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

[Bin Meng]

On Fri, Feb 5, 2016 at 1:41 AM, Tom Rini <trini at konsulko.com> wrote:
> On Wed, Feb 03, 2016 at 12:42:44PM +0800, Bin Meng wrote:
>> Hi Simon,
>>
>> On Wed, Feb 3, 2016 at 12:31 PM, Simon Glass <sjg at chromium.org> wrote:
>> > 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!
>> >
>>
>> Agreed. But guess Intel won't do that due to whatever backward
>> compatible reasons..
>>
>> >>
>> >>> 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.
>>
>> Isn't the limitation coming from UEFI itself? My understanding is that
>> we cannot boot a 64-bit EFI application from a 32-bit UEFI BIOS as
>> well.
>
> Well, how much are distros going to dislike having to provide 32bit EFI
> grub, for the x86 case here?
>

See a blog post "Don't ship 32-bit UEFI firmware on x86" here:
https://mjg59.dreamwidth.org/26734.html

Regards,
Bin