Re: [PATCHv1, RFC 0/8] Boot-time switching between 4- and 5-level paging

[Andy Lutomirski]

On Thu, May 25, 2017 at 4:24 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, May 25, 2017 at 1:33 PM, Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
>> Here' my first attempt to bring boot-time between 4- and 5-level paging.
>> It looks not too terrible to me. I've expected it to be worse.
>
> If I read this right, you just made it a global on/off thing.
>
> May I suggest possibly a different model entirely? Can you make it a
> per-mm flag instead?
>
> And then we
>
>  (a) make all kthreads use the 4-level page tables
>
>  (b) which means that all the init code uses the 4-level page tables
>
>  (c) which means that all those checks for "start_secondary" etc can
> just go away, because those all run with 4-level page tables.
>
> Or is it just much too expensive to switch between 4-level and 5-level
> paging at run-time?
>

Even ignoring expensiveness, I'm not convinced it's practical.  AFAICT
you can't atomically switch the paging mode and CR3, so either you
need some magic page table with trampoline that works in both modes
(which is presumably doable with some trickery) or you need to flip
paging off.  Good luck if an NMI hits in the mean time.  There was
code like that once upon a time for EFI mixed mode, but it got deleted
due to triple-faults.

Doing this in switch_mm() sounds painful.