Mixing it up with EFI mixed mode
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EFI mixed mode allows a 64-bit operating system to boot on devices with 32-bit EFI firmware. It works in two phases - boot transition and runtime thunking. During boot transition, the 64-bit kernel image contains both 32-bit and 64-bit entry points to support different boot loaders. Runtime thunking uses a dedicated kernel page table to map UEFI regions with virtual equals physical addresses, truncates pointers to 32-bits, and switches to 32-bit CPU mode to call UEFI services. Supported boot loaders include Grub2, Efilinux, and potentially rEFInd. Developers need to be aware that unsigned long no longer represents the natural pointer size and some UEFI
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Mixing it up with EFI mixed mode
- 1. 1 Mixing it up with EFI mixed mode Matt Fleming, SSG/OTC
- 2. 2 Debian 8 “Jessie” release “Support for mixed-mode EFI systems like the Intel Bay Trail: a 64-bit platform crippled with a 32-bit EFI firmware.”
- 3. 3 What is EFI mixed mode? • Fat 64-bit OS on small 32-bit EFI firmware • Implemented in the x86 kernel • Translates x86-64 ABI to i386 • Utilises full capabilities of CPU • Increases virtual address space
- 4. 4 How does it work? 2 Phases
- 5. 5 How does it work? 2 Phases Boot transition Runtime thunking
- 7. 7 Boot transition – Traditional entry point 64-bit kernel image efi64_stub_entry • Boot loaders always enter via native entry point • Legacy +512 bytes for 64-bit loaders 64-bit boot loader bootx64.efi
- 8. 8 Boot transition – Mixed mode entry points 64-bit kernel image efi64_stub_entry efi32_stub_entry • 2 entry points are available • No changes required to Linux boot loaders • All magic happens in the kernel • 1 kernel image for both scenarios 32-bit/64-bit boot loaders bootia32.efi 512bytes bootx64.efi
- 10. 10 Thunking at runtime • Dedicated kernel page table • UEFI regions are mapped with VA == PA • Physical addresses are passed to UEFI • Luckily never seen address above 4GB 0xffffffffffffffff 0x0 0xffffffff Virtual AddressesPhysical Addresses Kernel text/data
- 11. 11 Thunking at runtime 0x000000fde3c190 1. Convert pointers to physical address 0xffff8800fde3c190 2. Truncate to 32-bits
- 12. 12 Thunking at runtime 0xfde3c190 3. Push arguments onto stack (convert ABI) 4. Switch to 32-bit CPU mode and call UEFI 0x000000340fdece 0xfde45080 0x000000fde3c190 Stack 0x340fdece Registers
- 13. 13 Development gotchas • “unsigned long” no longer represents natural pointer size • Some UEFI runtime services not supported (UpdateCapsule()) • If memory existed above 4GB: kaboom!
- 14. 14 Supported boot loaders • Grub2 • Used by Debian, Fedora, Ubuntu • Efilinux • Kernel support developed using efilinux • rEFInd • Support unconfirmed • Syslinux
- 15. 15 A quick word on gummiboot • Gummiboot is an UEFI application loader not a boot loader • It has no understanding of the different Linux kernel entry points • It simply uses the UEFI boot services to load and run applications • Linux kernel masquerades as PE/COFF application
- 16. 16 Ingredients for rolling your own solution • 32-bit boot loader from the supported list • Drop it in EFI System Partition (ESP) at EFIBOOTBOOTIA32.EFI • Build your x86 kernel with the following options • CONFIG_EFI_STUB • CONFIG_EFI_MIXED • Profit!
- 17. 17 Q & A