Re: [Qemu-devel] [RFC 0/2] target-arm: Provide '-cpu host' when running KVM

[Peter Maydell]

On 26 August 2013 12:18, Gleb Natapov <address@hidden> wrote:
> On Sun, Aug 25, 2013 at 04:11:51PM +0100, Peter Maydell wrote:
>> A lot are feature bits that are simply mandatory
>> for any ARMv7 or later processor (which is what you need for KVM
>> to work at all).
> What if you want to emulate older arm arch for a guest?

I don't believe that was a design goal architecturally and it's
not a design goal for KVM/ARM either. If anybody actually
cares about that use case they're welcome to submit patches.
In any case, feature bits are probably the least of your worries
(in that if the guest looks at them and uses features not strictly
present on the older CPU it's still going to run, and if it doesn't
look at them then it doesn't care what the bit says); to pick
some random examples:
 * no way to make a v7 core provide v5 unaligned-access
   behaviour
 * a v6 OS is unlikely to be able to correctly manage/clean/
   invalidate the kind of cache a v7 CPU has
 * no support for the ARM v5 legacy pagetable format
There are no doubt others.

-- PMM