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SubjectRe: [RFC 21/55] KVM: arm64: Forward HVC instruction to the guest hypervisor
On 03/07/17 13:03, Will Deacon wrote:
> On Mon, Jul 03, 2017 at 11:51:26AM +0200, Christoffer Dall wrote:
>> On Mon, Jul 03, 2017 at 11:31:56AM +0200, Andrew Jones wrote:
>>> On Mon, Jul 03, 2017 at 11:08:50AM +0200, Christoffer Dall wrote:
>>>> On Mon, Jun 26, 2017 at 11:21:25AM -0400, Jintack Lim wrote:
>>>>> On Wed, Feb 22, 2017 at 6:47 AM, Christoffer Dall <> wrote:
>>>>>> On Mon, Jan 09, 2017 at 01:24:17AM -0500, Jintack Lim wrote:
>>>>>>> +/* We forward all hvc instruction to the guest hypervisor. */
>>>>>>> +int handle_hvc_nested(struct kvm_vcpu *vcpu)
>>>>>>> +{
>>>>>>> + return kvm_inject_nested_sync(vcpu, kvm_vcpu_get_hsr(vcpu));
>>>>>>> +}
>>>>>> I don't understand the logic here or in the caller above. Do we really
>>>>>> forward *all" hvc calls to the guest hypervisor now, so that we no
>>>>>> longer support any hypercalls from the VM? That seems a little rough
>>>>>> and probably requires some more discussions.
>>>>> So I think if we run a VM with the EL2 support, then all hvc calls
>>>>> from the VM should be forwarded to the virtual EL2.
>>>> But do we actually check if the guest has EL2 here? It seems you cann
>>>> handle_hvc_nested unconditionally when you have
>>>> OCNFIG_KVM_ARM_NESTED_HYP. I think that's what threw me off when first
>>>> reading your patch.
>>>>> I may miss something obvious, so can you (or anyone) come up with some
>>>>> cases that the host hypervisor needs to directly handle hvc from the
>>>>> VM with the EL2 support?
>>>> So I'm a little unsure what to say here. On one hand you are absolutely
>>>> correct, that architecturally if we emulated virtual EL2, then all
>>>> hypercalls are handled by the virtual EL2 (even hypercalls from virtual
>>>> EL2 which should become self-hypercalls).
>>>> On the other hand, an enlightened guest may want to use hypercalls to
>>>> the hypervisor for some reason, but that would require some numbering
>>>> scheme to separate the two concepts.
>>> Yes, I've been thinking that a KVM generic vcpu needs to be enlightened,
>>> and to use a hypercall to get the host cpu's errata. If we head down that
>>> road, then even a vcpu emulating EL2 would need to be able to this.
>> We could use SMC calls here a well, as the "conduit" as I believe the
>> ARM folks are calling it. We just need to agree somewhere (across
>> hypervisors preferably), that when you have virtual EL2, everything is
>> via SMC (even upcalls to a host hypervisor), and otherwise it's via HVC.
> Does that mean you require the CPU to implement EL3 if you want to use
> nested virtualisation?

The 8.3 spec has relaxed the use of SMC for the non-root hypervisor,
where the top-level hypervisor can trap SMCs from nested hypervisors,
irrespective of EL3 being implemented. It still cannot SMCs from an EL1
guest if EL3 is not implemented though...


Jazz is not dead. It just smells funny...

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