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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