On Thu, Jun 14, 2018, 6:08 PM Amaan Cheval <amaan.che...@gmail.com> wrote:
> Thanks for your input, everyone! I appreciate it! :) > > On Thu, Jun 14, 2018 at 11:25 AM, Chris Johns <chr...@rtems.org> wrote: > > On 14/06/2018 05:33, Joel Sherrill wrote: > >> On Wed, Jun 13, 2018, 6:57 PM Amaan Cheval <amaan.che...@gmail.com > >> <mailto:amaan.che...@gmail.com>> wrote: > >> > >> On Wed, Jun 13, 2018 at 9:35 PM, Gedare Bloom <ged...@rtems.org > >> <mailto:ged...@rtems.org>> wrote: > >> > On Wed, Jun 13, 2018 at 11:33 AM, Amaan Cheval < > amaan.che...@gmail.com > >> <mailto: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 > > > > 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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