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

[Paul Brook]

> >> With a new api, cpu_physical_memory_map() changes semantics.  It only
> >> returns pointers for static ram mappings.  Everything else is bounced
> >> which guarantees that an address can't change during DMA.
> >
> > Doesn't this mean that only the initial RAM is directly DMA-able?
> >
> > While memory hotplug(and unplug) may be an infrequent event, having the
> > majority of ram be hotplug seems much more likely.
> 
> Hotplug works fine for direct DMA'ing.  map/unmap would maintain a
> reference count on the registered RAM region and hot unplug would not be
> allowed until that reference dropped to zero.  For something like
> virtio, it means that the driver has to be unloaded in the guest before
> you hot unplug the region of memory if it happens to be using that
> region of memory for the ring storage.
> 
> 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?

Paul