Re: [PATCH v3 5/6] powerpc/64: Add support for out-of-line static calls

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 06/10/2022 à 20:22, Segher Boessenkool a écrit :
> On Thu, Oct 06, 2022 at 11:39:50AM +1100, Michael Ellerman wrote:
>> Christophe Leroy <christophe.leroy@csgroup.eu> writes:
>>> However, thinking out loudly, I'm wondering, could we make things any
>>> simpler when CONFIG_MODULES is not selected, or is that a too much
>>> corner case on PPC64 ?
>>
>> I'd say it's mostly a corner case.
>>
>> Obviously no distros ship with modules disabled.
>>
>> AFAIK even the stripped down kernels we use in CPU bringup have modules
>> enabled.
>>
>> So I think it's probably not worth worrying about, unless there's an
>> obvious and fairly simple optimisation.
> 
> Long ago I built kernels that fit together with the boot firmware and a
> root fs (busybox+dropbear essentially) in 4MB, but I doubt we can get
> close to that at all these days :-)

4MB, not easy. But 8M still achievable. Well our smaller board has 32M, 
we have thousands of it spread all over Europe and have to keep it up to 
date ....

> 
> What is the overhead if you enable modules but do not use them, these
> days?
> 

On the 8xx it is mainly the instruction TLB miss handler:

#ifdef CONFIG_MODULES
	mfcr	r11
	not.	r10, r10
#endif
	mfspr	r10, SPRN_M_TWB	/* Get level 1 table */
#ifdef CONFIG_MODULES
	blt+	3f
	rlwinm	r10, r10, 0, 20, 31
	oris	r10, r10, (swapper_pg_dir - PAGE_OFFSET)@ha
3:
	mtcr	r11
#endif


And also some patches which have a interesting impact, like commit 
cb3ac45214c0 ("powerpc/code-patching: Don't call 
is_vmalloc_or_module_addr() without CONFIG_MODULES")


On the other hand, if we want one day to replace nftables by BPF jitted 
iptables, CONFIG_MODULES will be required. So what will be the 
trade-off, don't know yet because BPF is not yet cross-compile friendly.

Christophe