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

[Auger Eric <eric.auger@redhat.com>]

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

Whould it be better if we would pass something like kvm-type=48bGPA?
Otherwise I can decode another virt machine option (min_vm_phys_shift)
in kvm_type callback.

Thanks

Eric


> 
> Thanks,
> drew
>