Re: [PATCH] riscv: deprecate CONFIG_MMU=n

[Damien Le Moal <dlemoal@kernel.org>]

On 2024/02/26 6:59, Clément Léger wrote:
> 
> 
> On 26/02/2024 15:57, Damien Le Moal wrote:
>> On 2024/02/26 6:06, 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
>>> 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@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.
>>
>> 2 years from now is 2026...
> 
> Guess I'm a bit tired -_-'

Off-by-1 bug. It happens :)

> 
>>
>>> +
>>>  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)
>>
>> 	if (!IS_ENABLED(CONFIG_MMU))
>>
>> would be more elegant here...
> 
> Sure.
> 
>>
>>> +	pr_err("RISC-V NOMMU support is deprecated and scheduled for removal by the beginning of 2027\n");
>>
>> Why pr_err() ? pr_warn() seems more appropriate.
> 
> Agreed.
> 
> Thanks,
> 
> Clément
> 
>>
>>> +#endif
>>>  }
>>>  
>>>  bool arch_cpu_is_hotpluggable(int cpu)
>>

-- 
Damien Le Moal
Western Digital Research


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