2c84466ad8
Before of this patch, we supported two methods to address a boot device:
1. Specifying root=/dev/hdXY, where X is a-z letter which corresponds to
a boot device, and Y as number from 1 to 16, to indicate the partition
number, which can be omitted to instruct the kernel to use a raw device
rather than a partition on a raw device.
2. Specifying root=PARTUUID: with a GUID string of a GUID partition. In
case of existing storage device with GPT partitions, this is most likely
the safest option to ensure booting from persistent storage.
While option 2 is more advanced and reliable, the first option has 2
caveats:
1. The string prefix "/dev/hd" doesn't mean anything beside a convention
on Linux installations, that was taken into use in Serenity. In Serenity
we don't mount DevTmpFS before we mount the boot device on /, so the
kernel doesn't really access /dev anyway, so this convention is only a
big misleading relic that can easily make the user to assume we access
/dev early on boot.
2. This convention although resemble the simple linux convention, is
quite limited in specifying a correct boot device across hardware setup
changes, so option 2 was recommended to ensure the system is always
bootable.
With these caveats in mind, this commit tries to fix the problem with
adding more addressing options as well as to remove the first option
being mentioned above of addressing.
To sum it up, there are 4 addressing options:
1. Hardware relative address - Each instance of StorageController is
assigned with a index number relative to the type of hardware it handles
which makes it possible to address storage devices with a prefix of the
commandset ("ata" for ATA, "nvme" for NVMe, "ramdisk" for Plain memory),
and then the number for the parent controller relative hardware index,
another number LUN target_id, and a third number for LUN disk_id.
2. LUN address - Similar to the previous option, but instead we rely on
the parent controller absolute index for the first number.
3. Block device major and minor numbers - by specifying the major and
minor numbers, the kernel can simply try to get the corresponding block
device and use it as the boot device.
4. GUID string, in the same fashion like before, so the user use the
"PARTUUID:" string prefix and add the GUID of the GPT partition.
For the new address modes 1 and 2, the user can choose to also specify a
partition out of the selected boot device. To do that, the user needs to
append the semicolon character and then add the string "partX" where X
is to be changed for the partition number. We start counting from 0, and
therefore the first partition number is 0 and not 1 in the kernel boot
argument.
2.1 KiB
Name
Boot Device Addressing - addressing the correct boot device to use.
Synopsis
Serenity's kernel can select the boot device at boot time, based on the root boot parameter.
This functionality is used to control which boot device is selected to be used for all further boot process operations.
Description
The kernel root boot parameter takes the form of root={value}, where the ={value}
trailer can be set to specific prefixes to indicate the boot device preference.
Addressing options
The user can choose to use addressing based on synthetic unix concepts:
block0:0
This is especially useful in static hardware setups, so the user can choose to use
either a raw StorageDevice or partition block device. The 0,0 selection is the MAJOR,MINOR
numbers of the device.
However, when there's knowledge of the hardware arrangement of raw StorageDevices,
it could be valuable to use addressing based on hardware-relative interface-specific "location"
to address raw StorageDevices:
ata0:0:0 [First ATA controller, ATA first primary channel, master device]
nvme0:0 [First NVMe Controller, First NVMe Namespace]
ramdisk0 [First Ramdisk]
When the logical arrangement is known, using (absolute) LUNs is the easiest option as it doesn't rely on using unix device numbers or hardware-relative location:
lun0:0:0 - first device on the first channel of the first controller to be enumerated
Note on selecting partitions from raw StorageDevices
All the addressing options above support selecting a partition device, given that
the selected device is a StorageDevice and not a DiskPartition device:
nvme0;part0
lun0:0:0;part0
The only exception to this is when choosing a BlockDevice. As such,
trying to specify block0:0;part0, for example, will lead to a kernel panic,
as an invalid boot device parameter.
Selecting a specific partition based on known GUID
For GPT partitions, passing PARTUUID: and the GUID of the partition can be used
to select a GPT partition. Although it could be slower to find the corresponding
partition, it is the safest option available for persistent storage.