Re: [PATCH 05/10] KVM: x86: MMU: Use for_each_rmap_spte macro instead of pte_list_walk()

From: Takuya Yoshikawa
Date: Sun Nov 15 2015 - 21:51:03 EST

On 2015/11/14 18:20, Marcelo Tosatti wrote:

The actual issue is this: a higher level page that had, under its children,
no out of sync pages, now, due to your addition, a child that is unsync:

initial state:

final state:

level1 -x-> level2 -x-> level3

Where -x-> are the links created by this pagefault fixing round.

If _any_ page under you is unsync (not necessarily the ones this
pagefault is accessing), you have to mark parents unsync.

I understand this, but I don't think my patch will break this.

What kvm_mmu_mark_parents_unsync() does is:

for each p_i in sp->parent_ptes rmap chain

Then, mark_unsync() finds the parent sp including that p_i to
set ->unsync_child_bitmap and increment ->unsync_children if
necessary. It may also call kvm_mmu_mark_parents_unsync()

I understand we need to tell the parents "you have an unsync
child/descendant" until this information reaches the top level
by that recursive calls.

But since these recursive calls cannot come back to the starting sp,
the child->parent graph has no loop, each mark_unsync(p_i) will not
be affected by other parents in that sp->parent_ptes rmap chain,
from which we started the recursive calls.

As the following code shows, my patch does mark_unsync(parent_pte)
separately, and then mmu_page_add_parent_pte(vcpu, sp, parent_pte):

- } else if (sp->unsync)
+ if (parent_pte)
+ mark_unsync(parent_pte);
+ } else if (sp->unsync) {
+ if (parent_pte)
+ mark_unsync(parent_pte);
+ }
+ mmu_page_add_parent_pte(vcpu, sp, parent_pte);

So, as you worried, during each mark_unsync(p_i) is processed,
this parent_pte does not exist in that sp->parent_ptes rmap chain.

But as I explained above, this does not change anything about what
each mark_unsync(p_i) call does, so keeps the original behaviour.

By the way, I think "kvm_mmu_mark_parents_unsync" and "mark_unsync"
do not tell what they actually do well. When I first saw the names,
I thought they would just set the parents' sp->unsync.

To reflect the following meaning better, it should be
propagate_unsync(_to_parents) or something:

Tell the parents "you have an unsync child/descendant"
until this unsync information reaches the top level


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