[Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options

[Paul Brook <paul@codesourcery.com>]

> >> The key difference is that these regions are created and destroyed
> >> rarely and in such a way that the destruction is visible to the guest.
> >
> > So you're making ram unmap an asynchronous process, and requiring that
> > the address space not be reused until that umap has completed?
> 
> It technically already would be.  If you've got a pending DMA
> transaction and you try to hot unplug badness will happen.  This is
> something that is certainly exploitable.

Hmm, I guess we probably want to make this work with all mappings then. DMA to 
a ram backed PCI BAR (e.g. video ram) is certainly feasible.
Technically it's not the unmap that causes badness, it's freeing the 
underlying ram.

For these reasons I'm tempted to push the refcounting down to the ram 
allocation level. This has a couple of nice properties.

Firstly we don't care about dynamic allocation any more. We just say that 
mapping changes may not effect active DMA transactions. If virtio chooses to 
define that the vring DMA transaction starts when the device is enabled and 
ends when disabled, that's fine by me.  This probably requires revisiting the 
memory barrier issues - barriers are pointless if you don't guarantee cache 
coherence (i.e. no bounce buffers).

Secondly, ram deallocation is not guest visible. The guest visible parts 
(memory unmapping) can happen immediately, and we avoid a whole set of 
unplug/replug race conditions. We may want to delay the completion of a 
monitor hotplug command until the actual deallocation occurs, but that's a 
largely separate issue.

Paul