[PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[Will Deacon]

Big-endian arm64 configurations are vanishingly rare, yet we still claim
to support them in Linux despite very limited testing or visible
interest. Supporting big-endian adds unnecessary burden to reviewers and
contributors which, without any known active users, is hard to justify.
For example, recent work to improve our futex routines and to implement
nested virtualisation support is non-trivially complicated by having to
support both big- and little-endianness.

Back in 2019 [1], it was claimed that Huawei were using arm64 big-endian
machines in their telecommunication products but I don't know whether
that's still the case and certainly haven't seen any patch contributions
to help support or maintain it.

Make CPU_BIG_ENDIAN depend on BROKEN as an initial deprecation step
towards its removal.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Hanjun Guo <guohanjun@huawei.com>
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/linux-arm-kernel/73701e9f-bee1-7ae8-2277-7a3576171cd4@huawei.com/ [1]
Signed-off-by: Will Deacon <will@kernel.org>
---

Cc'ing Guenter as a heads-up in case he needs to turn down his testing
to avoid this causing a false regression report.

Cc'ing Hanjun and Jonathan for clarity on the telecommunication
situation.

 arch/arm64/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index e9bbfacc35a6..5ac670d41604 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1493,7 +1493,7 @@ choice
 config CPU_BIG_ENDIAN
 	bool "Build big-endian kernel"
 	# https://github.com/llvm/llvm-project/commit/1379b150991f70a5782e9a143c2ba5308da1161c
-	depends on AS_IS_GNU || AS_VERSION >= 150000
+	depends on (AS_IS_GNU || AS_VERSION >= 150000) && BROKEN
 	help
 	  Say Y if you plan on running a kernel with a big-endian userspace.
 
-- 
2.51.0.470.ga7dc726c21-goog

Re: [PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[Ard Biesheuvel]

On Fri, 19 Sept 2025 at 20:40, Will Deacon <will@kernel.org> wrote:
>
> Big-endian arm64 configurations are vanishingly rare, yet we still claim
> to support them in Linux despite very limited testing or visible
> interest. Supporting big-endian adds unnecessary burden to reviewers and
> contributors which, without any known active users, is hard to justify.
> For example, recent work to improve our futex routines and to implement
> nested virtualisation support is non-trivially complicated by having to
> support both big- and little-endianness.
>
> Back in 2019 [1], it was claimed that Huawei were using arm64 big-endian
> machines in their telecommunication products but I don't know whether
> that's still the case and certainly haven't seen any patch contributions
> to help support or maintain it.
>
> Make CPU_BIG_ENDIAN depend on BROKEN as an initial deprecation step
> towards its removal.
>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Hanjun Guo <guohanjun@huawei.com>
> Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Cc: Guenter Roeck <linux@roeck-us.net>
> Link: https://lore.kernel.org/linux-arm-kernel/73701e9f-bee1-7ae8-2277-7a3576171cd4@huawei.com/ [1]
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>

Acked-by: Ard Biesheuvel <ardb@kernel.org>

> Cc'ing Guenter as a heads-up in case he needs to turn down his testing
> to avoid this causing a false regression report.
>
> Cc'ing Hanjun and Jonathan for clarity on the telecommunication
> situation.
>
>  arch/arm64/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index e9bbfacc35a6..5ac670d41604 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1493,7 +1493,7 @@ choice
>  config CPU_BIG_ENDIAN
>         bool "Build big-endian kernel"
>         # https://github.com/llvm/llvm-project/commit/1379b150991f70a5782e9a143c2ba5308da1161c
> -       depends on AS_IS_GNU || AS_VERSION >= 150000
> +       depends on (AS_IS_GNU || AS_VERSION >= 150000) && BROKEN
>         help
>           Say Y if you plan on running a kernel with a big-endian userspace.
>
> --
> 2.51.0.470.ga7dc726c21-goog
>

Re: [PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[Marc Zyngier]

