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

[Marc Zyngier]

On 2013-08-14 07:32, Alexander Graf wrote:
> On 13.08.2013, at 20:03, Peter Maydell wrote:
>
> > These patches add support to target-arm for '-cpu host'.
> > The general semantics are the same as for ppc and x86 (ie "whatever
> > the host kernel can support that looks basically like the host
> > CPU"), but the mechanism is a little different.
> >
> > The kernel API (currently still proposed rather than implemented,
> > hence the RFC nature of this patchset) is that the existing
> > KVM_ARM_VCPU_INIT supports a new 'target' value KVM_ARM_TARGET_HOST,
> > which it treats as "whatever you are". On the userspace side,
> > we use this if the kernel supports it. If it doesn't then we
> > know the kernel must be an A15-on-A15 only one, and so can
> > safely implement '-cpu host' with KVM_ARM_TARGET_CORTEX_A15.
>
> How do you know what core specific registers QEMU can expect from
> this particular CPU? Imagine ARM changes the MMU implementation in 
> the
> next ARMv8 CPU. With -cpu host you wouldn't be able to support 
> gdbstub
> anymore, because you wouldn't know whether the MMU format is the old
> or the new one.

Well, we're already there. A15 supports both classic page tables and 
LPAE.

You can discover which one is being used by inspecting the guest, and 
I'd
fully expect future versions of the architecture to expose bits in the 
ID
registers indicating the various supported extensions.

        M.
--
Fast, cheap, reliable. Pick two.