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

[Simon Glass]

Hi Bin,

On 2 February 2016 at 08:02, Bin Meng <bmeng.cn at gmail.com> wrote:
> Hi Albert,
>
> On Tue, Feb 2, 2016 at 5:53 PM, Albert ARIBAUD
> <albert.u.boot at aribaud.net> wrote:
>> Hello Bin and Simon,
>>
>> On Tue, 2 Feb 2016 15:25:48 +0800, Bin Meng <bmeng.cn at gmail.com> wrote:
>>> 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)
>>
>>> >>> 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.
>>
>> Bin: not sure what you mean by "if MRC is written in pure C" -- there
>> is no C construct that can even approach the mrc instruction, which can
>> only be emitted through an asm statement.
>
> You are exposed as an ARM guy :-) I was talking about an Intel term
> Memory Reference Code which are a amount of magic codes to initialize
> system RAM.
>
>>
>>> > 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.
>>
>> Simon: seems like a sensible approach, as it does not mix 32 and 64
>> bits in one "build artefact", plus it seems logical in that SPL's
>> role is to get the platform ready for U-Boot; switching from
>> power-on32-bit mode to 64-bit mode belongs quite "naturally" in SPL.
>>
>>> 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.
>>
>> Some U-Boot users who might want to get rid of x86 32-bit code in
>> x86 64-bit platforms just like in the past some people must have wanted
>> to get rid of real-mode 16-bit x86 code in order to run pure 32-bit; the
>
> Yep, but unfortunately we still cannot get rid of real-mode 16-bit x86
> code even today :(
>
>> idea is that if you can do with as well as without a feature, then that
>> feature is potential dead code, and is candidate for removal, all the
>> more when that feature partly collides with another feature, as here
>> where 32-bit and 64-bit support sort of overlap partially.
>>
>
> I wonder if some day these processors (arm, x86, whatever else?) will
> come out of reset in the 64-bit mode directly. No more any legacy
> modes. At that time, 64-bit mode bootloader is definitely a must.

ARM does. Not sure if Intel will, but they should IMO!

>
>> Now, we can wait until x86 32-bit is really dead (as in "not used
>> except in a few legacy projects whose engineers' children are about to
>> retire") and then scrape dead code parts which no one really understands
>> any more, or we can try and anticipate and replace code while we still
>> have a grasp of what it does. I personally like the idea of anticipating
>> better.
>>
>> Just in case, note that I do not mean x86 32-bit support should be
>> removed from U-Boot now or later. I mean that if we can make x86 64-bit
>> support in U-Boot less and less dependent on x86 32-bit support, then I
>> think we should, so that the day we completely drop x86 32-bit support,
>> x86 64-bit support will be (as) unaffected (as possible).
>>
>
> I agree with the philosophy here. But I sense this might be too
> anticipating as there are some other tasks to do for U-Boot 32-bit
> like ACPI and SMM. 32-bit is enough for now, unless we want to access
>>4GB memory in U-Boot shell?

Yes, I suppose there are more important things. The 32/64-bit split
bothers me. For example with the EFI loader series, U-Boot runs in
32-bit mode so can only run a 32-bit EFI application (e.g. grub). That
seems like an annoying limitation. We don't have that limitation when
booting a kernel.

But I'm not sure it is very important - just something I was thinking about.

Regards,
Simon