[RFC PATCH] x86/mm: Disable PTI for kernel_ident_mapping_init()

[Dave Hansen]

On 11/25/24 10:53, David Woodhouse wrote:
>> I think we have a lot of software-available space in the page table
>> pointer entries. What would folks think if we set a special bit in those
>> p4d entries that said:
>>
>> 	"I don't need to be propagated to
>> 	the user portion of the page tables."
>>
>> It would obviously get set in this code that you're trying to fix. It
>> might _also_ be able to be set in in "_USR", like here:
>>
>> #define _KERNPG_TABLE_NOENC  (__PP|__RW|   0|___A|   0|___D|   0|   0)
>> #define _PAGE_TABLE_NOENC    (__PP|__RW|_USR|___A|   0|___D|   0|   0)
>>
>> like:
>>
>> #define _USR _PAGE_USER|_PAGE_SW_WHATEVER
> In fact, do we even need a separate bit? Any PTE without the _PAGE_USER
> bit set clearly doesn't need to be mirrored into the user page
> tables...?

I can't think of any exceptions where this would break off the top of my
head. It seems too simple to work. ;)