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

----- 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