[PATCH] riscv: deprecate CONFIG_MMU=n

[Clément Léger]

On 26/02/2024 16:14, Samuel Holland wrote:
> On 2024-02-26 8:06 AM, Clément Léger wrote:
>> Deprecation of NOMMU support for riscv was discussed during LPC 2023
>> [1]. Reasons for this involves lack of users as well as maintenance
>> efforts to support this mode. psABI FDPIC specification also never
>> made it upstream and last public messages of this development seems to
>> date back from 2020 [2]. Plan the deprecation to be done in 2 years from
> 
> What are the criteria for delaying/canceling the removal? NOMMU support doesn't
> rot nearly as fast as XIP; static PIE ELF works and is well specified; and as
> mentioned at LPC, there are some users, even if "just for fun".

Hi Samuel,

I was actually developing some feature that encountered NOMMU build
failures and I was reminded that NOMMU was discussed to be deprecated
during last LPC. I guess I could also mark XIP as deprecated. The
rationale behind delaying is to let some users to manifest themselves
before a full removal. But this can still be discussed of course, this
patch was also meant to trigger such feedback.

> 
>> now. Mark the Kconfig option as deprecated by adding a new dummy option
>> which explicitly displays the deprecation in case of CONFIG_MMU=n. This option
>> is selected indirectly by CONFIG_RISCV_M_MODE since an option can not
>> select another one directly with a "select" in case of such CONFIG=n.
>> Additionally, display a pr_err() message at boot time in case of NOMMU
>> build to warn about upcoming deprecation.
>>
>> Link: https://lpc.events/event/17/contributions/1478/ [1]
>> Link: https://groups.google.com/a/groups.riscv.org/g/sw-dev/c/ZjYUJswknQ4/m/WYRRylTwAAAJ [2]
>> Signed-off-by: Clément Léger <cleger at rivosinc.com>
>>
>> ---
>>  arch/riscv/Kconfig        | 8 ++++++++
>>  arch/riscv/kernel/setup.c | 4 ++++
>>  2 files changed, 12 insertions(+)
>>
>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>> index bffbd869a068..8da58c102d3f 100644
>> --- a/arch/riscv/Kconfig
>> +++ b/arch/riscv/Kconfig
>> @@ -221,6 +221,7 @@ config ARCH_MMAP_RND_COMPAT_BITS_MAX
>>  # set if we run in machine mode, cleared if we run in supervisor mode
>>  config RISCV_M_MODE
>>  	bool
>> +	select NOMMU
>>  	default !MMU
>>  
>>  # set if we are running in S-mode and can use SBI calls
>> @@ -236,6 +237,13 @@ config MMU
>>  	  Select if you want MMU-based virtualised addressing space
>>  	  support by paged memory management. If unsure, say 'Y'.
>>  
>> +config NOMMU
>> +	depends on !MMU
>> +	bool "NOMMU kernel (DEPRECATED)"
>> +	help
>> +	  NOMMU kernel is deprecated and is scheduled for removal by
>> +	  the beginning of 2027.
>> +
> 
> The idiomatic way to display this kind of warning is a comment directive:
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index d4e890fb5b5a..b736440ce0f7 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -248,6 +248,9 @@ config MMU
>           Select if you want MMU-based virtualised addressing space
>           support by paged memory management. If unsure, say 'Y'.
> 
> +comment "NOMMU support is deprecated (scheduled for removal in 2027)"
> +       depends on !MMU
> +

Thanks, I was not able to find any example of such usage.

Clément

>  config PAGE_OFFSET
>         hex
>         default 0xC0000000 if 32BIT && MMU
> 
> Regards,
> Samuel
> 
>>  config PAGE_OFFSET
>>  	hex
>>  	default 0xC0000000 if 32BIT && MMU
>> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
>> index 4f73c0ae44b2..8799816ef0a6 100644
>> --- a/arch/riscv/kernel/setup.c
>> +++ b/arch/riscv/kernel/setup.c
>> @@ -295,6 +295,10 @@ void __init setup_arch(char **cmdline_p)
>>  	riscv_set_dma_cache_alignment();
>>  
>>  	riscv_user_isa_enable();
>> +
>> +#if !defined(CONFIG_MMU)
>> +	pr_err("RISC-V NOMMU support is deprecated and scheduled for removal by the beginning of 2027\n");
>> +#endif
>>  }
>>  
>>  bool arch_cpu_is_hotpluggable(int cpu)
>