tashaboot: arm64 bringup, semihosting payload loader

consoles on the pl011 the dtb points at, walks the memory node and
chosen, loads an arm64 kernel Image over semihosting and jumps to it
with the documented register state. string functions, printf and
libfdt are vendored from lk (lk2nd) and u-boot so the libc surface is
the one those projects already proved. built and booted on qemu virt,
payload exits clean through semihosting.

1 files changed, 94 insertions, 0 deletions

diff --git a/arch/arm64/kernel/start.S b/arch/arm64/kernel/start.S
new file mode 100644
index 0000000..776744c
--- /dev/null
+++ b/arch/arm64/kernel/start.S
@@ -0,0 +1,94 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * tashaboot arm64 entry. firmware drops us here, usually at EL2 on
+ * qemu virt. we park the secondaries, drop to EL1, set the stack and
+ * hand the dtb pointer to C.
+ *
+ * Copyright (C) 2026 Bradley Morgan <brads@mainlining.org>
+ */
+
+#include <asm/macro.h>
+
+.section .text.boot
+.globl _start
+_start:
+	b	reset
+
+	.balign 8
+.globl _text_base
+_text_base:
+	.quad	0x40000000
+
+reset:
+	/* keep the dtb pointer before anything clobbers x0 */
+	mov	x19, x0
+
+	/* park secondary cores, they have nothing to do yet */
+	mrs	x0, mpidr_el1
+	and	x0, x0, #0xff
+	cbnz	x0, park
+
+	/* figure out which EL we got */
+	mrs	x0, CurrentEL
+	lsr	x0, x0, #2
+	cmp	x0, #3
+	b.eq	from_el3
+	cmp	x0, #2
+	b.eq	from_el2
+	cmp	x0, #1
+	b.eq	el1_entry
+	b	park
+
+from_el3:
+	/* EL3 is a dead end for us, drop through the EL2 path if we can */
+	b	park
+
+from_el2:
+	/* EL1 must be aarch64 */
+	mov	x0, #(1 << 31)
+	msr	hcr_el2, x0
+
+	/* scrub the EL2 state so EL1 starts clean */
+	msr	cptr_el2, xzr
+	msr	hstr_el2, xzr
+	mrs	x0, cnthctl_el2
+	orr	x0, x0, #(3 << 0)
+	msr	cnthctl_el2, x0
+	msr	vpidr_el2, xzr
+
+	mov	x0, #0x3c5
+	msr	scr_el3, x0
+
+	/* no MMU, no caches at EL1 yet */
+	msr	sctlr_el1, xzr
+	isb
+
+	/* SPSR for EL1h with interrupts masked */
+	mov	x0, #0x3c5
+	msr	spsr_el2, x0
+	adr	x0, el1_entry
+	msr	elr_el2, x0
+	eret
+
+el1_entry:
+	/* stack for the bootloader, grows down from the image end */
+	ldr	x0, =__image_end
+	mov	sp, x0
+
+	/* clear bss */
+	ldr	x0, =__bss_start
+	ldr	x1, =__bss_end
+1:	cmp	x0, x1
+	b.hs	2f
+	str	xzr, [x0], #8
+	b	1b
+2:
+
+	/* dtb pointer into C arg 0 */
+	mov	x0, x19
+	bl	tashaboot_main
+
+	/* if main returns there is nothing sensible to do */
+park:
+	wfe
+	b	park