugs the kernel caught, both AAPCS clobbers in the final
jump. Cache maintenance was called after the register setup, x0-x18
are caller saved, so tb_flush_dcache_all() wiped the dtb pointer and
the kernel spun in setup_machine_fdt() with an invalid device tree
blob. The flush helpers also clobbered x1 (u-boot's void call
convention left mov x1, x0 in cache.S) which handed the kernel a wild
x0. The arguments ride in x20/x21 across the cache calls now, callee
saved, and the register setup is the last thing before the branch.

What is missing on purpose: no SMP bringup (secondary cores park),
no PSCI, no initrd or root filesystem handoff, single serial
console. Those come next.

Signed-off-by: Bradley Morgan <brads@mainlining.org>

1 files changed, 43 insertions, 0 deletions

diff --git a/arch/arm64/kernel/boot.S b/arch/arm64/kernel/boot.S
new file mode 100644
index 0000000..615be06
--- /dev/null
+++ b/arch/arm64/kernel/boot.S
@@ -0,0 +1,43 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * boot.S - the final jump to the payload. x0 = dtb, x1 = x2 = x3 = 0,
+ * MMU and caches off, D cache flushed, I cache invalidated. that is
+ * the whole contract from Documentation/arch/arm64/booting.rst.
+ *
+ * Copyright (C) 2026 Bradley Morgan <brads@mainlining.org>
+ */
+
+#include <asm/linkage.h>
+
+.pushsection .text.tb_boot_linux, "ax"
+ENTRY(tb_boot_linux)
+	/* ep in x0, dtb in x1, per the kernel boot protocol */
+
+	/*
+	 * cache maintenance first, register setup last. x0-x18 are
+	 * caller saved per the AAPCS, the flush helpers are free to
+	 * clobber them, so the args ride in x20/x21 across the calls.
+	 */
+	mov	x20, x0		/* entry point */
+	mov	x21, x1		/* dtb */
+
+	bl	tb_flush_dcache_all
+	bl	tb_invalidate_icache_all
+
+	mov	x8, x20
+	mov	x0, x21
+	mov	x1, xzr
+	mov	x2, xzr
+	mov	x3, xzr
+
+	/* MMU off, caches off, the kernel sets up its own state */
+	mrs	x9, sctlr_el1
+	bic	x9, x9, #(1 << 0)	/* M, MMU */
+	bic	x9, x9, #(1 << 2)	/* C, D-cache */
+	bic	x9, x9, #(1 << 12)	/* I, I-cache */
+	msr	sctlr_el1, x9
+	isb
+
+	br	x8
+ENDPROC(tb_boot_linux)
+.popsection