Re: [PATCH] riscv: misaligned: Make enabling delegation depend on BROKEN

[Vivian Wang <wangruikang@iscas.ac.cn>]

On 3/30/26 20:59, Conor Dooley wrote:

> On Mon, Mar 30, 2026 at 02:47:15PM +0800, Vivian Wang wrote:
>> The unaligned access emulation code in Linux has various deficiencies.
>> For example, it doesn't emulate vector instructions [1], and doesn't
>> emulate KVM guest accesses. Therefore, requesting misaligned exception
>> delegation with SBI FWFT actually regresses userspace and KVM guest
>> behavior. Until Linux can handle it properly, guard these sbi_fwft_set()
>> calls behind RISCV_SBI_FWFT_DELEGATE_MISALIGNED, which in turn depends
>> on BROKEN.
>>
>> The rest of the existing code proceeds as before, except as if
>> SBI_FWFT_MISALIGNED_EXC_DELEG is not available, to handle any remaining
>> address misaligned exceptions on a best-effort basis. The KVM SBI FWFT
>> implementation is also not touched, but it is disabled if the firmware
>> emulates unaligned accesses.
>>
>> Cc: stable@vger.kernel.org
>> Fixes: cf5a8abc6560 ("riscv: misaligned: request misaligned exception from SBI")
>> Reported-by: Songsong Zhang <U2FsdGVkX1@gmail.com> # KVM
>> Link: https://lore.kernel.org/linux-riscv/38ce44c1-08cf-4e3f-8ade-20da224f529c@iscas.ac.cn/ [1]
>> Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>
>> ---
>> Clément: Sorry to call what you did broken, but it really is breaking
>> on real hardware out there. I think this is the right way for now.
>> ---
>>  arch/riscv/Kconfig                   | 14 ++++++++++++++
>>  arch/riscv/kernel/traps_misaligned.c |  2 +-
>>  2 files changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>> index 90c531e6abf5..8ad1f13c170e 100644
>> --- a/arch/riscv/Kconfig
>> +++ b/arch/riscv/Kconfig
>> @@ -941,6 +941,20 @@ config RISCV_VECTOR_MISALIGNED
>>  	help
>>  	  Enable detecting support for vector misaligned loads and stores.
>>  
>> +config RISCV_SBI_FWFT_DELEGATE_MISALIGNED
>> +	bool "Request firmware delegation of unaligned access exceptions"
>> +	depends on RISCV_SBI
>> +	depends on BROKEN
> Making it hard to enable I think makes a lot of sense, given the issues
> you're reporting but I tacking on BROKEN will remove effectively all
> build coverage of it* and will definitely stop almost anyone using it.
> Should it be just made NONPORTABLE with the text about being incomplete
> expanded a wee bit to say what is broken so that people can make a
> decision?

Having thought about it more, I think NONPORTABLE does make sense. There
is still a noticible amount of older non-H and non-RVV hardware out
there that remain quite popular, and on these machines this will work
fine, any further bugs notwithstanding.

I'll change it to NONPORTABLE in v2, and, as you said, expand on known
issues.

Thanks,
Vivian "dramforever" Wang