On Fri, 19 Sep 2025 19:40:25 +0100,
Will Deacon <will@kernel.org> wrote:
> 
> Big-endian arm64 configurations are vanishingly rare, yet we still claim
> to support them in Linux despite very limited testing or visible
> interest. Supporting big-endian adds unnecessary burden to reviewers and
> contributors which, without any known active users, is hard to justify.
> For example, recent work to improve our futex routines and to implement
> nested virtualisation support is non-trivially complicated by having to
> support both big- and little-endianness.
> 
> Back in 2019 [1], it was claimed that Huawei were using arm64 big-endian
> machines in their telecommunication products but I don't know whether
> that's still the case and certainly haven't seen any patch contributions
> to help support or maintain it.
> 
> Make CPU_BIG_ENDIAN depend on BROKEN as an initial deprecation step
> towards its removal.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Hanjun Guo <guohanjun@huawei.com>
> Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Cc: Guenter Roeck <linux@roeck-us.net>
> Link: https://lore.kernel.org/linux-arm-kernel/73701e9f-bee1-7ae8-2277-7a3576171cd4@huawei.com/ [1]
> Signed-off-by: Will Deacon <will@kernel.org>

It's about time.

Acked-by: Marc Zyngier <maz@kernel.org>

	M.

-- 
Jazz isn't dead. It just smells funny.

Re: [PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[Catalin Marinas]

On Fri, Sep 19, 2025 at 07:40:25PM +0100, Will Deacon wrote:
> Big-endian arm64 configurations are vanishingly rare, yet we still claim
> to support them in Linux despite very limited testing or visible
> interest. Supporting big-endian adds unnecessary burden to reviewers and
> contributors which, without any known active users, is hard to justify.
> For example, recent work to improve our futex routines and to implement
> nested virtualisation support is non-trivially complicated by having to
> support both big- and little-endianness.
> 
> Back in 2019 [1], it was claimed that Huawei were using arm64 big-endian
> machines in their telecommunication products but I don't know whether
> that's still the case and certainly haven't seen any patch contributions
> to help support or maintain it.
> 
> Make CPU_BIG_ENDIAN depend on BROKEN as an initial deprecation step
> towards its removal.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Hanjun Guo <guohanjun@huawei.com>
> Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Cc: Guenter Roeck <linux@roeck-us.net>
> Link: https://lore.kernel.org/linux-arm-kernel/73701e9f-bee1-7ae8-2277-7a3576171cd4@huawei.com/ [1]
> Signed-off-by: Will Deacon <will@kernel.org>

Acked-by: Catalin Marinas <catalin.marinas@arm.com>

Re: [PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[Guenter Roeck]

On 9/19/25 11:40, Will Deacon wrote:
> Big-endian arm64 configurations are vanishingly rare, yet we still claim
> to support them in Linux despite very limited testing or visible
> interest. Supporting big-endian adds unnecessary burden to reviewers and
> contributors which, without any known active users, is hard to justify.
> For example, recent work to improve our futex routines and to implement
> nested virtualisation support is non-trivially complicated by having to
> support both big- and little-endianness.
> 
> Back in 2019 [1], it was claimed that Huawei were using arm64 big-endian
> machines in their telecommunication products but I don't know whether
> that's still the case and certainly haven't seen any patch contributions
> to help support or maintain it.
> 
> Make CPU_BIG_ENDIAN depend on BROKEN as an initial deprecation step
> towards its removal.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Hanjun Guo <guohanjun@huawei.com>
> Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Cc: Guenter Roeck <linux@roeck-us.net>
> Link: https://lore.kernel.org/linux-arm-kernel/73701e9f-bee1-7ae8-2277-7a3576171cd4@huawei.com/ [1]
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> 
> Cc'ing Guenter as a heads-up in case he needs to turn down his testing
> to avoid this causing a false regression report.

Cool, one more architecture to drop. I wonder if I should drop arm64 big endian testing
for all kernel releases. That would simplify my test infrastructure substantially,
and it would avoid useless regressions due to bitrot for older kernel releases
if no one is using it anyway. Thoughts ?

