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For some HVA, their pte are zero, without page fault

From: 李玮
Subject: For some HVA, their pte are zero, without page fault
Date: Sat, 12 Jun 2021 17:05:52 +0800

Background: qemu-4.2.0 qemu-system-i386,  host os is linux-kernel 4.19.193 ubuntu desktop

start qemu-system-i386 with its seabios, during the bios process, there are two store operations: store 0x6ffc, store 0xd4ad0 by store_helper.
0x6ffc and 0xd4ad0 are GVA, their HVA maybe are 7f65efe06ffc, 7f65efed4ad0.
Then I wrote a kernel module to try to get their HPA.

static inline void QEMU_ALWAYS_INLINE
store_helper(CPUArchState *env, target_ulong addr, uint64_t val,
             TCGMemOpIdx oi, uintptr_t retaddr, MemOp op)
    haddr = (void *)((uintptr_t)addr + entry->addend);
    store_memop(haddr, val, op);
++    print_entry_hpa(haddr);

void  print_entry_hpa() // in kernel module
    hpa_pgd = pgd_offset(mm, hpa);
    printk("pgd: %llx\n", pgd_val(*hpa_pgd));
    hpa_pud = pud_offset(&__p4d(pgd_val(*hpa_pgd)), hpa);
    printk("pud: %llx\n", pud_val(*hpa_pud));
    hpa_pmd = pmd_offset(hpa_pud, hpa);
    printk("pmd: %llx\n", pmd_val(*hpa_pmd));
    hpa_pte = pte_offset_kernel(hpa_pmd, hpa);
    printk("pte: %llx\n", pte_val(*hpa_pte));

However, I found that their PTE are both 0.

[19772.167927] pgd: 800000021c310067
[19772.167928] pud: 20fe22067
[19772.167929] pmd: 80000002142008e7
[19772.167929] pte: 0

Then I wrote a line of printk in kernel:/arch/x86/mm/fault.c, which is the interrupt entry function of page fault.

static noinline void
__do_page_fault(struct pt_regs *regs, unsigned long error_code,
unsigned long address)
    struct vm_area_struct *vma;
    struct task_struct *tsk;
    struct mm_struct *mm;
    vm_fault_t fault, major = 0;
    u32 pkey;
++    if((address & 0xFFFFF) == 0xd4ad0)
++       printk("__do_page_fault addr: %lx\n", address);
    tsk = current;
    mm = tsk->mm;

But in dmesg I caught nothing that means the zero pte didn't trigger a page fault.
It's so strange, the value of store 0x6ffc and store 0xd4ad0 is not zero and the qemu runs normally.

SO what's the problem?

Thanks a lot!   
Wei Li

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