Re: Question about OS initialization at UEFI firmware (x86)

[Rafael Machado <rafaelrodrigues.machado@gmail.com>]

Thanks a lot guys.

Now things make sense.

Em qui, 5 de jan de 2017 às 11:18, Laszlo Ersek <lersek@redhat.com>
escreveu:

> On 01/05/17 13:16, Alcantara, Paulo wrote:
> >> -----Original Message-----
> >> From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf Of
> >> Rafael Machado
> >> Sent: quinta-feira, 5 de janeiro de 2017 10:00
> >> To: edk2-devel@lists.01.org
> >> Subject: [edk2] Question about OS initialization at UEFI firmware (x86)
> >>
> >> Hi everyone
> >>
> >> I was taking a look at how the OS boots after the firmware and
> bootloader
> >> are done.
> >>
> >> To understand this I started to take a look at the linux source code,
> and the
> >> strange is that I saw some bios legacy interrupts being called.
> >>
> >> The flow I checked is this:
> >>
> >> void main(void)  -->  linux/arch/x86/boot/main.c
> >>      int detect_memory(void) -->  linux/arch/x86/boot/memory.c
> >>            static int detect_memory_e820(void) -->
> >> linux/arch/x86/boot/memory.c
> >>                    intcall(0x15, &ireg, &oreg) -->
> linux/arch/x86/boot/memory.c
> >>
> >>
> >> At the last call the value of ireg is this one:
> >>
> >> ireg.ax  = 0xe820;
> >> ireg.cx  = sizeof buf;
> >> ireg.edx = SMAP;
> >> ireg.di  = (size_t)&buf;
> >>
> >>
> >> As we can see this is done so the OS knows the memory map, so the OS can
> >> do all its magic.
> >>
> >> Finally, my question is:
> >>
> >> How could linux, or any other OS, boot on a system with UEFI firmware
> that
> >> does not have CSM (compatibility support module) ?
> >
> > The code you pasted above seems to be executed when booting Linux on
> > PC BIOS firmware. See below.
> >
> >> I consider that some parts of the hypothetical OS need to be written to
> call
> >> some UEFI protocols. Am I right ?
> >
> > As far as I know, there are currently two ways of booting Linux on
> > UEFI firmware:
> >
> > 1) The OS loader (bootloader as a PE/COFF image) uses the EFI handover
> > protocol to boot the Linux kernel image. What the loader does is
> > basically to find the entry point offset (handover_offset) in that
> > image and jump to it. The entry point conforms to ABI defined in UEFI
> > spec.
> >
> > 2) The kernel may be built as PE/COFF binary (UEFI image) so the
> firmware can
> > directly boot it at BDS without any external OS loader.
> >
> > You might want to look at how OVMF boots up Linux through QEMU's
> command-line parameter "-kernel" by using EFI handover protocol.
>
> See also:
>
> https://lwn.net/Articles/632528/
>
>