[U-Boot] 64-bit x86 U-Boot?

[Bin Meng]

Hi Simon,

On Tue, Feb 2, 2016 at 11:58 AM, Simon Glass <sjg at chromium.org> wrote:
> +Bin (sorry, meant to copy you before)
>
> Hi Albert,
>
> On 1 February 2016 at 00:14, Albert ARIBAUD <albert.u.boot at aribaud.net> wrote:
>> Hello Simon,
>>
>> On Sun, 31 Jan 2016 19:20:31 -0700, Simon Glass <sjg at chromium.org>
>> wrote:
>>> Hi Bin,
>>>
>>> At present U-Boot supports booting a 64-bit kernel directly. It can
>>> also be loaded as a 64-bit payload from EFI. But it cannot be built as
>>> a 64-bit boot loader.
>>>
>>> I took a bit of a look at this. It looks like we need to stay in
>>> 32-bit mode until the FSP is loaded. Also, to get to 64-bit mode I'm
>>> pretty sure we need page tables, which means we need somewhere to put
>>> them!
>>>

Yes unfortunately that is the FSP limitation.

>>> Looking at the board_f init sequence, it seems that arch_cpu_init() is
>>> the earlist we could switch to 64-bit. We'd need to grab some memory
>>> from somewhere to do this - I wonder if this can be CAR? There is no
>>> SRAM on x86 chips I think.
>>>
>>> For non-FSP devices we don't init the RAM until much later -
>>> dram_init(). That means that a significant portion of the init
>>> sequence would be 32-bit code. I'm not sure that will work.
>>>

I believe we can do dram_init() in 64-bit mode as well if MRC is
written in pure C.

>>> I suppose one option is to only go to 64-bit mode when relocating. But
>>> then we end up with lots of code that needs to run in 32-bit and
>>> 64-bit.
>>>
>>> Do you have any ideas on this?
>>
>> How about starting with implementing the last option, i.e. switch to 64
>> bits when DDR is available, mainline that, then progressively work your
>> way toward an earlier switch?
>
> The tricky thing is that I'm not sure we can run 32-bit code in 64-bit
> mode or vice-versa. The instruction set is similar but there are data
> size settings that probably make it incompatible.
>

No, it's not 100% compatible. We have to switch back and forth unfortunately.

> I wonder whether we might need to resort to SPL for the 32-bit
> portion, and jump to a 64-bit U-Boot from there? Tegra does something
> similar to that.
>

What's the benefit of doing a 64-bit bootloader? Intel's UEFI BIOS has
a 32-bit and 64-bit version, and has caused some troubles for the next
stage loader (bootia32.efi vs. bootx64.efi). I know for PowerPC, a
64-bit U-Boot does not exist as 32-bit U-Boot can load 32-bit and
64-bit kernel, just like what we have for x86. 64-bit U-Boot was only
seen on ARMv8, but that's the architecture limitation I believe, and
we have to do that.

Regards,
Bin