Re: [Qemu-arm] [Qemu-devel] [RFC 5/6] hw/arm/virt: support kvm_type property

[Thomas Huth]

On 03.07.2018 14:31, Auger Eric wrote:
> Hi Drew,
> 
> On 07/03/2018 01:55 PM, Andrew Jones wrote:
>> On Wed, Jun 20, 2018 at 03:07:32PM +0200, Eric Auger wrote:
>>> The kvm-type property currently is used to pass
>>> a user parameter to KVM_CREATE_VM. This matches
>>> the way KVM/ARM expects to pass the max_vm_phys_shift
>>> parameter.
>>>
>>> This patch adds the support or the kvm-type property in
>>> machvirt and also implements the machine class kvm_type()
>>> callback so that it either returns the kvm-type value
>>> provided by the user or returns the max_vm_phys_shift
>>> exposed by KVM.
>>>
>>> for instance, the usespace can use the following option to
>>> instantiate a 42b IPA guest: -machine kvm-type=42
>>
>> 'kvm-type' is a very generic name. It looks like you're creating a KVM
>> VM of type 42 (which I assume is the ultimate KVM VM that answers the
>> meaning to Life, the Universe, and Everything), but it's not obvious
>> how it relates to physical address bits. Why not call this property
>> something like 'min_vm_phys_shift'? Notice the 'min' in the name,
>> because this is where the user is stating what the minimum number of
>> physical address bits required for the VM is. IIUC, if KVM supports
>> more, then it shouldn't be a problem.
> 
> Well I agree with you that using kvm-type=42 is not very nice.
> 
> On the other hand the current kernel API to pass the VM GPA address size
> is though the KVM_CREATE_VM kvm_type argument.
> 
> in accel/kvm/kvm-all.c there is all the infrastructure to fetch the
> generic machine kvm-type machine option and decode it into type, which
> is passed to KVM_CREATE_VM.
> 
> "
>     kvm_type = qemu_opt_get(qemu_get_machine_opts(), "kvm-type");
>     if (mc->kvm_type) {
>         type = mc->kvm_type(ms, kvm_type);
>     } else if (kvm_type) {
>         ret = -EINVAL;
>         fprintf(stderr, "Invalid argument kvm-type=%s\n", kvm_type);
>         goto err;
>     }
> 
>     do {
>         ret = kvm_ioctl(s, KVM_CREATE_VM, type);
>     } while (ret == -EINTR);
> "
> 
> This infrastructure already is used in hw/ppc/spapr.c

FWIW: The ppc code uses "kvm-type" to select the KVM implementation in
the kernel, since there are two implementations: kvm-pr (which is a
trap-and-emulate implementation) and kvm-hv (which is a
hardware-accelerated implementation). If you now introduce kvm-type for
ARM, too, but with a completely different meaning, I think that could
rather be confusing for the users...?

 Thomas