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

[Dou Liyang]

Hi Dave,

At 07/11/2018 10:02 PM, Dave Anderson wrote:
> 
> 
> ----- 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.

OK, I will compare them carefully.

> (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

Yes, I agree.

> 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?

Did you mean that we actually don't need to do x86_64_task_uses_5level()
in user task?

> 
>> 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?
> 

Actually, I can't found a user task using 4-leve paging in the kernel
which is running with 5-level page tables. Do you have some examples?

Let me investigate more.

Thank,
	dou

> Dave
> 
>   
> 
> 
>