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Date: Thu, 30 Mar 2017 00:28:36 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] issues of region cache and iommu reset
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>, Peter Xu <peterx@redhat.com>, qemu devel <qemu-devel@nongnu.org>, Cornelia Huck <cornelia.huck@de.ibm.com>

On Wed, Mar 29, 2017 at 10:09:10AM +0200, Paolo Bonzini wrote:
> 
> 
> On 29/03/2017 10:00, Jason Wang wrote:
> > 
> > 
> > 1) vtd was reset first
> > 
> > 2) during the reset of virtio-net-pci #1, deletion of msix subregion
> > will cause a commit of all memory listeners
> > 
> > 3) virito-net-pci #2's region cache will be update, but since vtd has
> > already been reset, it can't get a valid mappings here
> >
> > Any ideas on how to fix this? Need region cache be aware of IOMMU/IOTLB
> > state in this case? Or can we simply reset IOMMU as the last one?
> 
> Something like this?
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 03592c5..73e69ac 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -176,6 +176,10 @@ err_used:
>      address_space_cache_destroy(&new->desc);
>  err_desc:
>      g_free(new);
> +    atomic_rcu_set(&vq->vring.caches, NULL);
> +    if (old) {
> +        call_rcu(old, virtio_free_region_cache, rcu);
> +    }
>  }
> 
>  /* virt queue functions */

I would be worried about call_rcu here - this means
something can hang on to and use the old cache,
and reset really must act as a sync/flush point.


> Paolo