64-bit guests on 64-bit host without hardware virtualization

[xpclient]

Currently VirtualBox 4 requires hardware virtualization for 64-bit guests under any host OS. Is it easier to virtualize 64-bit guests under 64-bit hosts than 32-bit hosts with and without hardware virtualization or is it the same? MANY random CPU models from Intel don't support hardware virtualization so there is no way for those who are stuck with such processors to ever use 64-bit guests.

[Sasquatch]

The reason hardware-V is required is because the overhead would be too great to emulate 64 bit. Booting such a system would take a long time and lots of resources. Using hardware-V, this barrier can be overcome, as the actual 64 bit extension can be exposed that way.

[mark rumsey]

Perhaps this is more an issue of timing that pure technical difficulties. At the moment there is little need to emulate a 64bit processor. As Sasquatch says, emulation puts a huge overhead on the host, and I suspect it is many times greater than for emulating a 32bit guest, so machines would crawl. So, there are significant gains to be had using hardware virtualisation. The only time it becomes a problem at the moment is if the host hardware does not support hardware virtualisation. This is getting rarer and rarer and really only affects entry level machines that probably aren't really up to 64bit guests anyway. So, its a non-starter. However, that may change in the future. We do not know what will happen with the next generation of processors. Will they be 128bit with 64bit support but no 32 bit subsystems? Will they stay at 64bit but become pure 64bit and be unable to run32bit? Will they become some sort of odd highly parallel hybrid that won't execute 64bit code as we know it? At this stage we just don't know. But, if something like that that were on the horizon there may be a case for having a 64bit emulator. For example, consider a future server processor was pure 128 but (i.e. could not run 64 or 32bit code). It would be accompanied by a 128bit operating system and a suite of 128 bit applications and together they may create the most phenomenal servers ever built. However, most servers run some legacy applications, and since these could be 64bit (or even 32bit) that would be a big problem. Enter VBox with a 64bit processor emulator to save the day. This is, of course, pure speculation. 64bit emulation may happen, it may not. The only thing for certain is if it does ever happen it won't be for a long, long time!