[Crash-utility] [PATCH v2] x86_64: Make the conversion between 4level and 5level paging automatically

[Dave Anderson]

----- Original Message -----
> Hi Dave,
> 
> At 07/11/2018 03:33 AM, Dave Anderson wrote:
> > 
> > The final phase of support would be making this work:
> > 
> >    static int
> >    x86_64_task_uses_5level(struct task_context *tc)
> >    {
> >            return FALSE;
> >    }
> > 
> > Have you had a chance to look at how that can be done?
> > 
> 
> I have no idea to check if 5level paging is used in User-space.
> 
> You gave me some methods, and I have try:
> 
>    -The MAP_FIXED flag
>    -check if the contents of the page directory or p4d is 'flods'

It would be most helpful if you can compare two tasks, one with 5-level
and one without, and see if there's anything obviously different.
(I don't know what "flods" means)

The documentation in 5level-paging.txt is not very clear, but it does
state that MAP_FIXED is not required:

  But userspace can ask for allocation from full address space by
  specifying hint address (with or without MAP_FIXED) above 47-bits.

  If hint address set above 47-bit, but MAP_FIXED is not specified, we try
  to look for unmapped area by specified address. If it's already
  occupied, we look for unmapped area in *full* address space, rather than
  from 47-bit window.

That seems to imply that when the kernel is using 5-level page tables,
then the user-space tasks are also using them -- regardless whether
the task has requested an allocation or not.  In other words, if a task
that is already running requests memory in the high address space, its
page tables have already been set up.  I wouldn't think that an mmap()
request could change the top-level page table mappings on the fly, 
correct?

> Now, I found a rough way. As We all know, in x86_64_uvtop_level4(),
> the user virtual address is either 4-level or 5-level paging. Can we
> do like that:
> 
>    if (machdep->flags & VM_5LEVEL)
>        1) Using 5-level parsing code...
>        2) Check if it is success
>           If failed, Using 4-level parsing code...
>    else
>        Always using 4-level parsing code...
> 
>    3) go on...

Have you actually confirmed that a user task using 4-level page tables
will fail "vtop" or "vm -p" if the kernel is running with 5-level page
tables? 

Dave