Re: [kvm-devel] [Qemu-devel] Re: [PATCH 1/6] Use correct types to enable > 2G support

[Daniel P. Berrange]

On Fri, Feb 01, 2008 at 11:53:02AM -0600, Anthony Liguori wrote:
> Ian Jackson wrote:
> > Anthony Liguori writes ("[Qemu-devel] Re: [kvm-devel] [PATCH 1/6] Use 
> > correct types to enable > 2G support"):
> >   
> >> The alternative is to change all the places that assume phys_ram_base + 
> >> PA which I don't like very much.
> >>     
> >
> > We would ideally like to do this for Xen, at least in the places we
> > care about.  (Xen uses less of the qemu tree than KVM, I think.)
> >   
> 
> Support for the map cache in the Xen tree is a rather big change that 
> I'm not going to attempt to support it in this patch series.
> 
> I'd rather preserve the phys_ram_base + PA assumption because it allows 
> us to be able to do support > 1 page DMA operations for our virtual IO 
> drivers.  If you break the assumption that physically contiguous memory 
> in the guest is virtual contiguous memory in the host, things get pretty 
> ugly.

Well Xen i386 has no choice but to use the map cache, since PAE lets 
i386 guests have as much as 100 GB of memory & there's no way you can
map that into QEMU's 32-bit userspace. So if virt IO has a dependancy
on contigious memory access in QEMU its not going to play nice with
Xen.

Dan.
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