Mirrors/opensbi
3
0
Fork 1
mirror of https://github.com/riscv-software-src/opensbi.git synced 2026-09-20 15:21:42 +01:00
Code Issues Packages Projects Releases Wiki Activity

firmware: fw_base.S: override a0, a1 and a2 from the firmware header

Browse Source 
Wire up the flags word and the three override values in the OpenSBI
firmware header. When the previous booting stage sets one of the
FW_HEADER_FLAGS_OVERRIDE_A[012] bits, the cold boot path replaces the
matching register with the value stored in the header before doing
anything else with the boot arguments.

This lets a previous booting stage which cannot pass the boot arguments
in registers hand them over by patching a few words in the firmware
image instead. For example, a booting stage running on a dedicated boot
processor can load the OpenSBI image, patch the header with the hart id,
the DTB address and, for FW_DYNAMIC, the address of a struct
fw_dynamic_info it built somewhere in DRAM, and then release the hart
that runs OpenSBI, without ever being able to set up that hart's
registers itself.

The flags word is zero in a freshly built image, so nothing is
overridden and every existing booting stage keeps passing a0, a1 and a2
in registers as before.

Suggested-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260918081313.659655-3-zong.li@sifive.com
Signed-off-by: Anup Patel <anup@brainfault.org>
This commit is contained in:
Zong Li
2026-09-18 16:52:39 +05:30
committed by Anup Patel
parent 1854901adc
commit f83d6a730b
1 changed files with 31 additions and 0 deletions
Download Patch File Download Diff File Expand all files Collapse all files
firmware/fw_base.S
+31
View File
@@ -109,6 +109,37 @@ _fw_header_override_a2:
/* Reserved, pads the header up to FW_HEADER_SIZE bytes */ /* Reserved, pads the header up to FW_HEADER_SIZE bytes */
.fill (FW_HEADER_SIZE - FW_HEADER_RESERVED_OFFSET), 1, 0 .fill (FW_HEADER_SIZE - FW_HEADER_RESERVED_OFFSET), 1, 0
_start_real: _start_real:
/*
* Override a0, a1 and a2 with the values from the firmware header.
*
* This is for a previous booting stage which cannot pass the boot
* arguments in registers, but can patch the firmware image before
* jumping to it. Nothing is overridden when the previous booting
* stage left the flags word alone, so the registers passed by all
* existing booting stages are used as-is.
*
* This runs before relocation, so only PC-relative addressing is
* used. t0, t1 and t2 are free to use here: the boot arguments live
* in a0-a4 and every other register is reset further down the cold
* boot path.
*/
lla t0, _fw_header_flags
lw t0, (t0)
andi t1, t0, FW_HEADER_FLAGS_OVERRIDE_A0
beqz t1, _skip_override_a0
csrr a0, CSR_MHARTID
_skip_override_a0:
andi t1, t0, FW_HEADER_FLAGS_OVERRIDE_A1
beqz t1, _skip_override_a1
lla t2, _fw_header_override_a1
REG_L a1, (t2)
_skip_override_a1:
andi t1, t0, FW_HEADER_FLAGS_OVERRIDE_A2
beqz t1, _skip_override_a2
lla t2, _fw_header_override_a2
REG_L a2, (t2)
_skip_override_a2:
/* Find preferred boot HART id */ /* Find preferred boot HART id */
MOV_3R s0, a0, s1, a1, s2, a2 MOV_3R s0, a0, s1, a1, s2, a2
call fw_boot_hart call fw_boot_hart