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

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 14 May 2020 14:53:24 -0400
Collin Walling <walling@linux.ibm.com> wrote:

> 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 :)
> 

Yes; if done correctly, it should be easy to hack something up for tcg
as well, if we want it.