Thanks,
Guenter

> 
> Cc'ing Hanjun and Jonathan for clarity on the telecommunication
> situation.
> 
>   arch/arm64/Kconfig | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index e9bbfacc35a6..5ac670d41604 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1493,7 +1493,7 @@ choice
>   config CPU_BIG_ENDIAN
>   	bool "Build big-endian kernel"
>   	# https://github.com/llvm/llvm-project/commit/1379b150991f70a5782e9a143c2ba5308da1161c
> -	depends on AS_IS_GNU || AS_VERSION >= 150000
> +	depends on (AS_IS_GNU || AS_VERSION >= 150000) && BROKEN
>   	help
>   	  Say Y if you plan on running a kernel with a big-endian userspace.
>   

Re: [PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[Will Deacon]

On Fri, 19 Sep 2025 19:40:25 +0100, Will Deacon wrote:
> Big-endian arm64 configurations are vanishingly rare, yet we still claim
> to support them in Linux despite very limited testing or visible
> interest. Supporting big-endian adds unnecessary burden to reviewers and
> contributors which, without any known active users, is hard to justify.
> For example, recent work to improve our futex routines and to implement
> nested virtualisation support is non-trivially complicated by having to
> support both big- and little-endianness.
> 
> [...]

Applied to arm64 (for-next/misc), thanks!

[1/1] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN
      https://git.kernel.org/arm64/c/1cf89b6bf660

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev

Re: [PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[Will Deacon]

On Wed, Sep 24, 2025 at 09:09:16AM -0700, Guenter Roeck wrote:
> On 9/19/25 11:40, Will Deacon wrote:
> > Big-endian arm64 configurations are vanishingly rare, yet we still claim
> > to support them in Linux despite very limited testing or visible
> > interest. Supporting big-endian adds unnecessary burden to reviewers and
> > contributors which, without any known active users, is hard to justify.
> > For example, recent work to improve our futex routines and to implement
> > nested virtualisation support is non-trivially complicated by having to
> > support both big- and little-endianness.
> > 
> > Back in 2019 [1], it was claimed that Huawei were using arm64 big-endian
> > machines in their telecommunication products but I don't know whether
> > that's still the case and certainly haven't seen any patch contributions
> > to help support or maintain it.
> > 
> > Make CPU_BIG_ENDIAN depend on BROKEN as an initial deprecation step
> > towards its removal.
> > 
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: Marc Zyngier <maz@kernel.org>
> > Cc: Ard Biesheuvel <ardb@kernel.org>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Hanjun Guo <guohanjun@huawei.com>
> > Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > Cc: Guenter Roeck <linux@roeck-us.net>
> > Link: https://lore.kernel.org/linux-arm-kernel/73701e9f-bee1-7ae8-2277-7a3576171cd4@huawei.com/ [1]
> > Signed-off-by: Will Deacon <will@kernel.org>
> > ---
> > 
> > Cc'ing Guenter as a heads-up in case he needs to turn down his testing
> > to avoid this causing a false regression report.
> 
> Cool, one more architecture to drop. I wonder if I should drop arm64 big endian testing
> for all kernel releases. That would simplify my test infrastructure substantially,
> and it would avoid useless regressions due to bitrot for older kernel releases
> if no one is using it anyway. Thoughts ?

It's entirely up to you, but I'd suggest leaving it running until 6.18
is out and then turning it all off if nobody has chimed in to say
they're using this stuff.

Will

Re: [PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[Guenter Roeck]

On 9/25/25 06:01, Will Deacon wrote:
> On Wed, Sep 24, 2025 at 09:09:16AM -0700, Guenter Roeck wrote:
>> On 9/19/25 11:40, Will Deacon wrote:
>>> Big-endian arm64 configurations are vanishingly rare, yet we still claim
>>> to support them in Linux despite very limited testing or visible
>>> interest. Supporting big-endian adds unnecessary burden to reviewers and
>>> contributors which, without any known active users, is hard to justify.
>>> For example, recent work to improve our futex routines and to implement
>>> nested virtualisation support is non-trivially complicated by having to
>>> support both big- and little-endianness.
>>>
>>> Back in 2019 [1], it was claimed that Huawei were using arm64 big-endian
>>> machines in their telecommunication products but I don't know whether
>>> that's still the case and certainly haven't seen any patch contributions
>>> to help support or maintain it.
>>>
>>> Make CPU_BIG_ENDIAN depend on BROKEN as an initial deprecation step
>>> towards its removal.
>>>
>>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>>> Cc: Marc Zyngier <maz@kernel.org>
>>> Cc: Ard Biesheuvel <ardb@kernel.org>
>>> Cc: Arnd Bergmann <arnd@arndb.de>
>>> Cc: Hanjun Guo <guohanjun@huawei.com>
>>> Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
>>> Cc: Guenter Roeck <linux@roeck-us.net>
>>> Link: https://lore.kernel.org/linux-arm-kernel/73701e9f-bee1-7ae8-2277-7a3576171cd4@huawei.com/ [1]
>>> Signed-off-by: Will Deacon <will@kernel.org>
>>> ---
>>>
>>> Cc'ing Guenter as a heads-up in case he needs to turn down his testing
>>> to avoid this causing a false regression report.
>>
>> Cool, one more architecture to drop. I wonder if I should drop arm64 big endian testing
>> for all kernel releases. That would simplify my test infrastructure substantially,
>> and it would avoid useless regressions due to bitrot for older kernel releases
>> if no one is using it anyway. Thoughts ?
> 
> It's entirely up to you, but I'd suggest leaving it running until 6.18
> is out and then turning it all off if nobody has chimed in to say
> they're using this stuff.
> 

