Re: [PATCH 08/11] KVM: MMU: use page track for non-leaf shadow pages

[Kai Huang]

On 12/15/2015 05:10 PM, Xiao Guangrong wrote:
> On 12/15/2015 03:52 PM, Kai Huang wrote:
>
> > >   static bool __mmu_gfn_lpage_is_disallowed(gfn_t gfn, int level,
> > > @@ -2140,12 +2150,18 @@ static struct kvm_mmu_page 
> > > *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
> > >       hlist_add_head(&sp->hash_link,
> > > &vcpu->kvm->arch.mmu_page_hash[kvm_page_table_hashfn(gfn)]);
> > >       if (!direct) {
> > > -        if (rmap_write_protect(vcpu, gfn))
> > > +        /*
> > > +         * we should do write protection before syncing pages
> > > +         * otherwise the content of the synced shadow page may
> > > +         * be inconsistent with guest page table.
> > > +         */
> > > +        account_shadowed(vcpu->kvm, sp);
> > > +
> > > +        if (level == PT_PAGE_TABLE_LEVEL &&
> > > +              rmap_write_protect(vcpu, gfn))
> > >               kvm_flush_remote_tlbs(vcpu->kvm);
> > I think your modification is good but I am little bit confused here. 
> > In account_shadowed, if
> > sp->role.level > PT_PAGE_TABLE_LEVEL, the sp->gfn is write protected, 
> > and this is reasonable. So why
> > write protecting the gfn of PT_PAGE_TABLE_LEVEL here?
>
> Because the shadow page will become 'sync' that means the shadow page 
> will be synced
> with the page table in guest. So the shadow page need to be 
> write-protected to avoid
> the guest page table is changed when we do the 'sync' thing.
>
> The shadow page need to be write-protected to avoid that guest page 
> table is changed
> when we are syncing the shadow page table. See kvm_sync_pages() after 
> doing
> rmap_write_protect().
I see. So why are you treat PT_PAGE_TABLE_LEVEL gfn separately here? why 
this cannot be done in account_shadowed, as you did for upper level sp? 
Actually I am thinking whether account_shadowed is overdoing things. 
From it's name it should only *account* shadow sp, but now it also does 
write protection and disable large page mapping.

Thanks,
-Kai
> > >   /*
> > >    * remove the guest page from the tracking pool which stops the 
> > > interception
> > >    * of corresponding access on that page. It is the opposed 
> > > operation of
> > > @@ -134,20 +160,12 @@ void kvm_page_track_remove_page(struct kvm 
> > > *kvm, gfn_t gfn,
> > >       struct kvm_memory_slot *slot;
> > >       int i;
> > > -    WARN_ON(!check_mode(mode));
> > > -
> > >       for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> > >           slots = __kvm_memslots(kvm, i);
> > >           slot = __gfn_to_memslot(slots, gfn);
> > >           spin_lock(&kvm->mmu_lock);
> > > -        update_gfn_track(slot, gfn, mode, -1);
> > > -
> > > -        /*
> > > -         * allow large page mapping for the tracked page
> > > -         * after the tracker is gone.
> > > -         */
> > > -        kvm_mmu_gfn_allow_lpage(slot, gfn);
> > > +        kvm_slot_page_track_remove_page_nolock(kvm, slot, gfn, mode);
> > Looks you need to merge this part with patch 1, as you are modifying
> > kvm_page_track_{add,remove}_page here, which are introduced in your 
> > patch 1.
>
> Indeed, it is better.

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