stand/man/loader.efi.8

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.Dd September 4, 2022
.Dt LOADER.EFI 8
.Os
.Sh NAME
.Nm loader.efi
.Nd UEFI kernel loader
.Sh DESCRIPTION
On UEFI systems,
.Nm
loads the kernel.
.Pp
.Xr boot1.efi 8
is used to load
.Nm
when it is placed within a UFS or ZFS file system.
Alternatively,
.Nm
is used directly when configured with
.Xr efibootmgr 8 ,
or when placed directly as the default boot program as described in
.Xr uefi 8 .
When a system is built using
.Xr bsdinstall 8 ,
.Nm
will be used directly.
.Ss Console Considerations
The EFI BIOS provides a generic console.
In
.Nm
this is selected by specifying
.Dq efi
using the
.Dv console
variable.
.Nm
examines the
.Dv 8be4df61-93ca-11d2-aa0d-00e098032b8c-ConOut
UEFI environment variable to guess what the
.Dq efi
console points to.
.Nm
will output its prompts and menus to all the places specified by ConOut.
However, the
.Fx
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
.Xr rc 8
system, and prompts for things like
.Xr geli 8
passwords.
If
.Nm
finds a video device first, then
.Nm
tells the kernel to use the video console as primary.
Likewise, if a serial device is first in the
.Dv ConOut
list, the serial port will be the primary console.
.Pp
If there is no
.Dv ConOut
variable, both serial and video are attempted.
.Nm
uses the
.Dq efi
console for the video (which may or may not work) and
.Dq comconsole
for the serial on
.Dv 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.
.Pp
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
.Dv console
to
.Dq comconsole .
The default port is
.Dv COM1
with an I/O address of 0x3f8.
.Dv comconsole_port
is used to set this to a different port address.
.Dv comconsole_speed
is used to set the of the serial port (the default is 9600).
If you have
.Dv console
set to
.Dq efi,comconsole
you will get output on both the EFI console and the serial port.
If this causes a doubling of characters, set
.Dv console
to
.Dq efi ,
since your EFI BIOS is redirecting to the serial port already.
.Pp
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
.Nm
does not have an ACPI parser, so it cannot convert that to an I/O port.
The
.Fx
kernel initializes its consoles before it can decode ACPI resources.
The
.Fx
kernel will look at the
.Dv hw.uart.console
variable to set its serial console.
Its format should be described in
.Xr uart 4
but is not.
Set it to
.Dq io:0x3f8,br:115200
with the proper port address.
PCI or memory mapped ports are beyond the scope of this man page.
.Pp
The serial ports are assigned as follows on IBM PC compatible systems:
.Bl -column -offset indent ".Sy Windows Name" ".Sy I/O Port Address" ".Sy Typical FreeBSD device"
.It Sy Windows Name Ta Sy I/O Port Address Ta Sy Typical FreeBSD device
.It COM1 Ta 0x3f8 Ta Pa /dev/uart0
.It COM2 Ta 0x2f8 Ta Pa /dev/uart1
.It COM3 Ta 0x3e8 Ta Pa /dev/uart2
.It COM4 Ta 0x2e8 Ta Pa /dev/uart3
.El
Though
.Dv COM3
and
.Dv COM4
can vary.
.Pp
.Ss 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
.Dq yes
or
.Dq 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.
.Bl -column -offset indent ".Sy boot flag" ".Sy loader variable" ".Sy Kernel RB_ flag"
.It Sy boot flag Ta Sy loader variable Ta Sy Kernel RB_ flag
.It Fl a Ta Dv boot_askme Ta Va RB_ASKNAME
.It Fl c Ta Dv boot_cdrom Ta Va RB_CDROM
.It Fl d Ta Dv boot_ddb Ta Va RB_KDB
.It Fl r Ta Dv boot_dfltroot Ta Va RB_DFLTROOT
.It Fl D Ta Dv boot_multiple Ta Va RB_MULTIPLE
.It Fl m Ta Dv boot_mute Ta Va RB_MUTE
.It Fl g Ta Dv boot_gdb Ta Va RB_GDB
.It Fl h Ta Dv boot_serial Ta Va RB_SERIAL
.It Fl p Ta Dv boot_pause Ta Va RB_PAUSE
.It Fl P Ta Dv boot_probe Ta Va RB_PROBE
.It Fl s Ta Dv boot_single Ta Va RB_SINGLE
.It Fl v Ta Dv boot_verbose Ta Va RB_VERBOSE
.El
And the following flags determine the primary console:
.Bl -column -offset indent ".Sy Flags" ".Sy Kernel Flags" ".Sy Kernel Consoles" ".Sy Primary Console"
.It Sy Flags Ta Sy Kernel Flags Ta Sy Kernel Consoles Ta Sy Primary Console
.It none Ta 0 Ta Video Ta Video
.It Fl h Ta RB_SERIAL Ta Serial Ta Serial
.It Fl D Ta RB_MULTIPLE Ta Serial, Video Ta Video
.It Fl Dh Ta RB_SERIAL | RB_MULTIPLE Ta Serial, Video Ta Serial
.El
.Pp
.Nm
does not implement the probe
.Fl P
functionality where we use the video console if a keyboard is connected and a
serial console otherwise.