Makes sense. I'll do that.

Thanks,
Guenter

Re: [PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[Hanjun Guo]

Hi Will,

On 2025/9/20 2:40, Will Deacon wrote:
> Big-endian arm64 configurations are vanishingly rare, yet we still claim
> to support them in Linux despite very limited testing or visible
> interest. Supporting big-endian adds unnecessary burden to reviewers and
> contributors which, without any known active users, is hard to justify.
> For example, recent work to improve our futex routines and to implement
> nested virtualisation support is non-trivially complicated by having to
> support both big- and little-endianness.
> 
> Back in 2019 [1], it was claimed that Huawei were using arm64 big-endian
> machines in their telecommunication products but I don't know whether
> that's still the case and certainly haven't seen any patch contributions
> to help support or maintain it.
> 
> Make CPU_BIG_ENDIAN depend on BROKEN as an initial deprecation step
> towards its removal.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Hanjun Guo <guohanjun@huawei.com>
> Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Cc: Guenter Roeck <linux@roeck-us.net>
> Link: https://lore.kernel.org/linux-arm-kernel/73701e9f-bee1-7ae8-2277-7a3576171cd4@huawei.com/ [1]
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> 
> Cc'ing Guenter as a heads-up in case he needs to turn down his testing
> to avoid this causing a false regression report.
> 
> Cc'ing Hanjun and Jonathan for clarity on the telecommunication
> situation.

Sorry for the late reply, the current products are still using the ARM64 
big-endian architecture on Linux, but the future evolution will not.

It will take quite a while (for years) for the end of the product life
cycle, please keep it as BROKEN.

Thanks
Hanjun

Re: [PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[J. Neuschäfer]

On Fri, Sep 19, 2025 at 07:40:25PM +0100, Will Deacon wrote:
> Big-endian arm64 configurations are vanishingly rare, yet we still claim
> to support them in Linux despite very limited testing or visible
> interest. Supporting big-endian adds unnecessary burden to reviewers and
> contributors which, without any known active users, is hard to justify.
> For example, recent work to improve our futex routines and to implement
> nested virtualisation support is non-trivially complicated by having to
> support both big- and little-endianness.
> 
> Back in 2019 [1], it was claimed that Huawei were using arm64 big-endian
> machines in their telecommunication products but I don't know whether
> that's still the case and certainly haven't seen any patch contributions
> to help support or maintain it.
> 
> Make CPU_BIG_ENDIAN depend on BROKEN as an initial deprecation step
> towards its removal.

As of this month (September 2025) there's a new community project [1,2]
to revive aarch64_be. Jens Reidel (CC'd) and I are involved in it. We've
been fixing several aarch64_be-related bugs, but mostly[3] in userspace,
because the kernel support is pretty solid.

So, just so it doesn't go unmentioned, there is interest in keeping
big-endian ARM64 alive.

