Date: Wed, 20 Sep 2017 21:35:13 -0600 From: Ian Lepore <ian@freebsd.org> To: Jon Brawn <jon@brawn.org> Cc: FreeBSD Current <current@freebsd.org>, Warner Losh <imp@bsdimp.com> Subject: Re: Pre-filled RAM disk. Message-ID: <1505964913.73082.65.camel@freebsd.org> In-Reply-To: <CANCZdfrV5NaoOTTGUaXwaDXeoCcmjR3L1b9MnA6QF=__RUZanA@mail.gmail.com> References: <AEDBF95D-5D91-41A0-AEFF-56840FAB755C@brawn.org> <CANCZdfqGk6euOtHrBjvd=L%2BC5uaEeYomcW1xEJwYDadzjW6G6A@mail.gmail.com> <CANCZdfrV5NaoOTTGUaXwaDXeoCcmjR3L1b9MnA6QF=__RUZanA@mail.gmail.com>
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On Wed, 2017-09-20 at 21:23 -0600, Warner Losh wrote: > On Wed, Sep 20, 2017 at 9:16 PM, Warner Losh <imp@bsdimp.com> wrote: >=20 > >=20 > >=20 > >=20 > > On Wed, Sep 20, 2017 at 8:50 PM, Jon Brawn <jon@brawn.org> wrote: > >=20 > > >=20 > > > Wotcha! > > >=20 > > > I work for Arm for my sins, and in my spare time I=A2ve 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=A2t the same thing as > > > QEMU, nor > > > is it just an FPGA, it=A2s 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=A2m not going to > > > talk about > > > for legal reasons, but one that was suggested was using FreeBSD > > > under > > > emulation. > > >=20 > > > 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=A2s DRB and the root > > > filesystem > > > memory image inside a light-weight bootloader wrapper, load that > > > at the > > > right offset into the emulator=A2s memory map, and twang the > > > virtual reset > > > line of the emulated processor. There=A2s some magic jiggery pokery > > > to get > > > console output from what the OS thinks is an AMBA UART, but > > > that=A2s about > > > size of it. > > >=20 > > > So, what does FreeBSD have to offer in the way of ramdisk > > > functionality? > > >=20 > > Yes. > >=20 > > See MD_ROOT and friends. > >=20 > The MFS_IMAGE kernel option has replaced this. >=20 > Warner And the documentation (such as it is) for MFS_IMAGE is in the md(4) manpage. =A0In a nutshell, it's a mechanism that lets you compile an existing filesystem image directly into the kernel and it is mounted as a memory filesystem at boot time. =A0Hopefully being contained within the kernel will make the problem of loading it at a fixed physical address go away for you. -- Ian
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