[GSoC - x86_64] Using FreeBSD's UEFI loader for RTEMS static binaries
Amaan Cheval
amaan.cheval at gmail.com
Thu Jun 14 17:25:07 UTC 2018
Sounds good!
For shutdown and whatnot, I imagine I'll need to get our ACPI support
out there too, so that may be on the backburner for a little bit.
Maybe this should be a new thread, but; what is the minimal BSP we
will be willing to merge upstream? I'm thinking that this process of
booting, initialization, and reaching the init task (but not having
full or possibly any console support) can be the minimal.
Is that a reasonable assumption?
On Thu, Jun 14, 2018 at 9:42 PM, Joel Sherrill <joel at rtems.org> wrote:
>
>
> On Thu, Jun 14, 2018, 6:08 PM Amaan Cheval <amaan.cheval at gmail.com> wrote:
>>
>> Thanks for your input, everyone! I appreciate it! :)
>>
>> On Thu, Jun 14, 2018 at 11:25 AM, Chris Johns <chrisj at rtems.org> wrote:
>> > On 14/06/2018 05:33, Joel Sherrill wrote:
>> >> On Wed, Jun 13, 2018, 6:57 PM Amaan Cheval <amaan.cheval at gmail.com
>> >> <mailto:amaan.cheval at gmail.com>> wrote:
>> >>
>> >> On Wed, Jun 13, 2018 at 9:35 PM, Gedare Bloom <gedare at rtems.org
>> >> <mailto:gedare at rtems.org>> wrote:
>> >> > On Wed, Jun 13, 2018 at 11:33 AM, Amaan Cheval
>> >> <amaan.cheval at gmail.com
>> >> <mailto:amaan.cheval at gmail.com>> wrote:
>> >> >> Hi!
>> >> >>
>> >> >> As we discussed in the last thread on the topic[1], I'm trying
>> >> to use
>> >> >> FreeBSD's loader.efi directly with RTEMS' generated static
>> >> binaries
>> >> >> (since FreeBSD's loader.efi has an ELF loader).
>> >> >>
>> >> >> In brief, I did this by:
>> >> >> - Installing FreeBSD in QEMU with UEFI firmware
>> >> >> - Confirming that FreeBSD's loader.efi is in fact used
>> >> >> - Replacing FreeBSD's ELF kernel with a "custom" kernel[2] with
>> >> an RTEMS ELF
>> >> >> - Verifying that the code running after FreeBSD's loader.efi is
>> >> in
>> >> >> fact the "RTEMS ELF" by attaching gdb to QEMU (the rtems ELF is
>> >> simply
>> >> >> a while(1) loop compiled with RTEMS' tools - see later on why I
>> >> can't
>> >> >> do something more elaborate)
>> >> >>
>> >> >> Some more details of the process I followed for testing this:
>> >> >> https://gist.github.com/AmaanC/42faa131ee97a1d6c4c7c25c29f0fde9z
>> >> >>
>> >> >> I think this method is superior to the PIC RTEMS method because:
>> >> >> - FreeBSD uses it
>> >> >> - RTEMS retains static ELF binaries, which can likely easily be
>> >> >> combined with a Multiboot header + protect mode starter code
>> >> >> - FreeBSD has methods to provide ACPI related hints to their ELF
>> >> >> kernel - this might make our implementation with regards to ACPI
>> >> >> simpler too
>> >
>> > I agree this is the best approach. In time we can host on our file
>> > server a
>> > package of FreeBSD binaries that boot an RTEMS kernel.
>> >
>> >> >>
>> >> >> Regarding some concerns Chris had with linker options and
>> >> whatnot,
>> >> >> here's what FreeBSD uses:
>> >> >>
>> >> https://www.freebsd.org/doc/en/books/arch-handbook/boot-kernel.html
>> >> >>
>> >> >> Here's what I used (with the code being a simple while(1) loop):
>> >> >> x86_64-rtems5-gcc ktest.c -c -nostdlib
>> >> >> x86_64-rtems5-ld ktest.o -e main -o kernel
>> >> >>
>> >
>> > Nice, this looks fine. It is normal for a bare metal piece of C code.
>> >
>> >> >>
>> >>
>> >> -------------------------------------------------------------------------------------
>> >> >>
>> >> >> What I need input on:
>> >> >> - Right now, we use the following RTEMS code for testing:
>> >> >>
>> >> >> int main() {
>> >> >> while(1) {}
>> >> >> }
>> >> >>
>> >> >
>> >> > It's not really an RTEMS code, it is a C program (ktest.c)
>> >> compiled
>> >> > with the RTEMS-flavored toolchain, right?
>> >>
>> >> Yeah, for now that's right. I'm going to conduct the same gdb based
>> >> debug-stepping style test for RTEMS setting boot_card as the entry
>> >> point soon - for now, it crashes QEMU with:
>> >>
>> >> qemu: fatal: Trying to execute code outside RAM or ROM at
>> >> 0x00000000000b0000
>> >>
>> >> RAX=00000000006004c0 RBX=00000000006003d8 RCX=0000000037f36000
>> >> RDX=0000000000400000
>> >> RSI=0000000004000000 RDI=0000000000000180 RBP=00000000006003d8
>> >> RSP=000000003c589fb8
>> >> ...
