Re: [PATCH v4] x86: add etc/phys-bits fw_cfg file

[Paolo Bonzini]

On Thu, Sep 22, 2022 at 7:13 PM Jim Mattson <jmattson@google.com> wrote:
> > Treating 40 as invalid and continue to use the current conservative
> > heuristic, otherwise treat phys-bits as valid might work.  Obvious
> > corner case is that it'll not catch broken manual configurations
> > (host-phys-bits=off,phys-bits=<larger-than-host>), only the broken
> > default.  Not sure how much of a problem that is in practice, maybe
> > it isn't.
> >
> > I think I still prefer to explicitly communicate a reliable phys-bits
> > value to the guest somehow.
>
> On x86 hardware, KVM is incapable of emulating a guest physical width
> that differs from the host physical width. There isn't support in the
> hardware for it.

Indeed, everything else is a userspace bug. Especially since here
we're talking of host_maxphyaddr < guest_maxphyaddr, which is
completely impossible.

Paolo