Console Considerations

The EFI BIOS provides a generic console. In loader.efi this is selected by specifying “efi” using the console variable. loader.efi examines the 8be4df61-93ca-11d2-aa0d-00e098032b8c-ConOut UEFI environment variable to guess what the “efi” console points to. loader.efi will output its prompts and menus to all the places specified by ConOut. However, the FreeBSD kernel has a limitation when more than one console is present. The kernel outputs to all configured consoles. Only the primary console will get the log messages from the rc(8) system, and prompts for things like geli(8) passwords. If loader.efi finds a video device first, then loader.efi tells the kernel to use the video console as primary. Likewise, if a serial device is first in the ConOut list, the serial port will be the primary console.

If there is no ConOut variable, both serial and video are attempted. loader.efi uses the “efi” console for the video (which may or may not work) and “comconsole” for the serial on COM1 at the default baud rate. The kernel will use a dual console, with the video console primary if a UEFI graphics device is detected, or the serial console as primary if not.

On x86 platforms, if you wish to redirect the loader's output to a serial port when the EFI BIOS doesn't support it, or to a serial port that isn't the one the EFI BIOS redirects its output to, set console to “comconsole”. The default port is COM1 with an I/O address of 0x3f8. comconsole_port is used to set this to a different port address. comconsole_speed is used to set the of the serial port (the default is 9600). If you have console set to “efi,comconsole” you will get output on both the EFI console and the serial port. If this causes a doubling of characters, set console to “efi”, since your EFI BIOS is redirecting to the serial port already.

If your EFI BIOS redirects the serial port, you may need to tell the kernel which address to use. EFI uses ACPI's UID to identify the serial port, but loader.efi does not have an ACPI parser, so it cannot convert that to an I/O port. The FreeBSD kernel initializes its consoles before it can decode ACPI resources. The FreeBSD kernel will look at the hw.uart.console variable to set its serial console. Its format should be described in uart(4) but is not. Set it to “io:0x3f8,br:115200” with the proper port address. PCI or memory mapped ports are beyond the scope of this man page.

The serial ports are assigned as follows on IBM PC compatible systems:

Primary Console

The primary console is set using the boot flags. These command line arguments set corresponding flags for the kernel. These flags can be controlled by setting loader environment variables to “yes” or “no”. Boot flags may be set on the command line to the boot command. Inside the kernel, the RB_ flags are used to control behavior, sometimes in architecturally specific ways and are included to aid in discovery of any behavior not covered in this document.

loader.efi does not implement the probe -P functionality where we use the video console if a keyboard is connected and a serial console otherwise.

Staging Slop

The kernel must parse the firmware memory map tables to know what memory it can use. Since it must allocate memory to do this, loader.efi ensures there's extra memory available, called “slop”, after everything it loads (the kernel, modules and metadata) for the kernel to bootstrap the memory allocator.

By default, amd64 reserves 8MB. The staging_slop command allows for tuning the slop size. It takes a single argument, the size of the slop in bytes.

amd64 Nocopy

loader.efi will load the kernel into memory that is 2MB aligned below 4GB. It cannot load to a fixed address because the UEFI firmware may reserve arbitrary memory for its use at runtime. Prior to FreeBSD 13.1, kernels retained the old BIOS-boot protocol of loading at exactly 2MB. Such kernels must be copied from their loaded location to 2MB prior starting them up. The copy_staging command is used to enable this copying for older kernels. It takes a single argument which can be one of

disable

Force-disable copying staging area to 2M.

enable

Force-enable copying staging area to 2M.

auto

Selects the behaviour based on the kernel's capability of boostraping from non-2M physical base. The kernel reports this capability by exporting the symbol kernphys.

Arm64 loaders have operated in the ‘nocopy’ mode from their inception, so there is no copy_staging command on that platform. Riscv, 32-bit arm and arm64 have always loaded at any 2MB aligned location, so do not provide copy_staging.

amd64 Faults

Because it executes in x86 protected mode, the amd64 version of loader.efi is susceptible to CPU faults due to programmer mistakes and memory corruption. To make debugging such faults easier, amd64 loader.efi can provide detailed reporting of the CPU state at the time of the fault.

The grab_faults command installs a handler for faults directly in the IDT, avoiding the use of the UEFI debugging interface EFI_DEBUG_SUPPORT_PROTOCOL.RegisterExceptionCallback(). That interface is left available for advanced debuggers in the UEFI environment. The ungrab_faults command tries to deinstall the fault handler, returning TSS and IDT CPU tables to their pre-installation state. The fault command produces a fault in the loader.efi environment for testing purposes, by executing the ud2 processor instruction.

EFI System Partition

loader.efi is installed on the ESP (EFI System Partition) in one of the following locations:

efi/boot/bootXXX.efi

The default location for any EFI loader (see uefi(8) for values to replace ‘XXX’ with).

efi/freebsd/loader.efi

The location reserved specifically for the FreeBSD EFI loader.

The default location for the ESP mount point is documented in hier(7).

Updating loader.efi on the ESP

The following examples shows how to install a new loader.efi on the ESP.

First, find the partition of type “efi”:

# gpart list | grep -Ew '(Name|efi)'
1. Name: nvd0p1
   type: efi
2. Name: nvd0p2
3. Name: nvd0p3
4. Name: nvd0p4
1. Name: nvd0

The name of the ESP on this system is nvd0p1.

Second, let's mount the ESP, copy loader.efi to the special location reserved for FreeBSD EFI loaders, and unmount once finished:

# mount_msdosfs /dev/nvd0p1 /boot/efi
# cp /boot/loader.efi /boot/efi/efi/freebsd/loader.efi
# umount /boot/efi