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Date:      Wed, 20 Sep 2017 21:16:42 -0600
From:      Warner Losh <imp@bsdimp.com>
To:        Jon Brawn <jon@brawn.org>
Cc:        FreeBSD Current <current@freebsd.org>
Subject:   Re: Pre-filled RAM disk.
Message-ID:  <CANCZdfqGk6euOtHrBjvd=L%2BC5uaEeYomcW1xEJwYDadzjW6G6A@mail.gmail.com>
In-Reply-To: <AEDBF95D-5D91-41A0-AEFF-56840FAB755C@brawn.org>
References:  <AEDBF95D-5D91-41A0-AEFF-56840FAB755C@brawn.org>

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On Wed, Sep 20, 2017 at 8:50 PM, Jon Brawn <jon@brawn.org> wrote:

> Wotcha!
>
> I work for Arm for my sins, and in my spare time I=E2=80=99ve been playin=
g 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=E2=80=99t the same thing as=
 QEMU, nor
> is it just an FPGA, it=E2=80=99s something in the middle - you compile th=
e 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 woul=
d
> typically thrash the snot out of some particular aspect of the
> architecture, such as memory sharing amongst multiple processor cores. No=
w,
> we would like to use other operating systems that behave differently to
> Linux, there are some obvious candidates that I=E2=80=99m not going to ta=
lk 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 ca=
n
> pre-load root from a file in the emulation environment. In the Linux
> environment we would package the kernel, it=E2=80=99s DRB and the root fi=
lesystem
> memory image inside a light-weight bootloader wrapper, load that at the
> right offset into the emulator=E2=80=99s memory map, and twang the virtua=
l reset
> line of the emulated processor. There=E2=80=99s some magic jiggery pokery=
 to get
> console output from what the OS thinks is an AMBA UART, but that=E2=80=99=
s about
> size of it.
>
> So, what does FreeBSD have to offer in the way of ramdisk functionality?
>

Yes.

See MD_ROOT and friends.

Warner



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