>> >>
>> >> I see that it reaches that stage even from some code it ought not
>> >> to
>> >> be executing, so I'll look into what that may be about.
>>
>> It was quite simple, really - my stub doesn't define
>> _CPU_Context_restore yet - rtems_initialize_executive calls that
>> function expecting it to never return, but when it does, we lose
>> control and just start running code from virtual address 0 (or
>> possibly whatever happens to be on the stack as the return instruction
>> pointer).
>>
>> What we _do_ know is a positive sign, though - an actual RTEMS static
>> binary does seem to be loaded just fine, and starts executing too,
>> until we call _CPU_Context_restore and lose control.
>>
>> Next up: I'll work on the context-switching code to move past this,
>
>
> You will need context initialize and restore to get through initialization
> to the first task. You might as well complete all of the methods related to
> non-FP contexts and see if you can run base_sp. It may even call shutdown so
> you can do that.
>
> But getting to a user init task would be success and return to printk.
>
>> and then we can follow the original plan in my proposal
>> (context-switching, basic IRQ, idle thread based clock driver,
>> printk/console support - I'd like to get to the console driver as soon
>> as is viable - I could work on it directly outside of the RTEMS static
>> binary, using the "ktest" style kernel I mentioned earlier, but I
>> think we'd rather make progress directly on the BSP first).
>>
>> >
>> > Hmm.
>> >
>> >>
>> >> >
>> >> > It would be nice to get an RTEMS x86-64 BSP to start, at least to
>> >> > confirm that you reach _start, and then even you can try to make
>> >> it to
>> >> > the "boot_card" startup sequence.
>> >>
>> >> Right, I'll aim to have that working soon (using boot_card as the
>> >> entry, since "_start" usually does the bootloader stuff that we're
>> >> now
>> >> offloading to FreeBSD, and then calls boot_card anyway).
>> >>
>> >>
>> >> To be consistent with other BSPs, I have a start.c on the Deos BSPs. It
>> >> fetches
>> >> the boot arguments which are passed to boot_card() and does some other
>> >> setup
>> >> specific to Deos.
>> >>
>> >> No need to do this now but there is a good reason to follow the
>> >> pattern. Start
>> >> doesn't have to be in assembly.
>>
>> Noted for the future, thanks!
>>
>> >>
>> >>
>> >> >
>> >> >> That's literally it, because we have no access to standard
>> >> libraries,
>> >> >> and loader.efi calls ExitBootServices, after which we can't just
>> >> >> easily directly access video memory (at 0xb8000 for eg.) to
>> >> print to
>> >> >> the screen. The way FreeBSD handles this is by initializing the
>> >> >> console and printing to that - I haven't been able to easily
>> >> port that
>> >> >> yet.
>> >> >>
>> >> >> The question is - should I start with that effort (i.e. bringing
>> >> >> printk console functionality to RTEMS) the way FreeBSD does?
>> >> This way,
>> >> >> we skip the bootloader for now by simply using the one built on
>> >> the
>> >> >> real FreeBSD - if the console prints and more elaborate linking
>> >> tests
>> >> >> work fine, we can be certain that this works. If _not_, I
>> >> believe the
>> >> >> console initialization code will likely still remain the same
>> >> since
>> >> >> we'll want to do it similar to how FreeBSD does it.
>> >> >>
>> >> >
>> >> > I think this approach to getting a console to work may be
>> >> reasonable,
>> >> > assuming the FreeBSD console is not much more complicated than
>> >> what
>> >> > RTEMS needs. ...
>> >>
>> >> I can't say about this yet, but I'll look into it (and perhaps
>> >> simplifying it as we port it if it _is_ too complicated).
>> >>
>> >
>> > It has been a couple of years but I think FreeBSD contains some of the
>> > Intel
>> > code to interface to UEFI and via this you can get to the UEFI console.
>> > This
>> > should be easy but it comes with a side effect.
>> >
>> > UEFI boots in graphics mode and so it's console on a PC is a slow scroll
>> > one. On
>> > boards like a Minnow using the UEFI console has the advantage of being
>> > able to
>> > support any redirection UEFI has enabled such as a serial port. The
>> > disadvantage
>> > of this is performance and overhead. In time this may be a boot option.
>> >
>> > What I am not sure is the boundary between UEFI and the kernel and what
>> > is
>> > enabled or available when the kernel is loaded.
>>
>>
>> That's good information, thank you! I'll look into it as I can - for
>> now, can we settle on these for next steps?
>>
>> - We're using FreeBSD's loader.efi - to do so, we just need our BSP to
>> generate static ELFs, so nothing needs to go in the source tree
>> - I'll focus on the context-switching code for the BSP next, aiming to
>> get it to actually reach bsp_start - once that's done, we can focus on
>> the console output (this means that until then, verifying the progress
>> will likely still be done through emulators and debuggers).
>>
>> Let me know!
>>
>> >
>> >> >
>> >> >> What do you think?
>> >> >>
>> >
>> > Awesome work.
>> > Thanks
>> > Chris
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