Best regards,
J. Neuschäfer

[1]: https://aarch64.be/ (binary packages)
[2]: https://codeberg.org/neuschaefer/porting/src/branch/main/aarch64_be.md
     (status and documentation)
[3]: https://lore.kernel.org/lkml/20250726235646.254730-1-adrian@mainlining.org/

Re: [PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[Catalin Marinas]

On Mon, Sep 29, 2025 at 02:18:55AM +0200, J. Neuschäfer wrote:
> On Fri, Sep 19, 2025 at 07:40:25PM +0100, Will Deacon wrote:
> > Big-endian arm64 configurations are vanishingly rare, yet we still claim
> > to support them in Linux despite very limited testing or visible
> > interest. Supporting big-endian adds unnecessary burden to reviewers and
> > contributors which, without any known active users, is hard to justify.
> > For example, recent work to improve our futex routines and to implement
> > nested virtualisation support is non-trivially complicated by having to
> > support both big- and little-endianness.
> > 
> > Back in 2019 [1], it was claimed that Huawei were using arm64 big-endian
> > machines in their telecommunication products but I don't know whether
> > that's still the case and certainly haven't seen any patch contributions
> > to help support or maintain it.
> > 
> > Make CPU_BIG_ENDIAN depend on BROKEN as an initial deprecation step
> > towards its removal.
> 
> As of this month (September 2025) there's a new community project [1,2]
> to revive aarch64_be. Jens Reidel (CC'd) and I are involved in it. We've
> been fixing several aarch64_be-related bugs, but mostly[3] in userspace,
> because the kernel support is pretty solid.
> 
> So, just so it doesn't go unmentioned, there is interest in keeping
> big-endian ARM64 alive.

It's nice to see it gets testing but is there any actual beyond you and
Jens (i.e. in production somewhere like Huawei's case)? We want to
assess whether it's worth the kernel maintenance and testing hassle and
for how long.

-- 
Catalin

Re: [PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[J. Neuschäfer]

On Mon, Sep 29, 2025 at 11:52:48AM +0100, Catalin Marinas wrote:
> On Mon, Sep 29, 2025 at 02:18:55AM +0200, J. Neuschäfer wrote:
> > On Fri, Sep 19, 2025 at 07:40:25PM +0100, Will Deacon wrote:
> > > Big-endian arm64 configurations are vanishingly rare, yet we still claim
> > > to support them in Linux despite very limited testing or visible
> > > interest. Supporting big-endian adds unnecessary burden to reviewers and
> > > contributors which, without any known active users, is hard to justify.
> > > For example, recent work to improve our futex routines and to implement
> > > nested virtualisation support is non-trivially complicated by having to
> > > support both big- and little-endianness.
> > > 
> > > Back in 2019 [1], it was claimed that Huawei were using arm64 big-endian
> > > machines in their telecommunication products but I don't know whether
> > > that's still the case and certainly haven't seen any patch contributions
> > > to help support or maintain it.
> > > 
> > > Make CPU_BIG_ENDIAN depend on BROKEN as an initial deprecation step
> > > towards its removal.
> > 
> > As of this month (September 2025) there's a new community project [1,2]
> > to revive aarch64_be. Jens Reidel (CC'd) and I are involved in it. We've
> > been fixing several aarch64_be-related bugs, but mostly[3] in userspace,
> > because the kernel support is pretty solid.
> > 
> > So, just so it doesn't go unmentioned, there is interest in keeping
> > big-endian ARM64 alive.
> 
> It's nice to see it gets testing but is there any actual beyond you and
> Jens (i.e. in production somewhere like Huawei's case)? We want to
> assess whether it's worth the kernel maintenance and testing hassle and
> for how long.

We don't have any commercial or large deployments, no. It's currently
one build host and manual testing on additional machines.

I'd be sad to see big-endian arm64 support removed because it's an
opportunity to test for endian-related bugs (in various packages, not
just the kernel) on widely available and reasonably fast hardware.

I understand the maintenance cost argument, and I'm fine with the
CONFIG_BROKEN dependency, even if it might be applied to features that
cause problems in the future (such as nested virt) — it's easy to work
around, and features can be fixed as needed. A complete removal will
likely be harder to revert, though, especially once code simplifications
are made based on supporting only little-endian. That's my concern.


