Re: [Qemu-devel] -cpu host (was Re: KVM call minutes for 2013-08-06)

[Peter Maydell]

On 8 August 2013 13:51, Peter Maydell <address@hidden> wrote:
> For ARM you can't get at feature info of the host from userspace
> (unless you want to get into parsing /proc/cpuinfo), so my current
> idea is to have KVM_ARM_VCPU_INIT support a target-cpu-type
> which means "whatever host CPU is".

To expand on this for the 64 bit situation:
 * although in theory we could support a 32-bit-compiled QEMU
   binary on a 64-bit host kernel, I think there's not much
   need for it
 * if you run a 64-bit QEMU on a 64-bit host and ask VCPU_INIT
   for a 'host' CPU, you get a 64 bit CPU
 * you can add the feature flag '32 bit VM please' when
   making the VCPU_INIT call, which gets you the same
   host CPU but forced into 32 bit mode (this flag & behaviour
   exist in the kernel today) -- in QEMU I guess we have a
   '-cpu host32' which drives this, or possibly add support
   for "-cpu host,+32bitvm" style syntax.

NB that the API for reading and writing registers isn't the
same for "64 bit CPU in 32 bit mode" as for a native 32 bit
CPU -- the view of the guest that QEMU sees in the former
case is the same view that a 64 bit hypervisor sees of a
32 bit guest. I think that to avoid huge ifdefs it will
be cleaner to have
 target-arm/kvm.c [common functions]
 target-arm/kvm32.c [init_vcpu, get_registers, etc for 32 bit]
 target-arm/kvm64.c [ditto, 64 bit]

and configure only sets CONFIG_KVM for aarch64-on-aarch64
and arm-on-arm.

-- PMM