linux-kernel - Re: [PATCH 4/4] riscv: Allow NOMMU kernels to run in S-mode

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Message-ID: <21e03db9-efb7-4e26-9b0d-4931afd62194@sifive.com>
Date: Tue, 27 Feb 2024 13:02:11 -0600
From: Samuel Holland <samuel.holland@...ive.com>
To: Conor Dooley <conor.dooley@...rochip.com>
Cc: Palmer Dabbelt <palmer@...belt.com>, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] riscv: Allow NOMMU kernels to run in S-mode

Hi Conor,

On 2024-02-27 6:24 AM, Conor Dooley wrote:
> On Mon, Feb 26, 2024 at 04:34:49PM -0800, Samuel Holland wrote:
>> For ease of testing, it is convenient to run NOMMU kernels in supervisor
>> mode. The only required change is to offset the kernel load address,
>> since the beginning of RAM is usually reserved for M-mode firmware.
>>
>> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
>> ---
>>
>>  arch/riscv/Kconfig | 15 ++++++++++-----
>>  1 file changed, 10 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>> index ef53c00470d6..0dc09b2ac2f6 100644
>> --- a/arch/riscv/Kconfig
>> +++ b/arch/riscv/Kconfig
>> @@ -64,7 +64,7 @@ config RISCV
>>  	select ARCH_WANTS_THP_SWAP if HAVE_ARCH_TRANSPARENT_HUGEPAGE
>>  	select BINFMT_FLAT_NO_DATA_START_OFFSET if !MMU
>>  	select BUILDTIME_TABLE_SORT if MMU
>> -	select CLINT_TIMER if !MMU
>> +	select CLINT_TIMER if RISCV_M_MODE
>>  	select CLONE_BACKWARDS
>>  	select COMMON_CLK
>>  	select CPU_PM if CPU_IDLE || HIBERNATION || SUSPEND
>> @@ -220,8 +220,12 @@ 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
>> -	default !MMU
>> +	bool "Build a kernel that runs in machine mode"
>> +	depends on !MMU
>> +	default y
>> +	help
>> +	  Select this option if you want to run the kernel in M-mode,
>> +	  without the assistance of any other firmware.
>>  
>>  # set if we are running in S-mode and can use SBI calls
>>  config RISCV_SBI
>> @@ -238,8 +242,9 @@ config MMU
>>  
>>  config PAGE_OFFSET
>>  	hex
>> -	default 0xC0000000 if 32BIT && MMU
>> -	default 0x80000000 if !MMU
>> +	default 0x80000000 if !MMU && RISCV_M_MODE
>> +	default 0x80200000 if !MMU
>> +	default 0xc0000000 if 32BIT
>>  	default 0xff60000000000000 if 64BIT
> 
> The first default seen with a passing condition is the default chosen,
> right?

Yes, exactly. It's not required for the conditions to all be disjoint.

Regards,
Samuel