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

[Paolo Bonzini]

On 8/30/22 16:42, Peter Xu wrote:
> 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?

Okay, so I thought about it some more and it's quite tricky.

Strictly speaking, the synchronization is just between userspace and 
kernel. The fact that the actual producer of dirty pages is in another 
CPU is a red herring, because reset only cares about harvested pages.

In other words, the dirty page ring is essentially two ring buffers in 
one and we only care about the "harvested ring", not the "produced ring".

On the other hand, it may happen that userspace has set more RESET flags 
while the ioctl is ongoing:


    CPU0                     CPU1               CPU2
                                                fill gfn0
                                                store-rel flags for gfn0
                                                fill gfn1
                                                store-rel flags for gfn1
    load-acq flags for gfn0
    set RESET for gfn0
    load-acq flags for gfn1
    set RESET for gfn1
    do ioctl! ----------->
                             ioctl(RESET_RINGS)
                                                fill gfn2
                                                store-rel flags for gfn2
    load-acq flags for gfn2
    set RESET for gfn2
                             process gfn0
                             process gfn1
                             process gfn2
    do ioctl!
    etc.

The three load-acquire in CPU0 synchronize with the three store-release 
in CPU2, but CPU0 and CPU1 are only synchronized up to gfn1 and CPU1 may 
miss gfn2's fields other than flags.

The kernel must be able to cope with invalid values of the fields, and 
userspace will invoke the ioctl once more.  However, once the RESET flag 
is cleared on gfn2, it is lost forever, therefore in the above scenario 
CPU1 must read the correct value of gfn2's fields.

Therefore RESET must be set with a store-release, that will synchronize 
with a load-acquire in CPU1 as you suggested.

Paolo

> 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