Best regards,
J. Neuschäfer

Re: [PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[Marc Zyngier]

On Mon, 29 Sep 2025 21:16:33 +0100,
"J. Neuschäfer" <j.neuschaefer@gmx.net> wrote:
> 
> I understand the maintenance cost argument, and I'm fine with the
> CONFIG_BROKEN dependency, even if it might be applied to features that
> cause problems in the future (such as nested virt) — it's easy to work
> around, and features can be fixed as needed.

Not really. Cross-endianness support is completely incompatible with
nested virt *by design*, and can't be worked around by SW. You'd need
to introduce incompatible changes in the architecture to support this.

Yes, this is a shame, but also a sign that architecture and
implementations have moved on.

	M.

-- 
Without deviation from the norm, progress is not possible.

Re: [PATCH] arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

[A. Wilcox]

On Sep 19, 2025, at 13:40, Will Deacon <will@kernel.org> wrote:
> 
> Big-endian arm64 configurations are vanishingly rare, yet we still claim
> to support them in Linux despite very limited testing or visible
> interest. Supporting big-endian adds unnecessary burden to reviewers and
> contributors which, without any known active users, is hard to justify.
> For example, recent work to improve our futex routines and to implement
> nested virtualisation support is non-trivially complicated by having to
> support both big- and little-endianness.
> 
> Back in 2019 [1], it was claimed that Huawei were using arm64 big-endian
> machines in their telecommunication products but I don't know whether
> that's still the case and certainly haven't seen any patch contributions
> to help support or maintain it.
> 
> Make CPU_BIG_ENDIAN depend on BROKEN as an initial deprecation step
> towards its removal.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Hanjun Guo <guohanjun@huawei.com>
> Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Cc: Guenter Roeck <linux@roeck-us.net>
> Link: https://lore.kernel.org/linux-arm-kernel/73701e9f-bee1-7ae8-2277-7a3576171cd4@huawei.com/ [1]
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> 
> Cc'ing Guenter as a heads-up in case he needs to turn down his testing
> to avoid this causing a false regression report.
> 
> Cc'ing Hanjun and Jonathan for clarity on the telecommunication
> situation.
> 
> arch/arm64/Kconfig | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index e9bbfacc35a6..5ac670d41604 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1493,7 +1493,7 @@ choice
> config CPU_BIG_ENDIAN
> bool "Build big-endian kernel"
> # https://github.com/llvm/llvm-project/commit/1379b150991f70a5782e9a143c2ba5308da1161c
> - depends on AS_IS_GNU || AS_VERSION >= 150000
> + depends on (AS_IS_GNU || AS_VERSION >= 150000) && BROKEN
> help
>  Say Y if you plan on running a kernel with a big-endian userspace.
> 
> -- 
> 2.51.0.470.ga7dc726c21-goog


Hi Will (and others),

We at Wilcox Technologies Inc. have actually laid out multiple
projects and initiatives based on Linux aarch64_be support.  They
aren't yet ready for public release or consumption, but they are
definitely being actively planned and developed.  One of them is a
full port of a Linux distribution.  There have been successful runs
of Gentoo[1] as well.  The musl libc intends[2] support.

What would need to be done to help keep aarch64_be alive in Linux
while we continue our internal work and finish other projects?
Would standing up a CI builder help any?  Are there existing known
issues?  I've personally been experimenting with aarch64_be since
the early 5.x kernel series and I've never found kernel-level bugs,
only userland ones.  The kernel has been solid.

aarch64_be is quite useful to have as it allows developers to have
access to BE hardware "at home" without needing a S390x or PPC64
system that can cost many thousands of dollars.

If it's simply a matter of waiting for our initiatives to launch
to show actual interest and testing, then I can try to advocate for
those initiatives to be started sooner.

In summary, Linux aarch64_be works well on Raspberry Pi 3, RockPro
64, PINE A64, among other SBCs and SoCs.  We are looking forward
to multiple future developments using it and are willing to help
out where and how we can to ensure it is not removed from the Linux
kernel.

Best,
-Anna


[1]: https://github.com/rorybolt/Gentoo-aarch64_be
[2]: https://git.musl-libc.org/cgit/musl/commit/?id=dfc1a37c441fe271dfb19c7de62acadc73e255aa


--
Anna Wilcox (she/her)
SW Engineering: C++/Rust, DevOps, POSIX, Py/Ruby
Wilcox Technologies Inc.