On Wed, Sep 20, 2017 at 9:16 PM, Warner Losh <i...@bsdimp.com> wrote:

> On Wed, Sep 20, 2017 at 8:50 PM, Jon Brawn <j...@brawn.org> wrote:
>> Wotcha!
>> I work for Arm for my sins, and in my spare time I’ve been playing with
>> FreeBSD. In my day job I work with the CPU core validation team, and one of
>> the things we do is take the hardware design of a new core and run it on a
>> machine called an emulator. This emulator isn’t the same thing as QEMU, nor
>> is it just an FPGA, it’s something in the middle - you compile the hardware
>> design and download it to the emulator, and it can then run programs on
>> your design at about 1MHz. Which is lovely. Our main bread and butter is to
>> take such a design and get it to boot Arm Linux, a very cut down version,
>> and then run some tests hosted in the Linux environment. These tests would
>> typically thrash the snot out of some particular aspect of the
>> architecture, such as memory sharing amongst multiple processor cores. Now,
>> we would like to use other operating systems that behave differently to
>> Linux, there are some obvious candidates that I’m not going to talk about
>> for legal reasons, but one that was suggested was using FreeBSD under
>> emulation.
>> So, what is needed is someway of telling the operating system that it is
>> going to use a ram disk for its root filesystem, and that the ram disk is
>> going to be at a fixed physical address in the memory map. That way we can
>> pre-load root from a file in the emulation environment. In the Linux
>> environment we would package the kernel, it’s DRB and the root filesystem
>> memory image inside a light-weight bootloader wrapper, load that at the
>> right offset into the emulator’s memory map, and twang the virtual reset
>> line of the emulated processor. There’s some magic jiggery pokery to get
>> console output from what the OS thinks is an AMBA UART, but that’s about
>> size of it.
>> So, what does FreeBSD have to offer in the way of ramdisk functionality?
> Yes.
> See MD_ROOT and friends.

The MFS_IMAGE kernel option has replaced this.

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