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 > > >