[GSoC - x86_64] Using FreeBSD's UEFI loader for RTEMS static binaries
Amaan Cheval
amaan.cheval at gmail.com
Thu Jun 14 19:42:05 UTC 2018
Gotcha. We can leave an #error directive in what we merge initially too,
like we did for the i386 SMP support.
On Thu, Jun 14, 2018, 11:55 PM Gedare Bloom <gedare at rtems.org> wrote:
> On Thu, Jun 14, 2018 at 1:25 PM, Amaan Cheval <amaan.cheval at gmail.com>
> wrote:
> > 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?
> >
>
> The preferred "minimal" BSP is one with working console and clock
> driver. It would be OK to start merging once you reach the milestone
> of context switching to the Init task, though, and then add the
> console support, and then the clock driver, before you begin the
> other, more advanced BSP features.
>
> > 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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