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

[Laszlo Ersek <lersek@redhat.com>]

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/