linux-kernel - Re: [RFC 21/55] KVM: arm64: Forward HVC instruction to the guest hypervisor

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Message-ID: <55f84930-184a-c484-e3d5-21522d6a8cc9@arm.com>
Date:   Mon, 3 Jul 2017 13:35:37 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Will Deacon <will.deacon@....com>,
        Christoffer Dall <cdall@...aro.org>
Cc:     Andrew Jones <drjones@...hat.com>,
        Jintack Lim <jintack.lim@...aro.org>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        linux@...linux.org.uk, Catalin Marinas <catalin.marinas@....com>,
        vladimir.murzin@....com, Suzuki K Poulose <suzuki.poulose@....com>,
        mark.rutland@....com, james.morse@....com,
        lorenzo.pieralisi@....com, kevin.brodsky@....com,
        wcohen@...hat.com, shankerd@...eaurora.org, geoff@...radead.org,
        Andre Przywara <andre.przywara@....com>,
        Eric Auger <eric.auger@...hat.com>, anna-maria@...utronix.de,
        Shih-Wei Li <shihwei@...columbia.edu>,
        arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu, KVM General <kvm@...r.kernel.org>,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [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 <cdall@...aro.org> 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...

Thanks,

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