[GSoC - x86_64] Using FreeBSD's UEFI loader for RTEMS static binaries

[Amaan Cheval]

On Wed, Jun 13, 2018 at 9:35 PM, Gedare Bloom <gedare at rtems.org> wrote:
> On Wed, Jun 13, 2018 at 11:33 AM, Amaan Cheval <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
>>
>> 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
>>
>> -------------------------------------------------------------------------------------
>>
>> 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 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).

>
>> 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).

>
>> What do you think?
>>
>> Cheers,
>> Amaan
>>
>> [1] https://lists.rtems.org/pipermail/devel/2018-June/022052.html
>> [2] https://www.freebsd.org/doc/handbook/kernelconfig-building.html