Re: [PATCH v2 1/10] KVM: MMU: fix writable sync sp mapping

From: Xiao Guangrong
Date: Mon Jun 28 2010 - 05:44:33 EST

Avi Kivity wrote:

>> for_each_gfn_indirect_valid_sp(vcpu->kvm, s, gfn, node) {
>> + if (!can_unsync)
>> + return 1;
>> +
> What if the page is already unsync? We don't need write protection in
> this case.


The reason is when we sync children sps, we write-protected for all sps first,
list relevant code:

| static void mmu_sync_children(...)
| {
| ......
| for_each_sp(pages, sp, parents, i)
| protected |= rmap_write_protect(vcpu->kvm, sp->gfn); <==== A
| if (protected)
| kvm_flush_remote_tlbs(vcpu->kvm);
| for_each_sp(pages, sp, parents, i) {
| kvm_sync_page(vcpu, sp, &invalid_list); <==== B
| mmu_pages_clear_parents(&parents);
| }
| ......

For example:

SP1.pte[0] = P
SP2.gfn's pfn = P
[SP1.pte[0] = SP2.gfn's pfn]

At A point, SP1.gfn and SP2.gfn are write-protected.

At B point, if sync SP1 first, while it's synced. it will detect SP1.pte[0].gfn only has one unsync-sp,
that is SP2, so it will mapping it writable, then we sync SP2, we will set SP2 to sync page.

The final result is: SP2 is the sync page but SP2.gfn is writable.
Note: we not do write-protected in kvm_sync_page() anymore after commit: 95b4b26cfc
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