Re: [PATCH v1 1/5] KVM: arm64: Enable ring-based dirty memory tracking

[Peter Xu]

On Fri, Aug 26, 2022 at 04:28:51PM +0100, Marc Zyngier wrote:
> On Fri, 26 Aug 2022 11:58:08 +0100,
> Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> > 
> > On 8/23/22 22:35, Marc Zyngier wrote:
> > >> Heh, yeah I need to get that out the door. I'll also note that Gavin's
> > >> changes are still relevant without that series, as we do write unprotect
> > >> in parallel at PTE granularity after commit f783ef1c0e82 ("KVM: arm64:
> > >> Add fast path to handle permission relaxation during dirty logging").
> > > 
> > > Ah, true. Now if only someone could explain how the whole
> > > producer-consumer thing works without a trace of a barrier, that'd be
> > > great...
> > 
> > Do you mean this?
> >
> > void kvm_dirty_ring_push(struct kvm_dirty_ring *ring, u32 slot, u64 offset)
> 
> Of course not. I mean this:
> 
> static int kvm_vm_ioctl_reset_dirty_pages(struct kvm *kvm)
> {
> 	unsigned long i;
> 	struct kvm_vcpu *vcpu;
> 	int cleared = 0;
> 
> 	if (!kvm->dirty_ring_size)
> 		return -EINVAL;
> 
> 	mutex_lock(&kvm->slots_lock);
> 
> 	kvm_for_each_vcpu(i, vcpu, kvm)
> 		cleared += kvm_dirty_ring_reset(vcpu->kvm, &vcpu->dirty_ring);
> [...]
> }
> 
> and this
> 
> int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring)
> {
> 	u32 cur_slot, next_slot;
> 	u64 cur_offset, next_offset;
> 	unsigned long mask;
> 	int count = 0;
> 	struct kvm_dirty_gfn *entry;
> 	bool first_round = true;
> 
> 	/* This is only needed to make compilers happy */
> 	cur_slot = cur_offset = mask = 0;
> 
> 	while (true) {
> 		entry = &ring->dirty_gfns[ring->reset_index & (ring->size - 1)];
> 
> 		if (!kvm_dirty_gfn_harvested(entry))
> 			break;
> [...]
> 
> }
> 
> which provides no ordering whatsoever when a ring is updated from one
> CPU and reset from another.

Marc,

I thought we won't hit this as long as we properly take care of other
orderings of (a) gfn push, and (b) gfn collect, but after a second thought
I think it's indeed logically possible that with a reversed ordering here
we can be reading some garbage gfn before (a) happens butt also read the
valid flag after (b).

It seems we must have all the barriers correctly applied always.  If that's
correct, do you perhaps mean something like this to just add the last piece
of barrier?

===8<===
diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
index f4c2a6eb1666..ea620bfb012d 100644
--- a/virt/kvm/dirty_ring.c
+++ b/virt/kvm/dirty_ring.c
@@ -84,7 +84,7 @@ static inline void kvm_dirty_gfn_set_dirtied(struct kvm_dirty_gfn *gfn)
 
 static inline bool kvm_dirty_gfn_harvested(struct kvm_dirty_gfn *gfn)
 {
-       return gfn->flags & KVM_DIRTY_GFN_F_RESET;
+       return smp_load_acquire(&gfn->flags) & KVM_DIRTY_GFN_F_RESET;
 }
 
 int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring)
===8<===

Thanks,

-- 
Peter Xu