linux-kernel - Re: [PATCH 05/10] KVM: x86: MMU: Use for_each_rmap_spte macro instead of pte_list

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Date:	Sat, 14 Nov 2015 07:20:47 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Takuya Yoshikawa <yoshikawa_takuya_b1@....ntt.co.jp>
Cc:	pbonzini@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/10] KVM: x86: MMU: Use for_each_rmap_spte macro
 instead of pte_list_walk()

On Fri, Nov 13, 2015 at 07:47:28PM -0200, Marcelo Tosatti wrote:
> On Thu, Nov 12, 2015 at 08:52:45PM +0900, Takuya Yoshikawa wrote:
> > kvm_mmu_mark_parents_unsync() alone uses pte_list_walk(), witch does
> > nearly the same as the for_each_rmap_spte macro.  The only difference
> > is that is_shadow_present_pte() checks cannot be placed there because
> > kvm_mmu_mark_parents_unsync() can be called with a new parent pointer
> > whose entry is not set yet.
> > 
> > By calling mark_unsync() separately for the parent and adding the parent
> > pointer to the parent_ptes chain later in kvm_mmu_get_page(), the macro
> > works with no problem.
> > 
> > Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@....ntt.co.jp>
> > ---
> >  arch/x86/kvm/mmu.c | 36 +++++++++++++-----------------------
> >  1 file changed, 13 insertions(+), 23 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > index e8cfdc4..1691171 100644
> > --- a/arch/x86/kvm/mmu.c
> > +++ b/arch/x86/kvm/mmu.c
> > @@ -1007,26 +1007,6 @@ static void pte_list_remove(u64 *spte, unsigned long *pte_list)
> >  	}
> >  }
> >  
> > -typedef void (*pte_list_walk_fn) (u64 *spte);
> > -static void pte_list_walk(unsigned long *pte_list, pte_list_walk_fn fn)
> > -{
> > -	struct pte_list_desc *desc;
> > -	int i;
> > -
> > -	if (!*pte_list)
> > -		return;
> > -
> > -	if (!(*pte_list & 1))
> > -		return fn((u64 *)*pte_list);
> > -
> > -	desc = (struct pte_list_desc *)(*pte_list & ~1ul);
> > -	while (desc) {
> > -		for (i = 0; i < PTE_LIST_EXT && desc->sptes[i]; ++i)
> > -			fn(desc->sptes[i]);
> > -		desc = desc->more;
> > -	}
> > -}
> > -
> >  static unsigned long *__gfn_to_rmap(gfn_t gfn, int level,
> >  				    struct kvm_memory_slot *slot)
> >  {
> > @@ -1741,7 +1721,12 @@ static struct kvm_mmu_page *kvm_mmu_alloc_page(struct kvm_vcpu *vcpu,
> >  static void mark_unsync(u64 *spte);
> >  static void kvm_mmu_mark_parents_unsync(struct kvm_mmu_page *sp)
> >  {
> > -	pte_list_walk(&sp->parent_ptes, mark_unsync);
> > +	u64 *sptep;
> > +	struct rmap_iterator iter;
> > +
> > +	for_each_rmap_spte(&sp->parent_ptes, &iter, sptep) {
> > +		mark_unsync(sptep);
> > +	}
> >  }
> >  
> >  static void mark_unsync(u64 *spte)
> > @@ -2111,12 +2096,17 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
> 
> Faulting a spte, and one of the levels of sptes, either
> 
> 
> 		spte-1
> 		spte-2
> 		spte-3
> 
> has present bit clear. So we're searching for a guest page to shadow, with
> gfn "gfn".
> 
> >  		if (sp->unsync && kvm_sync_page_transient(vcpu, sp))
> >  			break;
> 
> If a shadow for gfn exists, but is unsync, sync guest-page ---to--> kvm
> sptes.
> 
> > -		mmu_page_add_parent_pte(vcpu, sp, parent_pte);
> 
> add "gfn" (actually its "struct kvm_mmu_page *sp" pointer) to
> the parent.
> >  		if (sp->unsync_children) {
> >  			kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
> >  			kvm_mmu_mark_parents_unsync(sp);
> 
> kvm_mmu_mark_parents_unsync relied on the links from current level all
> the way to top level to mark all levels unsync, so that on guest entry,
> KVM_REQ_MMU_SYNC is processed and any level is brought from guest -->
> kvm pages. This now fails, because you removed "mmu_page_add_parent_pte"
> (the link is not formed all the way to root).
> 
> Unless i am missing something, this is not correct.

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:
	level1 

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.

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