1 .. SPDX-License-Identifier: GPL-2.0+
2 .. Copyright (C) 2015 Google, Inc
6 This document provides information about U-Boot running on top of EFI, either
7 as an application or just as a means of getting U-Boot onto a new platform.
12 Running U-Boot on EFI is useful in several situations:
14 - You have EFI running on a board but U-Boot does not natively support it
15 fully yet. You can boot into U-Boot from EFI and use that until U-Boot is
18 - You need to use an EFI implementation (e.g. UEFI) because your vendor
19 requires it in order to provide support
21 - You plan to use coreboot to boot into U-Boot but coreboot support does
22 not currently exist for your platform. In the meantime you can use U-Boot
23 on EFI and then move to U-Boot on coreboot when ready
25 - You use EFI but want to experiment with a simpler alternative like U-Boot
30 Only x86 is supported at present. If you are using EFI on another architecture
31 you may want to reconsider. However, much of the code is generic so could be
34 U-Boot supports running as an EFI application for 32-bit EFI only. This is
35 not very useful since only a serial port is provided. You can look around at
36 memory and type 'help' but that is about it.
38 More usefully, U-Boot supports building itself as a payload for either 32-bit
39 or 64-bit EFI. U-Boot is packaged up and loaded in its entirety by EFI. Once
40 started, U-Boot changes to 32-bit mode (currently) and takes over the
41 machine. You can use devices, boot a kernel, etc.
46 First choose a board that has EFI support and obtain an EFI implementation
47 for that board. It will be either 32-bit or 64-bit. Alternatively, you can
48 opt for using QEMU [1] and the OVMF [2], as detailed below.
50 To build U-Boot as an EFI application (32-bit EFI required), enable CONFIG_EFI
51 and CONFIG_EFI_APP. The efi-x86_app config (efi-x86_app32_defconfig) is set up
52 for this. Just build U-Boot as normal, e.g.::
54 make efi-x86_app32_defconfig
57 To build U-Boot as an EFI payload (32-bit or 64-bit EFI can be used), enable
58 CONFIG_EFI, CONFIG_EFI_STUB, and select either CONFIG_EFI_STUB_32BIT or
59 CONFIG_EFI_STUB_64BIT. The efi-x86_payload configs (efi-x86_payload32_defconfig
60 and efi-x86_payload32_defconfig) are set up for this. Then build U-Boot as
63 make efi-x86_payload32_defconfig (or efi-x86_payload64_defconfig)
66 You will end up with one of these files depending on what you build for:
68 * u-boot-app.efi - U-Boot EFI application
69 * u-boot-payload.efi - U-Boot EFI payload application
74 QEMU is an emulator and it can emulate an x86 machine. Please make sure your
75 QEMU version is 6.0.0 or above to test this. You can run the payload with
79 cp /path/to/u-boot*.efi /tmp/efi
80 qemu-system-x86_64 -pflash edk2-x86_64-code.fd -hda fat:rw:/tmp/efi/
82 Add -nographic if you want to use the terminal for output. Once it starts
83 type 'fs0:u-boot-payload.efi' to run the payload or 'fs0:u-boot-app.efi' to
84 run the application. 'edk2-x86_64-code.fd' is the EFI 'BIOS'. QEMU already
85 ships both 32-bit and 64-bit EFI BIOS images. For 32-bit EFI 'BIOS' image,
86 use 'edk2-i386-code.fd'.
89 To try it on real hardware, put u-boot-app.efi on a suitable boot medium,
90 such as a USB stick. Then you can type something like this to start it::
92 fs0:u-boot-payload.efi
94 (or fs0:u-boot-app.efi for the application)
96 This will start the payload, copy U-Boot into RAM and start U-Boot. Note
97 that EFI does not support booting a 64-bit application from a 32-bit
98 EFI (or vice versa). Also it will often fail to print an error message if
101 You may find the script `scripts/build-efi.sh` helpful for building and testing
102 U-Boot on UEFI on QEMU. It also includes links to UEFI binaries dating from
105 See `Example run`_ for an example run.
109 Here follow a few implementation notes for those who want to fiddle with
110 this and perhaps contribute patches.
112 The application and payload approaches sound similar but are in fact
113 implemented completely differently.
117 For the application the whole of U-Boot is built as a shared library. The
118 efi_main() function is in lib/efi/efi_app.c. It sets up some basic EFI
119 functions with efi_init(), sets up U-Boot global_data, allocates memory for
120 U-Boot's malloc(), etc. and enters the normal init sequence (board_init_f()
123 Since U-Boot limits its memory access to the allocated regions very little
124 special code is needed. The CONFIG_EFI_APP option controls a few things
125 that need to change so 'git grep CONFIG_EFI_APP' may be instructive.
126 The CONFIG_EFI option controls more general EFI adjustments.
128 The only available driver is the serial driver. This calls back into EFI
129 'boot services' to send and receive characters. Although it is implemented
130 as a serial driver the console device is not necessarilly serial. If you
131 boot EFI with video output then the 'serial' device will operate on your
132 target devices's display instead and the device's USB keyboard will also
133 work if connected. If you have both serial and video output, then both
134 consoles will be active. Even though U-Boot does the same thing normally,
135 These are features of EFI, not U-Boot.
137 Very little code is involved in implementing the EFI application feature.
138 U-Boot is highly portable. Most of the difficulty is in modifying the
139 Makefile settings to pass the right build flags. In particular there is very
140 little x86-specific code involved - you can find most of it in
141 arch/x86/cpu. Porting to ARM (which can also use EFI if you are brave
142 enough) should be straightforward.
