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SubjectRe: [PATCH v5 1/2] mm: make dev_pagemap_mapping_shift() externally visible
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On 12/13/19 12:47 PM, Sean Christopherson wrote:
>> +unsigned long dev_pagemap_mapping_shift(unsigned long address,
>> + struct mm_struct *mm)
>> +{
>> + pgd_t *pgd;
>> + p4d_t *p4d;
>> + pud_t *pud;
>> + pmd_t *pmd;
>> + pte_t *pte;
>> +
>> + pgd = pgd_offset(mm, address);
>> + if (!pgd_present(*pgd))
>> + return 0;
>> + p4d = p4d_offset(pgd, address);
>> + if (!p4d_present(*p4d))
>> + return 0;
>> + pud = pud_offset(p4d, address);
>> + if (!pud_present(*pud))
>> + return 0;
>> + if (pud_devmap(*pud))
>> + return PUD_SHIFT;
>> + pmd = pmd_offset(pud, address);
>> + if (!pmd_present(*pmd))
>> + return 0;
>> + if (pmd_devmap(*pmd))
>> + return PMD_SHIFT;
>> + pte = pte_offset_map(pmd, address);
>> + if (!pte_present(*pte))
>> + return 0;
>> + if (pte_devmap(*pte))
>> + return PAGE_SHIFT;
>> + return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(dev_pagemap_mapping_shift);
>
> This is basically a rehash of lookup_address_in_pgd(), and doesn't provide
> exactly what KVM needs. E.g. KVM works with levels instead of shifts, and
> it would be nice to provide the pte so that KVM can sanity check that the
> pfn from this walk matches the pfn it plans on mapping.

One minor issue is that the levels for lookup_address_in_pgd() and for
KVM differ in name, although not in value. lookup uses PG_LEVEL_4K = 1.
KVM uses PT_PAGE_TABLE_LEVEL = 1. The enums differ a little too: x86
has a name for a 512G page, etc. It's all in arch/x86.

Does KVM-x86 need its own names for the levels? If not, I could convert
the PT_PAGE_TABLE_* stuff to PG_LEVEL_* stuff.

>
> Instead of exporting dev_pagemap_mapping_shift(), what about replacing it
> with a patch to introduce lookup_address_mm() and export that?
>
> dev_pagemap_mapping_shift() could then wrap the new helper (if you want),

I might hold off on that for now, since that helper is used outside of
x86, and I don't know if 'level' makes sense outside of x86.

Thanks,

Barret

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