[Crash-utility] Seek help about 5-level paging

[Kirill A. Shutemov]

On Mon, Jul 16, 2018 at 01:05:24AM +0000, Dou Liyang wrote:
> Dear Kirill,
> 
> Sorry to trouble you.
> 
> I am trying to make the Crash can parse the kernel with 5-level paging.
> 
> I met a problem, seek help.
> 
> IMO, all user-sapce tasks must using 5-level paging if the kernel
> has been in 5-level paging decided by '__pgtable_l5_enabled=1'. Correct?

There's no way to enable paging mode on per-task basis. It's enabled with
flag in CR4 therefore it's global.

> And, In the Documentation/x86/x86_64/5level-paging.txt:
> 
>   ...
>   To mitigate this, we are not going to allocate virtual address space
>   above 47-bit by default.
> 
>   But userspace can ask for allocation from full address space by
>   specifying hint address (with or without MAP_FIXED) above 47-bits.
>   ...
> 
> I guess it just means that some user-space tasks can't using the address
> above 47-bits, it doesn't mean that the tasks will back to use 4-level
> paging if the kernel has been in 5-level paging. Is it right?

Right.

-- 
 Kirill A. Shutemov