Very successful. We're now in the 3rd generation of Arm servers, all built to the same set of specifications with multiple vendor SoCs by tens of different OEM

Very successful. We're now in the 3rd generation of Arm servers, all built to the same set of specifications with multiple vendor SoCs by tens of different OEM/ODMs - SBSA (Server Base System Architecture) and SBBR (Server Base Boot Requirements). The goal of these specs was to make these servers as "boring" as possible, i.e. as similar to an x86 server so that neither OEM/ODMs nor IT guys have to be able to distinguish between supporting an Arm or an x86 servers. This means that you will be able to boot the same binary OS distribution on every machine. There are no more BSPs like in the old 32-bit Arm world. This means that the OS does not need to know anything about clock domains, pin muxes, GPIOs or DVFS beyond the standard facilities exposed via ACPI. Like x86, machine error handling is firmware-first. PCIe works the same way as on x86. AP core boot up and power off is abstracted through the PSCI (Power State Control Interface). TrustZone is not available for OS use and is purely used to implement resident portions of firmware (RAS error handling, PSCI, SDEI). For people asking why UEFI and why ACPI, the answer is very simple: because that's what 99% of all deployed servers use. Using something else is just a friction point for the OEMs, ODMs, IHVs and firmware vendors. It would also be a friction for anyone consuming these systems. Sometimes, you have to be the adult in the room and say that you don't need an "ideal" solution, but the existing solution will work. Plus, the UEFI+ACPI world only really works when you are able to make all systems look more or less the same, hiding all the nitty-gritty shitty bus accesses (I2C for power buttons, SPI for flash, GPIO etc) completely in the firmware, not for the OS to care about. The OpenPower ecosystem didn't see this at all, and they have an elegant firmware solution (hostboot + skiboot + petitboot), but... why bother? It's yet another way to boot, and it just makes their systems foreign to 99% of everyone making and using servers. The OpenPower booting was basically built for Google, but the reality of Google is very different from the realities of the world.