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SubjectRe: [PATCH v3 08/15] KVM: MMU: allow unmap invalid rmap out of mmu-lock
On Thu, Apr 18, 2013 at 07:22:23PM +0800, Xiao Guangrong wrote:
> On 04/18/2013 07:00 PM, Gleb Natapov wrote:
> > On Tue, Apr 16, 2013 at 02:32:46PM +0800, Xiao Guangrong wrote:
> >> pte_list_clear_concurrently allows us to reset pte-desc entry
> >> out of mmu-lock. We can reset spte out of mmu-lock if we can protect the
> >> lifecycle of sp, we use this way to achieve the goal:
> >>
> >> unmap_memslot_rmap_nolock():
> >> for-each-rmap-in-slot:
> >> preempt_disable
> >> kvm->arch.being_unmapped_rmap = rmapp
> >> clear spte and reset rmap entry
> >> kvm->arch.being_unmapped_rmap = NULL
> >> preempt_enable
> >>
> >> Other patch like zap-sp and mmu-notify which are protected
> >> by mmu-lock:
> >> clear spte and reset rmap entry
> >> retry:
> >> if (kvm->arch.being_unmapped_rmap == rmap)
> >> goto retry
> >> (the wait is very rare and clear one rmap is very fast, it
> >> is not bad even if wait is needed)
> >>
> > I do not understand what how this achieve the goal. Suppose that rmap
> > == X and kvm->arch.being_unmapped_rmap == NULL so "goto retry" is skipped,
> > but moment later unmap_memslot_rmap_nolock() does
> > vm->arch.being_unmapped_rmap = X.
>
> Access rmap is always safe since rmap and its entries are valid until
> memslot is destroyed.
>
> This algorithm protects spte since it can be freed in the protection of mmu-lock.
>
> In your scenario:
>
> ======
> CPU 1 CPU 2
>
> vcpu / mmu-notify access the RMAP unmap rmap out of mmu-lock which is under
> which is under mmu-lock slot-lock
>
> zap spte1
> clear RMAP entry
>
> kvm->arch.being_unmapped_rmap = NULL,
> do not wait
>
> free spte1
>
> set kvm->arch.being_unmapped_rmap = RMAP
> walking RMAP and do not see spet1 on RMAP
> (the entry of spte 1 has been reset by CPU 1)
and what prevents this from happening concurrently with "clear RMAP
entry"? Is it safe?

> set kvm->arch.being_unmapped_rmap = NULL
> ======
>
> That protect CPU 2 can not access the freed-spte.
>

--
Gleb.


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