Re: [Qemu-devel] [PATCH v2] piix: define a TOM register to report the base of PCI

[Ian Campbell]

I'm not sure about qemu-devel but on xen-devel the policy is not to top
post so please could you avoid doping so.

On Wed, 2013-02-27 at 09:49 +0000, Zhang, Xiantao wrote:
> > Given that Xen has at least two other mechanisms (xenstore and
> > hvmparams) for passing this sort of information around I'm not sure why
> > hacking the emulated i440fx device should be the preferred option.
> 
> Actually, even in hardware,  I believe there are many registers which
> are implemented with write-once attributes, and they are only used by
> firmware and reserved for OS.

The i440fx does not have this register (be it write once or otherwise),
which is my actual point -- you are adding a magic property to the
emulation of this device which the real hardware doesn't have. It isn't
really relevant that other hardware could implement write once
registers, that's obviously the case.

> In addition,  with this change,  it can benefit all VMMs (not just
> Xen) which use Qemu as device model.  If adopt xenstore way, perhaps
> other VMMs also have to write similar and duplicate logic for the same
> purpose.

Which other VMM? AIUI qemu/kvm doesn't have a requirement to communicate
this information from the VMM to qemu because qemu is the VMM and
controls all of the hardware resources.

Ian.