1Booting Xen 2=========== 3 4Xen follows the zImage protocol defined for 32-bit ARM Linux[1] and the 5Image protocol defined for ARM64 Linux[2]. 6 7In both cases the recommendation to boot in HYP/EL2 mode is a strict 8requirement for Xen. 9 10The exceptions to this on 32-bit ARM are as follows: 11 12 Xen does not require the machine type to be passed in r1. This 13 register is ignored (so may be invalid or the actual machine type). 14 15 Xen does not support the ATAG list and requires Device 16 Tree. Therefore r2 must point to the physical address of device tree 17 block (dtb) in system RAM. 18 19 NOTE: although Xen uses the zImage protocol there is no compression 20 actually used. This should be transparent to the bootloader. The 21 zImage protocol should still be used and not the stricter "raw 22 (non-zImage)" protocol described in arm/Booting. 23 24The exceptions to this on 64-bit ARM are as follows: 25 26 Xen binary should be loaded in memory below 10 TiB. 27 28Booting Guests 29-------------- 30 31Xen supports the legacy image header[3], zImage protocol for 32-bit 32ARM Linux[1] and Image protocol defined for ARM64[2]. 33 34Until Xen 4.17, in case of legacy image protocol, Xen ignored the load 35address and entry point specified in the header. This has now changed. 36 37Now, it loads the image at the load address provided in the header. 38And the entry point is used as the kernel start address. 39 40A deviation from uboot is that, Xen treats "load address == 0x0" as 41position independent execution (PIE). Thus, Xen will load such an image 42at an address it considers appropriate. Also, user cannot specify the 43entry point of a PIE image since the start address cennot be 44predetermined. 45 46Users who want to use Xen with statically partitioned domains, can provide 47the fixed non zero load address and start address for the dom0/domU kernel. 48The load address and start address specified by the user in the header must 49be within the memory region allocated by Xen. 50 51Also, it is to be noted that if user provides the legacy image header on 52top of zImage or Image header, then Xen uses the attributes of legacy 53image header to determine the load address, entry point, etc. 54 55 56Firmware/bootloader requirements 57-------------------------------- 58 59Xen relies on some settings the firmware has to configure in EL3 before starting Xen. 60 61* Xen must be entered in NS EL2 mode 62 63* The bit SCR_EL3.HCE (resp. SCR.HCE for 32-bit ARM) must be set to 1. 64 65 66[1] linux/Documentation/arm/booting.rst 67Latest version: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/arch/arm/booting.rst 68 69[2] linux/Documentation/arm64/booting.rst 70Latest version: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/arch/arm64/booting.rst 71 72[3] legacy format header 73Latest version: https://source.denx.de/u-boot/u-boot/-/blob/master/include/image.h#L315 74https://linux.die.net/man/1/mkimage 75