Re: [PATCH 06/12] arm64: assembler: Remove endianness helper macros

[Ard Biesheuvel]

On Thu, 20 Aug 2026, at 16:45, Will Deacon wrote:
> On Thu, Aug 20, 2026 at 04:27:06PM +0300, Ard Biesheuvel wrote:
>> 
>> On Thu, 20 Aug 2026, at 16:19, Will Deacon wrote:
>> > On Sun, Aug 16, 2026 at 10:42:40AM +0100, Will Deacon wrote:
>> >> On Tue, Aug 11, 2026 at 05:04:43PM +0200, Ard Biesheuvel wrote:
>> >> > On Tue, 11 Aug 2026, at 16:01, Will Deacon wrote:
>> >> > > diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
>> >> > > index 87a822e5c4ca..8951ce693552 100644
>> >> > > --- a/arch/arm64/kernel/head.S
>> >> > > +++ b/arch/arm64/kernel/head.S
>> >> > > @@ -138,8 +138,7 @@ SYM_CODE_START_LOCAL(record_mmu_state)
>> >> > >  	b.ne	0f
>> >> > >  	mrs	x19, sctlr_el2
>> >> > >  0:
>> >> > > -CPU_LE( tbnz	x19, #SCTLR_ELx_EE_SHIFT, 1f	)
>> >> > > -CPU_BE( tbz	x19, #SCTLR_ELx_EE_SHIFT, 1f	)
>> >> > > +	tbnz	x19, #SCTLR_ELx_EE_SHIFT, 1f
>> >> > >  	tst	x19, #SCTLR_ELx_C		// Z := (C == 0)
>> >> > >  	and	x19, x19, #SCTLR_ELx_M		// isolate M bit
>> >> > >  	csel	x19, xzr, x19, eq		// clear x19 if Z
>> >> > 
>> >> > There is some more code that can be removed here - see
>> >> > 2ced0f30a426c7301350681f838344d5aea711e3
>> >> 
>> >> Good spot, thanks! I'll do some more surgery at -rc1.
>> >
>> > Looking at this again, I'm not sure we can remove much here. I think we
>> > probably still want to force little-endian (i.e. clear the EE bit) if
>> > we're entered as big-endian. I've changed the following EOR to a BIC
>> > (see below), but I think that's about all we can do?
>> >
>> 
>> Well, the only case where we allow an active ID map is when doing EFI
>> boot, which is guaranteed to be little-endian. Since the kernel is now
>> also guaranteed to be little-endian, we should be able to simply kick
>> the CPU in LE mode right at the start, no? And simply ignore the case
>> of a BE bootloader entering with the MMU and caches enabled?
>
> Hmm, so why did we support this in the first place given that EFI is
> guaranteed to be little-endian? Or was that just because we wanted to
> handle the case of a big-endian kernel being loaded by EFI?
>

The original report is here:
https://lore.kernel.org/linux-arm-kernel/Y9FZsBEu8hSVVIA8@dev-arch.thelio-3990X/

So it would probably have been sufficient at the time to just kick
the CPU into LE mode - I don't remember why I added the additional
logic tbh.