linux-kernel - RE: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP fault for user address

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Date:   Thu, 1 Sep 2022 20:32:35 +0000
From:   "Kalra, Ashish" <Ashish.Kalra@....com>
To:     Borislav Petkov <bp@...en8.de>
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Subject: RE: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP
 fault for user address

[AMD Official Use Only - General]

Hello Boris,

>> It is basically an index into the 4K page within the hugepage mapped 
>> in the RMP table or in other words an index into the RMP table entry 
>> for 4K page(s) corresponding to a hugepage.

>So pte_index(address) and for 1G pages, pmd_index(address).

>So no reinventing the wheel if we already have helpers for that.

>Yes that makes sense and pte_index(address) is exactly what is required for 2M hugepages.

>Will use pte_index() for 2M pages and pmd_index() for 1G pages. 

Had a relook into this. 

As I mentioned earlier, this is computing an index into a 4K page within a hugepage mapping,
therefore, though pte_index() works for 2M pages, but pmd_index() will not work for 1G pages.

We basically need to do :
pfn |= (address >> PAGE_SHIFT) & mask;

where mask is the (number of 4K pages per hugepage) - 1

So this still needs the original code but with a fix for mask computation as following : 

static inline size_t pages_per_hpage(int level)
        return page_level_size(level) / PAGE_SIZE;
 }
        
static int handle_user_rmp_page_fault(struct pt_regs *regs, unsigned long error_code,
                                      unsigned long address)
 {      
       ... 
       pfn = pte_pfn(*pte);
        
        /* If its large page then calculte the fault pfn */
        if (level > PG_LEVEL_4K) {
+               /*
+                * index into the 4K page within the hugepage mapping
+                * in the RMP table
+                */
                unsigned long mask;
        
-               mask = pages_per_hpage(level) - pages_per_hpage(level - 1);
+              mask = pages_per_hpage(level) - 1;
                pfn |= (address >> PAGE_SHIFT) & mask;


Thanks,
Ashish