Re: [PATCH 4/7] kvm mmu: implement necessary data structures for second huge page accounting

Avi Kivity
2009-03-29T13:15:30Z

Joerg Roedel wrote:
>>
>>> +static int has_wrprotected_largepage(struct kvm *kvm, gfn_t gfn)
>>> +{
>>> + struct kvm_memory_slot *slot;
>>> + int *hugepage_idx;
>>> +
>>> + gfn = unalias_gfn(kvm, gfn);
>>> + slot = gfn_to_memslot_unaliased(kvm, gfn);
>>> + if (slot) {
>>> + hugepage_idx = slot_hugepage_idx(gfn, slot);
>>>
>>>
>> slot_largepage_idx() here?
>>
>> I don't think we ever write protect large pages, so why is this needed?
>>
>
> For 2mb pages we need to check if there is a write-protected 4k page in it
> before we map a 2mb page for writing. If there is any write-protected 4k
> page in a 2mb area this 2mb page is considered write-protected. These
> 'write-protected' 2mb pages are accounted in the account_shadow()
> function. This information is taken into account when we decide if we
> can map a guest 1gb page as a 1gb page on the host too.
>

account_shadowed() actually increments a hugepage write_count by 1 for
every 4K page, not 2M page, if I read the code correctly. The code I
commented on is right though.

The naming is confusing. I suggest
has_wrprotected_page_in_{large,huge}page(). although with the a level
parameter we can keep has_wrprotected_page().

btw, if we implement account_shadow() as you describe it (only account
hugepages on largepage transition 0->1 or 1->0) we save a potential
cacheline bounce on the hugepage write_count accounting array.

--
error compiling committee.c: too many arguments to function