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

[Dave Anderson]

----- Original Message -----
> 
> 
> ----- 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)

Ah, sorry, you mean if the top-level page directory folds back into itself.

So yes, I wonder if it would be obvious enough by checking whether the
top-level pgd folds back into itself, and if so, 4-levels of page tables
are in use?  But if that were the case, it seems that the 5-level translation
code would work on both types of user-space tasks.  I just don't understand
how it's supposed to work.

Dave


> 
> 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
> 
>  
>