[coreboot] why is firmware 32 bit as opposed to 64 bit

[Marc Jones]

Nice answer, Scott.

You pointed out everything I was going to say. I think this is
something we should put on the wiki in a FAQ. A 64bit ramstage has
questionable value, but a 64bit payload might be useful.

Marc


On Sun, Aug 10, 2014 at 4:20 PM, Scott Duplichan <scott at notabs.org> wrote:
> Vladimir 'φ-coder/phcoder' Serbinenko [mailto:phcoder at gmail.com] wrote"
>
>
> ]On 10.08.2014 21:06, John de la Garza wrote:
> ]> I understand that the calling functions in 32 bit C uses the stack and
> ]> this is why coreboot needs to use cache as RAM.  Doesn't 64 bit C use
> ]> registers to pass arguments to functions?  If this is the case why not
> ]> run in 64 bit mode?
> ]>
> ]> Also, even if cache as RAM is used and a stack is available, why not just
> ]> build a 64 bit binary?  What are the advantages to using a 32 bit binary?
> ]>
> ]>
> ]long mode (64-bit) needs paging table in RAM. So no 64-bit for preram
> ]binary. For rest it's theoretically possible but it's too much hassle
> ]for no benefit.
>
> The page table requirement is certainly a negative for x64 mode.
> Another is code size. Code grows by several percent when compiled
> for x64 mode. Use of x32 ABI could reduce the code size penalty,
> but page tables are still required.
>
> Most UEFI uses 32-bit mode until RAM is ready, then x64 mode
> after that. Unlike coreboot, UEFI doesn't prioritize code size
> minimization. Minimum code size is needed for fast boot, because
> the code must be read from the relatively slow flash chip before
> it can execute. Coreboot does prioritize fast boot. Switching
> coreboot to x64 mode would result in a measurable boot time
> degradation I believe.
>
> One advantage of x64 mode is easy use of DRAM structures larger
> then 4GB. Boot firmware is unlikely to benefit from this. Another
> benefit of x64 mode is use of additional registers. The additional
> registers can lead to faster code. But boot firmware is usually I/O
> bound, so it is unlikely the extra registers could lead to a
> measurable boot time reduction.
>
> Thanks,
> Scott
>
>
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