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