Re: [Qemu-devel] [Xen-devel] [RFC for 2.8 0/3] Drop support for 64 bit guests on 32 bit hosts

[Stefano Stabellini]

On Wed, 10 Aug 2016, Gerd Hoffmann wrote:
> On Di, 2016-08-09 at 16:55 +0100, Alex Bennée wrote:
> > Hi,
> > 
> > I'm proposing for the 2.8 cycle we officially drop supporting 64 bit
> > guests on 32 bit hosts. For most of the KVM targets it doesn't make
> > any sense anyway and for TCG it makes things harder (e.g. supporting
> > 64 bit atomics on a 32 bit platform). I'm not actually convinced
> > things actually work if built or that anyone relies on these
> > combinations. Consider these patches a way of flushing any such users
> > out ;-)
> 
> Adding xen-devel to Cc.
> 
> 64bit xen hypervisor with 32bit dom0 running 32bit qemu, providing
> device emulation for 64bit dumUs at least used to be a common setup
> years ago.  I have my doubts this is still the case in recent xen
> versions, but I guess we better ask the Xen guys ...
> 
> Can anyone clarify?

Hi Gerd, thanks for CC'ing xen-devel on this.

Although 32bit Dom0 deployments are not as common as they used to be,
they are still certainly a valid scenario. However for them to work,
QEMU doesn't really need to support TCG 32bit on 64bit. QEMU when used
with Xen as accelerator doesn't actually do any CPU emulation; there was
even a proposal to remove the CPU emulation completely from Xen machines
a couple of years back
(http://marc.info/?l=qemu-devel&m=139051862915170&w=2). I hope this
helps.