.Ss Additional Environment Variables
.Nm
loads some extra variables early in startup from
.Pa /efi/freebsd/loader.env
from the EFI partition. Only simple variables can be set here. It can be useful to specify the root filesystem:
.Bd -literal -offset indent
rootdev=disk0s1a
.Ed
.Ss 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,
.Nm
ensures there's extra memory available, called
.Dq slop ,
after everything it loads
.Po
the kernel, modules and metadata
.Pc
for the kernel to bootstrap the memory allocator.
.Pp
By default, amd64 reserves 8MB.
The
.Ic staging_slop
command allows for tuning the slop size.
It takes a single argument, the size of the slop in bytes.
.Ss amd64 Nocopy
.Nm
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
.Fx 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
.Ic copy_staging
command is used to enable this copying for older kernels.
It takes a single argument
which can be one of
.Bl -tag -width disable
.It Ar disable
Force-disable copying staging area to
.Ad 2M .
.It Ar enable
Force-enable copying staging area to
.Ad 2M .
.It Ar 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
.Va kernphys .
.El
.Pp
Arm64 loaders have operated in the
.Sq nocopy
mode from their inception, so there is no
.Ic copy_staging
command on that platform.
Riscv, 32-bit arm and arm64 have always loaded at any
.Ad 2MB
aligned location, so do not provide
.Ic copy_staging .
.Pp
.Bd -ragged -offset indent
.Sy Note.
BIOS loaders on i386 and amd64 put the staging area starting
at the physical address
.Ad 2M ,
then enable paging with identical mapping for the low
.Ad 1G .
The initial port of
.Nm
followed the same scheme for handing control to the kernel,
since it avoided modifications for the loader/kernel hand-off protocol,
and for the kernel page table bootstrap.
.Pp
This approach is incompatible with the UEFI specification,
and as a practical matter, caused troubles on many boards,
because UEFI firmware is free to use any memory for its own needs.
Applications like
.Nm
must only use memory explicitly allocated using boot interfaces.
The original way also potentially destroyed UEFI runtime interfaces data.
.Pp
Eventually,
.Nm
and the kernel were improved to avoid this problem.
.Ed
.Ss amd64 Faults
Because it executes in x86 protected mode, the amd64 version of
.Nm
is susceptible to CPU faults due to programmer mistakes and
memory corruption.
To make debugging such faults easier, amd64
.Nm
can provide detailed reporting of the CPU state at the time
of the fault.
.Pp
The
.Ic grab_faults
command installs a handler for faults directly in the IDT,
avoiding the use of the UEFI debugging interface
.Fn EFI_DEBUG_SUPPORT_PROTOCOL.RegisterExceptionCallback .
That interface is left available for advanced debuggers in
the UEFI environment.
The
.Ic ungrab_faults
command tries to deinstall the fault handler, returning TSS and IDT
CPU tables to their pre-installation state.
The
.Ic fault
command produces a fault in the
.Nm
environment for testing purposes, by executing the
.Ic ud2
processor instruction.
.Sh FILES
.Bl -tag -width "/boot/loader.efi"
.It Pa /boot/loader.efi
The location of the UEFI kernel loader within the system.
.El
.Ss EFI System Partition
.Nm
is installed on the ESP (EFI System Partition) in one of the following locations:
.Bl -tag -width "efi/freebsd/loader.efi"
.It Pa efi/boot/bootXXX.efi
The default location for any EFI loader
.Po see
.Xr uefi 8
for values to replace
.Ql XXX
with
.Pc .
.It Pa efi/freebsd/loader.efi
The location reserved specifically for the
.Fx
EFI loader.
.El
.Pp
The default location for the ESP mount point is documented in
.Xr hier 7 .
.Sh EXAMPLES
.Ss Updating loader.efi on the ESP
The following examples shows how to install a new
.Nm
on the ESP.
.Pp
First, find the partition of type
.Dq efi :
.Bd -literal -offset indent
# gpart list | grep -Ew '(Name|efi)'
1. Name: nvd0p1
   type: efi
2. Name: nvd0p2
3. Name: nvd0p3
4. Name: nvd0p4
1. Name: nvd0
.Ed
.Pp
The name of the ESP on this system is
.Pa nvd0p1 .
.Pp
Second, let's mount the ESP, copy
.Nm
to the special location reserved for
.Fx
EFI loaders, and unmount once finished:
.Bd -literal -offset indent
# mount_msdosfs /dev/nvd0p1 /boot/efi
# cp /boot/loader.efi /boot/efi/efi/freebsd/loader.efi
# umount /boot/efi
.Ed
.Sh SEE ALSO
.Xr loader 8 ,
.Xr uefi 8
.Sh BUGS
Systems that do not have a
.Dv ConOut
variable set are not conformant with the standard, and likely have unexpected
results.
.Pp
Non-x86 serial console handling is even more confusing and less well documented.
.Pp
Sometimes when the serial port speed isn't set, 9600 is used.
Other times the result is typically 115200 since the speed remains unchanged
from the default.