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

[Xiao Guangrong]

On 12/17/2015 10:44 AM, Kai Huang wrote:
> On 12/16/2015 04:39 PM, Xiao Guangrong wrote:
> > On 12/16/2015 03:51 PM, Kai Huang wrote:
> > > 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?
> >
> > non-leaf shadow pages are keepking write-protected which page fault handler can not fix write
> > access on it. And leaf shadow pages are not.
> My point is the original code didn't separate the two cases so I am not sure why you need to
> separate. Perhaps you want to make account_shadowed imply the non-leaf guest page table is
> write-protected while leaf page table is not.

That is why we get improvement after this patchset, we seep up the case for the write access
happens on non-leaf page tables. ;)

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