Re: [PATCH v2] KVM: x86/MMU: Do not check unsync status for root SP.

[Yu Zhang]

On Mon, Feb 08, 2021 at 12:36:57PM +0100, Paolo Bonzini wrote:
> On 07/02/21 13:22, Yu Zhang wrote:
> > In shadow page table, only leaf SPs may be marked as unsync.
> > And for non-leaf SPs, we use unsync_children to keep the number
> > of the unsynced children. In kvm_mmu_sync_root(), sp->unsync
> > shall always be zero for the root SP, , hence no need to check
> > it. Instead, a warning inside mmu_sync_children() is added, in
> > case someone incorrectly used it.
> > 
> > Also, clarify the mmu_need_write_protect(), by moving the warning
> > into kvm_unsync_page().
> > 
> > Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> 
> This should really be more of a Co-developed-by, and there are a couple
> adjustments that could be made in the commit message.  I've queued the patch
> and I'll fix it up later.

Indeed. Thanks for the remind, and I'll pay attention in the future. :)

B.R.
Yu

> 
> Paolo
> 
> > ---
> > Changes in V2:
> > - warnings added based on Sean's suggestion.
> > 
> >   arch/x86/kvm/mmu/mmu.c | 12 +++++++++---
> >   1 file changed, 9 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 86af582..c4797a00cc 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -1995,6 +1995,12 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu,
> >   	LIST_HEAD(invalid_list);
> >   	bool flush = false;
> > +	/*
> > +	 * Only 4k SPTEs can directly be made unsync, the parent pages
> > +	 * should never be unsyc'd.
> > +	 */
> > +	WARN_ON_ONCE(sp->unsync);
> > +
> >   	while (mmu_unsync_walk(parent, &pages)) {
> >   		bool protected = false;
> > @@ -2502,6 +2508,8 @@ int kvm_mmu_unprotect_page(struct kvm *kvm, gfn_t gfn)
> >   static void kvm_unsync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
> >   {
> > +	WARN_ON(sp->role.level != PG_LEVEL_4K);
> > +
> >   	trace_kvm_mmu_unsync_page(sp);
> >   	++vcpu->kvm->stat.mmu_unsync;
> >   	sp->unsync = 1;
> > @@ -2524,7 +2532,6 @@ bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
> >   		if (sp->unsync)
> >   			continue;
> > -		WARN_ON(sp->role.level != PG_LEVEL_4K);
> >   		kvm_unsync_page(vcpu, sp);
> >   	}
> > @@ -3406,8 +3413,7 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
> >   		 * mmu_need_write_protect() describe what could go wrong if this
> >   		 * requirement isn't satisfied.
> >   		 */
> > -		if (!smp_load_acquire(&sp->unsync) &&
> > -		    !smp_load_acquire(&sp->unsync_children))
> > +		if (!smp_load_acquire(&sp->unsync_children))
> >   			return;
> >   		write_lock(&vcpu->kvm->mmu_lock);
> > 
>