Re: [Qemu-devel] [RFC PATCH 01/10] configure: factor out list of supported Xen/KVM targets

[Stefano Stabellini]

On Mon, 17 Sep 2012, Paolo Bonzini wrote:
> Il 17/09/2012 19:02, Peter Maydell ha scritto:
> > If Xen requires 64 bit physaddrs it should probably just be asserting
> > this here, not randomly changing target_phys_bits. In fact all the
> > supported Xen target archs already have 64 bit physaddrs, so it's
> > harmless. But if there ever were a target Xen arch which didn't support
> > 64 bit phys addrs then the right approach would be to convert it to
> > do so regardless of whether we were using Xen or not.
> 
> No, Xen does not require 64-bit physaddrs, but it supports them even if
> you build it for 32-bit.  Memory is accessed through a "hole" (the
> mapcache) provided by the hypervisor, so that a 32-bit QEMU address
> space is enough to run 64-bit guests.  At least that's my understanding.

Xen requires 64 bit physaddrs because no matter what the specified arch
of the host or the guest at configure time is, the guest could still be
64 bit. In other words: the arch of the guest and the arch of the host
are irrelevant on Xen and physaddrs should always be 64 bit.