Re: [Qemu-devel] release retrospective, next release timing, numbering

[Laszlo Ersek]

On 04/29/18 16:56, Richard Henderson wrote:
> On 04/27/2018 02:01 PM, Michal Suchánek wrote:
>> Is there any reason why the 64bit emulator would not run on 32bit
>> system? The emulated 64bit system is .. emulated after all.
> 
> It does run, but it requires that the 32-bit host perform double-word
> arithmetic to emulate the 64-bit guest.  When the 64-bit guest is really a
> 32-bit guest in disguise, this carries a performance penalty.
> 
> If we ever stop caring about 32-bit hosts, the question becomes moot.

What about guest RAM size (more precisely, guest-phys address space)?

The x86_64 target might want to use tens of GBs of guest-phys address
space, e.g. for cold-plugged RAM, for DIMM hotplug, for 64-bit PCI MMIO
aperture. To my understanding, all of those have to be expressed with
host virtual addresses in the QEMU process (regardless of TCG vs. KVM).
But on a 32-bit host, the QEMU process only has 4GB HVA space.

Thanks
Laszlo