Think: when do I need to disable mmu/i-cache/d-cache?

stay armv8/armv9 Of aarch64 Under the architecture , Software startup process ：
BL1--->BL2--->BL31--->BL32--->BL33....

In different BL Mirror switching , Need to be disable MMU/i-cache/d-cache Do you ？
We use BL2 To BL31 For example , Discuss the scenario .

BL2—>BL31

BL2 load BL31 To the memory , And then jump to BL31, Then there are two scenarios to analyze ：

bl2 at EL3

• If bl2 yes EL3,BL31 It's also EL3, They use the same Transslation Regime, So in BL2 Jump BL31 When , need disable MMU/i-cache, as for d-cache There is no need for disabled, But if you are BL2 load Of BL31 Mirror image , So in load When finished mirroring , Need flush d-cache

bl2 at S-EL1

• If bl2 yes aarch64, that BL2 yes S-EL1,BL31 yes EL3, They have different Transslation Regime, So I Unwanted disable MMU/i-cache 了 , as for d-cache There is no need for disabled, But if you are BL2 load Of BL31 Mirror image , So in load When finished mirroring , Need flush d-cache
• If bl2 yes aarch32, that BL2 yes S-EL1,BL31 It's also S-EL1, They use the same Transslation Regime, So in BL2 Jump BL31 When , need disable MMU/i-cache, as for d-cache There is no need for disabled, But if you are BL2 load Of BL31 Mirror image , So in load When finished mirroring , Need flush d-cache

summary ：

• （1） When BL When the image is switched , Just consider whether disable MMU/i-cache, There is no need to consider whether disable d-cache;
• （2） When BL When the image is switched , If Transslation Regime There is a change , There is no need for disable MMU/i-cache
• （3） When BL When the image is switched , If Transslation Regime Nothing has changed , You need to disable MMU/i-cache
• （4） for example uboot To kernel Handoff , Satisfy step (3), So we need to disable MMU/i-cache

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Keep thinking about

During image switching ( When the program switches ), Why shut down MMU？ Why shut down i-cache？

First answer the first , Why shut down MMU？
Because the program 1 Manager Program 1 Page table of , It's impossible to manage programs 2 Page table of . Just entering the program 2 When , The jump points to the physical address , Therefore, it is required to enter the program 2 when , need disable MMU.

Answer the second question , Close everything i-cache Well ？
Because if it's just MMU,i-cache It can still be speculated , It may cache outdated instructions . Jump to program 2 Then point to , It could break down .