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
