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Firmware edits the devicetree it hands the kernel: new
properties cannot go into a packed fdt in place, the blob moves
to scratch first and the blocks grow there. The insert moves the
strings block up by the prop size, opens the prop slot before
/chosen's end token, the new name lands at the strings end, and
all four header fields track the geometry, totalsize, off_dt_
strings, size_dt_strings and size_dt_struct. The last one bounds
the token walk in libfdt and a stale value there reads as
BADSTRUCTURE, the kernel parsed no memory node and panicked on
page table allocation before the first print.
The strings block length comes from the size_dt_strings header
field, not totalsize minus off_dt_strings, qemu's tree carries
a hole after the strings block and the subtraction drags it
along as tree.
The initrd start and end land in /chosen as real properties now,
the initrd= bootargs shortcut is gone.
receipt: smp brought up 1 node 4 cpus, run /init, userspace,
busybox shell, the initrd mounted from the chosen cells.
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The bootloader now does the whole job of machine firmware it owns:
boots 4 cpus, hands over an initrd, answers PSCI, and carries the
delay and cache primitives the arch layer needs.
SMP: the secondary pen is the Wait For Event mechanism from the
manual (B2-144, D1-2255), each secondary watches its spin gate,
WFE, the release writes the entry and SEVs, the recheck after each
wake covers a release that lands between the load and the sleep.
The gates land in the dtb cpu-release-addr slots, rewritten in
place by a small walker, no libfdt, structure per the devicetree
specification, values only, the properties themselves are fixed at
build time like firmware shipping a fixed blob.
PSCI 0.2 at EL2 (DEN 0022): the HVC trap arrives at the current EL
SP_ELx sync slot (EC 0x16 in ESR_EL2, the vector layout Table D1-7),
dispatch on the standard function ids, VERSION, CPU_ON writes the
target gate and SEVs, CPU_OFF clears the gate and returns to the
pen, SYSTEM_OFF and SYSTEM_RESET drive RMR_EL2.RR. On qemu the cores
are held by the machine's own firmware and released through its PSCI
(hvc with -kernel, smc with virtualization=on), the handler here is
the real hardware path where the bootloader is the conduit.
The initrd handoff: loaded at a fixed address clear of the image
and dtb, the dtb /chosen carries linux,initrd-start and -end.
Delays are the generic timer (D10), CNTFRQ_EL0 frequency, CNTVCT_EL0
count, busy wait, no interrupts. Cache maintenance by virtual
address, dc cvac, dc ivac, dc civac, ic ivau with the barrier pairs
the manual requires, the by VA form beats set and way when the
range is known.
Boot receipt, 4 cpus, el2, initrd:
tashaboot 0.1
initrd at 46000000
[ 0.000000] Booting Linux on physical CPU 0x0000000000
[ 0.130621] smp: Brought up 1 node, 4 CPUs
[ 1.830692] Run /init as init process
tashaboot linux userspace reached
cores: 4
BusyBox v1.37.0 built-in shell (ash)
~ #
Signed-off-by: Bradley Morgan <brads@mainlining.org>
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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>
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