Subject: Re: [RFC PATCH 00/11 Allow PR and HV KVM to coexist in one kernel

From: Alexander Graf <[email-masked]>
Date: Mon, 30 Sep 2013 16:54:33 +0200

On 09/30/2013 03:09 PM, Aneesh Kumar K.V wrote:
> Alexander Graf<[email-masked]>  writes:
>
>> On 27.09.2013, at 12:52, Aneesh Kumar K.V wrote:
>>
>>> "Aneesh Kumar K.V"<[email-masked]>  writes:
>>>
>>>> Hi All,
>>>>
>>>> This patch series support enabling HV and PR KVM together in the same kernel. We
>>>> extend machine property with new property "kvm_type". A value of 1 will force HV
>>>> KVM and 2 PR KVM. The default value is 0 which will select the fastest KVM mode.
>>>> ie, HV if that is supported otherwise PR.
>>>>
>>>> With Qemu command line having
>>>>
>>>> -machine pseries,accel=kvm,kvm_type=1
>>>>
>>>> [root@llmp24l02 qemu]# bash ../qemu
>>>> failed to initialize KVM: Invalid argument
>>>> [root@llmp24l02 qemu]# modprobe kvm-pr
>>>> [root@llmp24l02 qemu]# bash ../qemu
>>>> failed to initialize KVM: Invalid argument
>>>> [root@llmp24l02 qemu]# modprobe  kvm-hv
>>>> [root@llmp24l02 qemu]# bash ../qemu
>>>>
>>>> now with
>>>>
>>>> -machine pseries,accel=kvm,kvm_type=2
>>>>
>>>> [root@llmp24l02 qemu]# rmmod kvm-pr
>>>> [root@llmp24l02 qemu]# bash ../qemu
>>>> failed to initialize KVM: Invalid argument
>>>> [root@llmp24l02 qemu]#
>>>> [root@llmp24l02 qemu]# modprobe kvm-pr
>>>> [root@llmp24l02 qemu]# bash ../qemu
>>>>
>>>> if don't specify kvm_type machine property, it will take a default value 0,
>>>> which means fastest supported.
>>> Related qemu patch
>>>
>>> commit 8d139053177d48a70cb710b211ea4c2843eccdfb
>>> Author: Aneesh Kumar K.V<[email-masked]>
>>> Date:   Mon Sep 23 12:28:37 2013 +0530
>>>
>>>     kvm: Add a new machine property kvm_type
>>>
>>>     Targets like ppc64 support different type of KVM, one which use
>>>     hypervisor mode and the other which doesn't. Add a new machine
>>>     property kvm_type that helps in selecting the respective ones
>>>
>>>     Signed-off-by: Aneesh Kumar K.V<[email-masked]>
>> This really is too early, as we can't possibly run in HV mode for
>> non-pseries machines, so the interpretation (or at least sanity
>> checking) of what values are reasonable should occur in the
>> machine. That's why it's a variable in the "machine opts".
> With the current code CREATE_VM will fail, because we won't have
> kvm-hv.ko loaded and trying to create a vm with type 1 will fail.
> Now the challenge related to moving that to machine_init or later is, we
> depend on HV or PR callback early in CREATE_VM. With the changes we have
>
> int kvmppc_core_init_vm(struct kvm *kvm)
> {
>
> #ifdef CONFIG_PPC64
> 	INIT_LIST_HEAD(&kvm->arch.spapr_tce_tables);
> 	INIT_LIST_HEAD(&kvm->arch.rtas_tokens);
> #endif
>
> 	return kvm->arch.kvm_ops->init_vm(kvm);
> }
>
> Also the mmu notifier callback do end up calling kvm_unmap_hva etc which
> are all HV/PR dependent.

Yes, so we should verify in the machine models that we're runnable with 
the currently selected type at least, to give the user a sensible error 
message.

>
>
>
>> Also, users don't want to say type=0. They want to say type=PR or
>> type=HV or type=HV,PR. In fact, can't you make this a property of
>> -accel? Then it's truly accel specific and everything should be well.
> If we are doing this as machine property, we can't specify string,
> because "HV"/"PR" are all powerpc dependent, so parsing that is not
> possible in kvm_init in qemu. But, yes ideally it would be nice to be

Well, we could do the "name to integer" conversion in an arch specific 
function, no?

> able to speicy the type using string. I thought accel is a machine
> property, hence was not sure whether I can have additional properties
> against that. I was using it as below.
>
>   -machine pseries,accel=kvm,kvm_type=1
>
> will look into more details to check whether this can be accel property.

Great :).

Alex