On Wed, Jun 13, 2018, 6:57 PM Amaan Cheval <amaan.che...@gmail.com> wrote:
> On Wed, Jun 13, 2018 at 9:35 PM, Gedare Bloom <ged...@rtems.org> wrote: > > On Wed, Jun 13, 2018 at 11:33 AM, Amaan Cheval <amaan.che...@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). > 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. > > > > >> 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 >
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