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

Joerg Roedel
2009-03-29T13:04:16Z

On Sun, Mar 29, 2009 at 02:45:44PM +0300, Avi Kivity wrote:
> Joerg Roedel wrote:
>> This patch adds the necessary data structures to take care of write
>> protections in place within a second huge page sized page.
>>
>> struct kvm_vcpu_arch {
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 9936b45..7d4162d 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -390,6 +390,15 @@ static int *slot_largepage_idx(gfn_t gfn, struct kvm_memory_slot *slot)
>> return &slot->lpage_info[idx].write_count;
>> }
>> +static int *slot_hugepage_idx(gfn_t gfn, struct kvm_memory_slot
>> *slot)
>> +{
>> + unsigned long idx;
>> +
>> + idx = (gfn / KVM_PAGES_PER_1G_PAGE) -
>> + (slot->base_gfn / KVM_PAGES_PER_1G_PAGE);
>> + return &slot->hpage_info[idx].write_count;
>> +}
>>
>
> A page level argument would remove the need for this duplication, as
> well as all the constants.
>
>> +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.

Joerg