The RAM disk driver is a way to use system memory as a block device. Instead of allocating memory, this driver allows a user to dynamically create RAM disk(s) in specific location(s) via the parameters address and size.
Remarks:
- If you are using this driver, you know what you are doing and you should at least reserve the memory that will be used in the device tree, else the operating system can behave abnormally and crash.
- There is no limitations about how many disks can be created, so be careful.
- Two RAM disks cannot use the same memory region (overlap).
- A RAM disk created cannot be removed, unless the driver is unloaded.
- RAM disk content is not erased at driver unload, so it means that you could retrieve the content after a warm reboot (as long the fsbl or uboot does not perform fancy operations in RAM).
- Tested on Linux kernel version 5.10.107 (Hardknott).
root@qemuarm64:~# insmod xrd.ko
[ 43.962180] xrd: loading out-of-tree module taints kernel.
[ 43.966036] loaded.
root@qemuarm64:~# echo "0x50000000" > /sys/module/xrd/parameters/address
root@qemuarm64:~# echo "0x1000000" > /sys/module/xrd/parameters/size
[ 48.082219] create RAM disk... (address: 0x50000000, size: 16777216).
[ 48.108744] RAM disk xram0 created.
root@qemuarm64:~# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xram0 251:0 0 16M 0 disk
vda 253:0 0 228.1M 0 disk /
root@qemuarm64:~# mkfs.ext4 /dev/xram0
mke2fs 1.46.1 (9-Feb-2021)
Creating filesystem with 4096 4k blocks and 4096 inodes
Allocating group tables: done
Writing inode tables: done
Creating journal (1024 blocks): done
Writing superblocks and filesystem accounting information: done
root@qemuarm64:~# mkdir mnt
root@qemuarm64:~# ls
mnt xrd.ko
root@qemuarm64:~# mount /dev/xram0 mnt/
[ 160.786314] EXT4-fs (xram0): mounted filesystem with ordered data mode. Opts: (null)
[ 160.791805] ext4 filesystem being mounted at /home/root/mnt supports timestamps until 2038 (0x7fffffff)
root@qemuarm64:~# echo "hello" > mnt/hello.txt
root@qemuarm64:~# cat mnt/hello.txt
helloConvert your dtb to dts.
dtc -I dtb -O dts -o qemu.dts qemu.dtbAdd a range in reserved-memory node (ex: xramdisk):
memory@40000000 {
reg = <0x00 0x40000000 0x00 0x20000000>;
device_type = "memory";
};
reserved-memory {
#address-cells = <2>;
#size-cells = <2>;
ranges;
xramdisk@0 {
no-map;
reg = <0x00 0x50000000 0x00 0x02000000>;
};
};Here we reserve 32M starting from 0x50000000 in available RAM memory (512M starting at 0x40000000).
Convert your dts to dtb.
dtc -I dts -O dtb -o qemu.dtb qemu.dtsWhen your device is up, we can check if the range is reserved.
qemuarm64 login: root
root@qemuarm64:~# cat /proc/iomem
09000000-09000fff : pl011@9000000
09000000-09000fff : 9000000.pl011 pl011@9000000
09010000-09010fff : pl031@9010000
09010000-09010fff : rtc-pl031
09030000-09030fff : pl061@9030000
10000000-3efeffff : pcie@10000000
10000000-1003ffff : 0000:00:01.0
10040000-10040fff : 0000:00:01.0
10041000-10041fff : 0000:00:03.0
40000000-4fffffff : System RAM
40200000-40faffff : Kernel code
40fb0000-4132ffff : reserved
41330000-4156ffff : Kernel data
48000000-48008fff : reserved
50000000-51ffffff : reserved -> xramdisk range
52000000-5fffffff : System RAM
5e000000-5effffff : reserved
5fe22000-5fee2fff : reserved
5fee3000-5fefffff : reserved
5ff02000-5ff02fff : reserved
5ff03000-5ff03fff : reserved
5ff04000-5ff0efff : reserved
5ff0f000-5fffffff : reserved
4010000000-401fffffff : PCI ECAM
8000000000-ffffffffff : pcie@10000000
8000000000-8000003fff : 0000:00:01.0
8000000000-8000003fff : virtio-pci-modern
8000004000-8000007fff : 0000:00:02.0
8000004000-8000007fff : virtio-pci-modern
8000008000-800000bfff : 0000:00:03.0
8000008000-800000bfff : virtio-pci-modern```Add -machine dumpdtb=qemu.dtb to your QEMU command, ex:
/home/red/distribution/hardknott/poky/build/tmp/work/x86_64-linux/qemu-helper-native/1.0-r1/recipe-sysroot-native/usr/bin/qemu-system-aarch64 \
-device virtio-net-pci,netdev=net0,mac=A4:B1:B1:03:A2:AF \
-netdev user,id=net0,hostfwd=tcp::2222-:22,hostfwd=tcp::2223-:23 \
-drive file=./files/core-image-minimal-qemuarm64.ext4,if=virtio,format=raw \
-object rng-random,filename=/dev/urandom,id=rng0 \
-device virtio-rng-pci,rng=rng0 \
-machine virt \
-cpu cortex-a57 -m 512 \
-serial mon:stdio -serial null \
-kernel ./files/Image \
-append 'root=/dev/vda rw highres=off console=ttyS0 mem=512M ip=dhcp console=ttyAMA0,115200 console=tty ' \
-nographic \
-machine dumpdtb=qemu.dtbremark: .dtb generated cannot be loaded by QEMU, you will need to convert it to dts, adn convert it back to dtb...
# Convert dtb to dts
dtc -I dtb -O dts -o qemu.dts qemu.dtb
# Convert dts to dtb
dtc -I dts -O dtb -o qemu.dtb qemu.dtsOnce the conversion is done, load the device tree by adding -dtb ./files/qemu.dtb to your QEMU command, ex:
/home/red/distribution/hardknott/poky/build/tmp/work/x86_64-linux/qemu-helper-native/1.0-r1/recipe-sysroot-native/usr/bin/qemu-system-aarch64 \
-device virtio-net-pci,netdev=net0,mac=A4:B1:B1:03:A2:AF \
-netdev user,id=net0,hostfwd=tcp::2222-:22,hostfwd=tcp::2223-:23 \
-drive file=./files/core-image-minimal-qemuarm64.ext4,if=virtio,format=raw \
-object rng-random,filename=/dev/urandom,id=rng0 \
-device virtio-rng-pci,rng=rng0 \
-machine virt \
-cpu cortex-a57 -m 512 \
-serial mon:stdio -serial null \
-kernel ./files/Image \
-append 'root=/dev/vda rw highres=off console=ttyS0 mem=512M ip=dhcp console=ttyAMA0,115200 console=tty ' \
-nographic \
-dtb ./files/qemu.dtb- https://blog.pankajraghav.com/2022/11/30/BLKRAM.html
- https://github.com/Panky-codes/blkram/blob/master/blkram.c
- https://static.lwn.net/images/pdf/LDD3/ch16.pdf
- https://xilinx-wiki.atlassian.net/wiki/spaces/A/pages/18841683/Linux+Reserved+Memory
- https://docs.u-boot.org/en/latest/develop/devicetree/dt_qemu.html
- https://tldp.org/LDP/lkmpg/2.6/html/x323.html
- https://www.gnu.org/software/libc/manual/html_node/Permission-Bits.html
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