Re: [PATCH 09/16] KVM: x86/mmu: Move private vs. shared check above slot validity checks

[Sean Christopherson <seanjc@google.com>]

On Mon, Mar 11, 2024, Xu Yilun wrote:
> On Fri, Mar 08, 2024 at 03:28:08PM -0800, Sean Christopherson wrote:
> > On Fri, Mar 08, 2024, Xu Yilun wrote:
> > > On Tue, Feb 27, 2024 at 06:41:40PM -0800, Sean Christopherson wrote:
> > > > @@ -4410,6 +4405,16 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> > > >  	fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;
> > > >  	smp_rmb();
> > > >  
> > > > +	/*
> > > > +	 * Check for a private vs. shared mismatch *after* taking a snapshot of
> > > > +	 * mmu_invalidate_seq, as changes to gfn attributes are guarded by the
> > > > +	 * invalidation notifier.
> > > 
> > > I didn't see how mmu_invalidate_seq influences gfn attribute judgement.
> > > And there is no synchronization between the below check and
> > > kvm_vm_set_mem_attributes(), the gfn attribute could still be changing
> > > after the snapshot.
> > 
> > There is synchronization.  If kvm_vm_set_mem_attributes() changes the attributes,
> > and thus bumps mmu_invalidate_seq, after kvm_faultin_pfn() takes its snapshot,
> > then is_page_fault_stale() will detect that an invalidation related to the gfn
> > occured and resume the guest *without* installing a mapping in KVM's page tables.
> > 
> > I.e. KVM may read the old, stale gfn attributes, but it will never actually
> > expose the stale attirubtes to the guest.
> 
> That makes sense!  I was just thinking of the racing for below few lines,
> 
> 	if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
> 		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> 		return -EFAULT;
> 	}
> 
> But the guarding is actually for the whole kvm_faultin_pfn(). It is the
> the same mechanism between getting old gfn attributes and getting old pfn.
> 
> I wonder if we could instead add some general comments at
> 
>    fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;
> 
> about the snapshot and is_page_fault_stale() thing.

Yeah, I'll add a comment.  The only reason not to add a comment is that, ideally,
the comment/documentation would live in common KVM code, not x86.  But this code
already has a few big comments about the mmu_notifier retry logic, one more
definitely won't hurt.