In this lesson we would be messing with the NVRAM variables (BIOS settings), i.e. variables that are persistent between boots.
As you remember after OVMF build we have these files:
$ ls -l Build/OvmfX64/RELEASE_GCC5/FV/OVMF*
-rw-r--r-- 1 kostr kostr 4194304 Jun 25 14:40 Build/OvmfX64/RELEASE_GCC5/FV/OVMF.fd
-rw-r--r-- 1 kostr kostr 3653632 Jun 25 14:40 Build/OvmfX64/RELEASE_GCC5/FV/OVMF_CODE.fd
-rw-r--r-- 1 kostr kostr 540672 Jun 25 14:38 Build/OvmfX64/RELEASE_GCC5/FV/OVMF_VARS.fd
OVMF_CODE.fd- code image (read-only)OVMF_VARS.fd- NVRAM variables image (read-write)OVMF.fd- combined image (OVMF_CODE.fd+OVMF_VARS.fd)
We can boot QEMU either with:
$ qemu-system-x86_64 -drive if=pflash,format=raw,file=Build/OvmfX64/RELEASE_GCC5/FV/OVMF.fd \
...
or with
$ qemu-system-x86_64 -drive if=pflash,format=raw,readonly,file=Build/OvmfX64/RELEASE_GCC5/FV/OVMF_CODE.fd \
-drive if=pflash,format=raw,file=Build/OvmfX64/RELEASE_GCC5/FV/OVMF_VARS.fd \
...
It was fine to use short form earlier, but in this lesson we would modify NVRAM variables, so it is best to use full form with a separate copy of OVMF_VARS.fd, so you could always revert things to their initial state.
Let's undo our modifications to the OvmfPkg package and rebuild it:
$ git restore OvmfPkg
$ build --platform=OvmfPkg/OvmfPkgX64.dsc --arch=X64 --buildtarget=RELEASE --tagname=GCC5
After that create a backup copy of OVMF_VARS.fd and run QEMU with it:
$ cp Build/OvmfX64/RELEASE_GCC5/FV/OVMF_VARS.fd ../
$ qemu-system-x86_64 -drive if=pflash,format=raw,readonly,file=Build/OvmfX64/RELEASE_GCC5/FV/OVMF_CODE.fd \
-drive if=pflash,format=raw,file=../OVMF_VARS.fd \
-drive format=raw,file=fat:rw:~/UEFI_disk \
-nographic \
-net none
Check boot variables with the help of bcfg boot dump:
Shell> bcfg boot dump
Option: 00. Variable: Boot0000
Desc - UiApp
DevPath - Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(462CAA21-7614-4503-836E-8AB6F4662331)
OVMF_VARS.fdiOptional- N
Option: 01. Variable: Boot0001
Desc - UEFI QEMU DVD-ROM QM00003
DevPath - PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
Optional- Y
Option: 02. Variable: Boot0002
Desc - UEFI QEMU HARDDISK QM00001
DevPath - PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
Optional- Y
Option: 03. Variable: Boot0003
Desc - EFI Internal Shell
DevPath - Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(7C04A583-9E3E-4F1C-AD65-E05268D0B4D1)
Optional- N
Besides showing boot options bcfg command can add/remove boot options or change their order.
You can check out help for bcfg via:
bcfg -? -b
Now let's try to add our InteractiveApp.efi to the boot options.
Shell> fs0:
FS0:\> bcfg boot add 4 InteractiveApp.efi "Interactive app"
Target = 0004.
bcfg: Add Boot0004 as 4
FS0:\> bcfg boot dump
Option: 00. Variable: Boot0000
Desc - UiApp
DevPath - Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(462CAA21-7614-4503-836E-8AB6F4662331)
Optional- N
Option: 01. Variable: Boot0001
Desc - UEFI QEMU DVD-ROM QM00003
DevPath - PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
Optional- Y
Option: 02. Variable: Boot0002
Desc - UEFI QEMU HARDDISK QM00001
DevPath - PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
Optional- Y
Option: 03. Variable: Boot0003
Desc - EFI Internal Shell
DevPath - Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(7C04A583-9E3E-4F1C-AD65-E05268D0B4D1)
Optional- N
Option: 04. Variable: Boot0004
Desc - Interactive app
DevPath - PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)/HD(1,MBR,0xBE1AFDFA,0x3F,0xFBFC1)/\InteractiveApp.efi
Optional- N
Now if you navigate to the Boot Manager you'll see our app:
As soon as you don't change OVMF_VARS.fd this option would be present even between QEMU restarts.
