Re: [RFC PATCH 17/17] KVM: PPC: Add an ioctl for userspace to select which platform to emulate

On 06/29/2011 02:58 PM, Josh Boyer wrote:

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 This makes me wonder if a similar thing might eventually be usable for
 running an i686 or x32 guest on an x86_64 KVM host.  I have no idea if
 that is even theoretically possible, but if it is it might be better to
 rename the ioctl to be architecture agnostic.

On x86 this is not required unless we want to "virtualize" pre-CPUID CPUs. Everything as of Pentium has a full bitmap of feature capabilities that KVM gets from user space, including information such as "Can we do 64-bit mode?".

Ah.  Thank you for the explanation.

To clarify a bit further, running an i686 guest on an x86_64 host is not only theoretically possible, but is done regularly.  First, x86_64 is backwards compatible with i686 (so you can install a 32-bit OS on 64-bit hardware), and second, you can impersonate 32-bit guest hardware on a 64-bit host.

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