Re: [PATCH v6 2/2] s390/kvm: diagnose 318 handling

[Collin Walling <walling@linux.ibm.com>]

On 5/14/20 2:37 PM, Thomas Huth wrote:
> On 14/05/2020 19.20, Collin Walling wrote:
>> On 5/14/20 5:53 AM, David Hildenbrand wrote:
>>> On 14.05.20 11:49, Janosch Frank wrote:
>>>> On 5/14/20 11:37 AM, David Hildenbrand wrote:
>>>>> On 14.05.20 10:52, Janosch Frank wrote:
>>>>>> On 5/14/20 9:53 AM, Thomas Huth wrote:
>>>>>>> On 14/05/2020 00.15, Collin Walling wrote:
>>>>>>>> DIAGNOSE 0x318 (diag318) is a privileged s390x instruction that must
>>>>>>>> be intercepted by SIE and handled via KVM. Let's introduce some
>>>>>>>> functions to communicate between userspace and KVM via ioctls. These
>>>>>>>> will be used to get/set the diag318 related information, as well as
>>>>>>>> check the system if KVM supports handling this instruction.
>>>>>>>>
>>>>>>>> This information can help with diagnosing the environment the VM is
>>>>>>>> running in (Linux, z/VM, etc) if the OS calls this instruction.
>>>>>>>>
>>>>>>>> By default, this feature is disabled and can only be enabled if a
>>>>>>>> user space program (such as QEMU) explicitly requests it.
>>>>>>>>
>>>>>>>> The Control Program Name Code (CPNC) is stored in the SIE block
>>>>>>>> and a copy is retained in each VCPU. The Control Program Version
>>>>>>>> Code (CPVC) is not designed to be stored in the SIE block, so we
>>>>>>>> retain a copy in each VCPU next to the CPNC.
>>>>>>>>
>>>>>>>> Signed-off-by: Collin Walling <walling@linux.ibm.com>
>>>>>>>> ---
>>>>>>>>  Documentation/virt/kvm/devices/vm.rst | 29 +++++++++
>>>>>>>>  arch/s390/include/asm/kvm_host.h      |  6 +-
>>>>>>>>  arch/s390/include/uapi/asm/kvm.h      |  5 ++
>>>>>>>>  arch/s390/kvm/diag.c                  | 20 ++++++
>>>>>>>>  arch/s390/kvm/kvm-s390.c              | 89 +++++++++++++++++++++++++++
>>>>>>>>  arch/s390/kvm/kvm-s390.h              |  1 +
>>>>>>>>  arch/s390/kvm/vsie.c                  |  2 +
>>>>>>>>  7 files changed, 151 insertions(+), 1 deletion(-)
>>>>>>> [...]
>>>>>>>> diff --git a/arch/s390/kvm/diag.c b/arch/s390/kvm/diag.c
>>>>>>>> index 563429dece03..3caed4b880c8 100644
>>>>>>>> --- a/arch/s390/kvm/diag.c
>>>>>>>> +++ b/arch/s390/kvm/diag.c
>>>>>>>> @@ -253,6 +253,24 @@ static int __diag_virtio_hypercall(struct kvm_vcpu *vcpu)
>>>>>>>>  	return ret < 0 ? ret : 0;
>>>>>>>>  }
>>>>>>>>  
>>>>>>>> +static int __diag_set_diag318_info(struct kvm_vcpu *vcpu)
>>>>>>>> +{
>>>>>>>> +	unsigned int reg = (vcpu->arch.sie_block->ipa & 0xf0) >> 4;
>>>>>>>> +	u64 info = vcpu->run->s.regs.gprs[reg];
>>>>>>>> +
>>>>>>>> +	if (!vcpu->kvm->arch.use_diag318)
>>>>>>>> +		return -EOPNOTSUPP;
>>>>>>>> +
>>>>>>>> +	vcpu->stat.diagnose_318++;
>>>>>>>> +	kvm_s390_set_diag318_info(vcpu->kvm, info);
>>>>>>>> +
>>>>>>>> +	VCPU_EVENT(vcpu, 3, "diag 0x318 cpnc: 0x%x cpvc: 0x%llx",
>>>>>>>> +		   vcpu->kvm->arch.diag318_info.cpnc,
>>>>>>>> +		   (u64)vcpu->kvm->arch.diag318_info.cpvc);
>>
>> Errr.. thought I dropped this message. We favored just using the
>> VM_EVENT from last time.
>>
>>>>>>>> +
>>>>>>>> +	return 0;
>>>>>>>> +}
>>>>>>>> +
>>>>>>>>  int kvm_s390_handle_diag(struct kvm_vcpu *vcpu)
>>>>>>>>  {
>>>>>>>>  	int code = kvm_s390_get_base_disp_rs(vcpu, NULL) & 0xffff;
>>>>>>>> @@ -272,6 +290,8 @@ int kvm_s390_handle_diag(struct kvm_vcpu *vcpu)
>>>>>>>>  		return __diag_page_ref_service(vcpu);
>>>>>>>>  	case 0x308:
>>>>>>>>  		return __diag_ipl_functions(vcpu);
>>>>>>>> +	case 0x318:
>>>>>>>> +		return __diag_set_diag318_info(vcpu);
>>>>>>>>  	case 0x500:
>>>>>>>>  		return __diag_virtio_hypercall(vcpu);
>>>>>>>
>>>>>>> I wonder whether it would make more sense to simply drop to userspace
>>>>>>> and handle the diag 318 call there? That way the userspace would always
>>>>>>> be up-to-date, and as we've seen in the past (e.g. with the various SIGP
>>>>>>> handling), it's better if the userspace is in control... e.g. userspace
>>>>>>> could also decide to only use KVM_S390_VM_MISC_ENABLE_DIAG318 if the
>>>>>>> guest just executed the diag 318 instruction.
>>>>>>>
>>>>>>> And you need the kvm_s390_vm_get/set_misc functions anyway, so these
>>>>>>> could also be simply used by the diag 318 handler in userspace?
>>
>> Pardon my ignorance, but I do not think I fully understand what exactly
>> should be dropped in favor of doing things in userspace.
>>
>> My assumption: if a diag handler is not found in KVM, then we
>> fallthrough to userspace handling?
> 
> Right, if you simply omit this change to diag.c, the default case
> returns -EOPNOTSUPP which then should cause an exit to userspace. You
> can then add the code in QEMU to handle_diag() in target/s390x/kvm.c
> instead.
> 
>  Thomas
> 

Very cool! Okay, I think this makes sense, then. I'll look into this.
Thanks for the tip.

@Conny, I assume this is what you meant as well? If so, ignore my
response I sent earlier :)

-- 
--
Regards,
Collin

Stay safe and stay healthy