We can go further and even place our app as a first boot source:
Shell> bcfg boot mv 4 0
Shell> bcfg boot dump
Option: 00. Variable: Boot0004
Desc - Interactive app
DevPath - PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)/HD(1,MBR,0xBE1AFDFA,0x3F,0xFBFC1)/\InteractiveApp.efi
Optional- N
Option: 01. Variable: Boot0000
Desc - UiApp
DevPath - Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(462CAA21-7614-4503-836E-8AB6F4662331)
Optional- N
Option: 02. Variable: Boot0001
Desc - UEFI QEMU DVD-ROM QM00003
DevPath - PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
Optional- Y
Option: 03. Variable: Boot0002
Desc - UEFI QEMU HARDDISK QM00001
DevPath - PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
Optional- Y
Option: 04. Variable: Boot0003
Desc - EFI Internal Shell
DevPath - Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(7C04A583-9E3E-4F1C-AD65-E05268D0B4D1)
Optional- N
Now if you reboot UEFI shell with a reset command, or rerun QEMU, you will need to pass our app before you can go to the BIOS menu:
BdsDxe: loading Boot0004 "Interactive app" from PciRoot(0x0)/Pci(0x1,0x1)/Ata(Primary,Master,0x0)/HD(1,MBR,0xBE1AFDFA,0x3F,0xFBFC1)/\InteractiveApp.efi
BdsDxe: starting Boot0004 "Interactive app" from PciRoot(0x0)/Pci(0x1,0x1)/Ata(Primary,Master,0x0)/HD(1,MBR,0xBE1AFDFA,0x3F,0xFBFC1)/\InteractiveApp.efi
Try to guess the secret symbol!
Input correct symbol, go to the Boot manager menu and run UEFI shell so we could delete our app from the boot sources:
FS0:\> bcfg boot dump
Option: 00. Variable: Boot0004
Desc - Interactive app
DevPath - PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)/HD(1,MBR,0xBE1AFDFA,0x3F,0xFBFC1)/\InteractiveApp.efi
Optional- N
Option: 01. Variable: Boot0000
Desc - UiApp
DevPath - Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(462CAA21-7614-4503-836E-8AB6F4662331)
Optional- N
Option: 02. Variable: Boot0001
Desc - UEFI QEMU DVD-ROM QM00003
DevPath - PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
Optional- Y
Option: 03. Variable: Boot0002
Desc - UEFI QEMU HARDDISK QM00001
DevPath - PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
Optional- Y
Option: 04. Variable: Boot0003
Desc - EFI Internal Shell
DevPath - Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(7C04A583-9E3E-4F1C-AD65-E05268D0B4D1)
Optional- N
FS0:\> bcfg boot rm 0
FS0:\> bcfg boot dump
Option: 00. Variable: Boot0000
Desc - UiApp
DevPath - Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(462CAA21-7614-4503-836E-8AB6F4662331)
Optional- N
Option: 01. Variable: Boot0001
Desc - UEFI QEMU DVD-ROM QM00003
DevPath - PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
Optional- Y
Option: 02. Variable: Boot0002
Desc - UEFI QEMU HARDDISK QM00001
DevPath - PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
Optional- Y
Option: 03. Variable: Boot0003
Desc - EFI Internal Shell
DevPath - Fv(7CB8BDC9-F8EB-4F34-AAEA-3EE4AF6516A1)/FvFile(7C04A583-9E3E-4F1C-AD65-E05268D0B4D1)
Optional- N