144 Use the 'reset' command to get back to EFI.
148 The payload approach is a different kettle of fish. It works by building
149 U-Boot exactly as normal for your target board, then adding the entire
150 image (including device tree) into a small EFI stub application responsible
151 for booting it. The stub application is built as a normal EFI application
152 except that it has a lot of data attached to it.
154 The stub application is implemented in lib/efi/efi_stub.c. The efi_main()
155 function is called by EFI. It is responsible for copying U-Boot from its
156 original location into memory, disabling EFI boot services and starting
157 U-Boot. U-Boot then starts as normal, relocates, starts all drivers, etc.
159 The stub application is architecture-dependent. At present it has some
160 x86-specific code and a comment at the top of efi_stub.c describes this.
162 While the stub application does allocate some memory from EFI this is not
163 used by U-Boot (the payload). In fact when U-Boot starts it has all of the
164 memory available to it and can operate as it pleases (but see the next
169 The payload can pass information to U-Boot in the form of EFI tables. At
170 present this feature is used to pass the EFI memory map, an inordinately
171 large list of memory regions. You can use the 'efi mem all' command to
172 display this list. U-Boot uses the list to work out where to relocate
175 Although U-Boot can use any memory it likes, EFI marks some memory as used
176 by 'run-time services', code that hangs around while U-Boot is running and
177 is even present when Linux is running. This is common on x86 and provides
178 a way for Linux to call back into the firmware to control things like CPU
179 fan speed. U-Boot uses only 'conventional' memory, in EFI terminology. It
180 will relocate itself to the top of the largest block of memory it can find
185 U-Boot drivers typically don't use interrupts. Since EFI enables interrupts
186 it is possible that an interrupt will fire that U-Boot cannot handle. This
187 seems to cause problems. For this reason the U-Boot payload runs with
188 interrupts disabled at present.
192 While the EFI application can in principle be built as either 32- or 64-bit,
193 only 32-bit is currently supported. This means that the application can only
194 be used with 32-bit EFI.
196 The payload stub can be build as either 32- or 64-bits. Only a small amount
197 of code is built this way (see the extra- line in lib/efi/Makefile).
198 Everything else is built as a normal U-Boot, so is always 32-bit on x86 at
204 This shows running with serial enabled (see `include/configs/efi-x86_app.h`)::
206 $ scripts/build-efi.sh -wsPr
207 Packaging efi-x86_app32
208 Running qemu-system-i386
210 BdsDxe: failed to load Boot0001 "UEFI QEMU HARDDISK QM00005 " from PciRoot(0x0)/Pci(0x3,0x0)/Sata(0x0,0xFFFF,0x0): Not Found
211 BdsDxe: loading Boot0002 "EFI Internal Shell" from Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(7C04A583-9E3E-4F1C-AD65-E05268D0B4D1)
212 BdsDxe: starting Boot0002 "EFI Internal Shell" from Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(7C04A583-9E3E-4F1C-AD65-E05268D0B4D1)
214 UEFI Interactive Shell v2.2
216 UEFI v2.70 (EDK II, 0x00010000)
218 FS0: Alias(s):HD0a65535a1:;BLK1:
219 PciRoot(0x0)/Pci(0x3,0x0)/Sata(0x0,0xFFFF,0x0)/HD(1,GPT,0FFD5E61-3B0C-4326-8049-BDCDC910AF72,0x800,0xB000)
221 PciRoot(0x0)/Pci(0x3,0x0)/Sata(0x0,0xFFFF,0x0)
223 Press ESC in 5 seconds to skip startup.nsh or any other key to continue.
224 Shell> fs0:u-boot-app.efi
225 U-Boot EFI App (using allocated RAM address 47d4000) key=8d4, image=06a6f610
229 U-Boot 2022.01-rc4 (Sep 19 2021 - 14:03:20 -0600)
231 CPU: x86, vendor Intel, device 663h
233 0: efi_media_0 PciRoot(0x0)/Pci(0x3,0x0)/Sata(0x0,0xFFFF,0x0)
234 1: <partition> PciRoot(0x0)/Pci(0x3,0x0)/Sata(0x0,0xFFFF,0x0)/HD(1,GPT,0FFD5E61-3B0C-4326-8049-BDCDC910AF72,0x800,0xB000)
235 Loading Environment from nowhere... OK
236 Model: EFI x86 Application
237 Hit any key to stop autoboot: 0
239 Partition Map for EFI device 0 -- Partition Type: EFI
241 Part Start LBA End LBA Name
245 1 0x00000800 0x0000b7ff "boot"
246 attrs: 0x0000000000000000
247 type: ebd0a0a2-b9e5-4433-87c0-68b6b72699c7
248 guid: 0ffd5e61-3b0c-4326-8049-bdcdc910af72
250 528384 u-boot-app.efi
260 This work could be extended in a number of ways:
264 - Add 64-bit application support (in progress)
266 - Figure out how to solve the interrupt problem
268 - Add more drivers to the application side (e.g.USB, environment access).
270 - Avoid turning off boot services in the stub. Instead allow U-Boot to make
271 use of boot services in case it wants to. It is unclear what it might want
272 though. It is better to use the app.
277 payload stub, application, support code. Mostly arch-neutral
280 x86 support code for running as an EFI application and payload
282 board/efi/efi-x86_app/efi.c
283 x86 board code for running as an EFI application
285 board/efi/efi-x86_payload
286 generic x86 EFI payload board support code
292 Ben Stoltz, Simon Glass
296 * [1] http://www.qemu.org
297 * [2] https://github.com/tianocore/tianocore.github.io/wiki/OVMF