linux-kernel - Re: [PATCH] riscv: mm: Update the comment of CONFIG_PAGE

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Message-ID: <A1D410BE9DD1128F+45f92224-dc0f-0bfd-d122-fbbcac48eaf3@tinylab.org>
Date:   Thu, 14 Sep 2023 14:51:43 +0800
From:   Song Shuai <songshuaishuai@...ylab.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, ajones@...tanamicro.com,
        alexghiti@...osinc.com, anup@...infault.org, samuel@...lland.org,
        rppt@...nel.org, suagrfillet@...il.com, panqinglin2020@...as.ac.cn,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: mm: Update the comment of CONFIG_PAGE_OFFSET



在 2023/9/14 12:59, Guo Ren 写道:
> On Wed, Aug 9, 2023 at 11:11 AM Song Shuai <songshuaishuai@...ylab.org> wrote:
>>
>> From: Song Shuai <suagrfillet@...il.com>
>>
>> Since the commit 011f09d12052 set sv57 as default for CONFIG_64BIT,
>> the comment of CONFIG_PAGE_OFFSET should be updated too.
>>
>> Fixes: 011f09d12052 ("riscv: mm: Set sv57 on defaultly")
>> Signed-off-by: Song Shuai <suagrfillet@...il.com>
>> ---
>>   arch/riscv/include/asm/page.h | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
>> index b55ba20903ec..53c00164c042 100644
>> --- a/arch/riscv/include/asm/page.h
>> +++ b/arch/riscv/include/asm/page.h
>> @@ -33,8 +33,8 @@
>>   #define PAGE_OFFSET            _AC(CONFIG_PAGE_OFFSET, UL)
> Why not remove CONFIG_PAGE_OFFSET and define PAGE_OFFSET_L2 ?
>
CONFIG_PAGE_OFFSET holds the first memory address regarding the 
different CONFIGs (MMU,32BIT,64BIT).

As for 64BIT, it actually represents "PAGE_OFFSET_L5" for Sv57 page mode 
defaultly. And the real page mode may be downgraded from kernel cmdline 
or restricted by the hardware, so the PAGE_OFFSET_L3/L4 was defined to 
set the real PAGE_OFFSET.

IIUC, the "PAGE_OFFSET_L2" you specified might be the "PAGE_OFFSET_L5" I 
previously mentioned,
If not, the PAGE_OFFSET_L2 semantically means the page offset for 
32BIT's default Sv32 page mode,and it has already been held in 
CONFIG_PAGE_OFFSET too.

So IMO there is no need to touch CONFIG_PAGE_OFFSET in this context.

Please correct me If I'm wrong.
>>   #endif
>>   /*
>> - * By default, CONFIG_PAGE_OFFSET value corresponds to SV48 address space so
>> - * define the PAGE_OFFSET value for SV39.
>> + * By default, CONFIG_PAGE_OFFSET value corresponds to SV57 address space so
>> + * define the PAGE_OFFSET value for SV48 and SV39.
>>    */
>>   #define PAGE_OFFSET_L4         _AC(0xffffaf8000000000, UL)
>>   #define PAGE_OFFSET_L3         _AC(0xffffffd800000000, UL)
>> --
>> 2.20.1
>>
> 
> 

-- 
Thanks
Song Shuai