Re: [PATCH 3/9] KVM: s390: pv: Add query interface

[Janosch Frank <frankja@linux.ibm.com>]

On 3/2/22 13:04, Claudio Imbrenda wrote:
> On Wed, 23 Feb 2022 09:20:01 +0000
> Janosch Frank <frankja@linux.ibm.com> wrote:
> 
>> Some of the query information is already available via sysfs but
>> having a IOCTL makes the information easier to retrieve.
>>
>> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
>> ---
>>   arch/s390/kvm/kvm-s390.c | 47 ++++++++++++++++++++++++++++++++++++++++
>>   include/uapi/linux/kvm.h | 23 ++++++++++++++++++++
>>   2 files changed, 70 insertions(+)
>>
>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>> index faa85397b6fb..837f898ad2ff 100644
>> --- a/arch/s390/kvm/kvm-s390.c
>> +++ b/arch/s390/kvm/kvm-s390.c
>> @@ -2217,6 +2217,34 @@ static int kvm_s390_cpus_to_pv(struct kvm *kvm, u16 *rc, u16 *rrc)
>>   	return r;
>>   }
>>   
>> +static int kvm_s390_handle_pv_info(struct kvm_s390_pv_info *info)
>> +{
>> +	u32 len;
>> +
>> +	switch (info->header.id) {
>> +	case KVM_PV_INFO_VM: {
>> +		len =  sizeof(info->header) + sizeof(info->vm);
>> +
>> +		if (info->header.len < len)
>> +			return -EINVAL;
> 
> so if userspace gives a smaller buffer, we fail?
> this means that if the struct grows in the future, existing software
> will break?

I've already answered this.
If we extend the struct, we can make this the lower bound and store a 
maximum of info->header.len or the length of the extended struct.

I.e. it would work like the QUI 0x100 rc.
Or we can add a new IOCTL which gives the KVM_PV_INFO_VM + new values.


The more interesting question is how we indicate more data and new IOCTL 
commands. Do we always bind them to a capability? Should we add a query 
in front of the KVM_PV_INFO_VM call which tells us the available calls?

> 
>> +
>> +		memcpy(info->vm.inst_calls_list,
>> +		       uv_info.inst_calls_list,
>> +		       sizeof(uv_info.inst_calls_list));
>> +
>> +		/* It's max cpuidm not max cpus so it's off by one */
>> +		info->vm.max_cpus = uv_info.max_guest_cpu_id + 1;
>> +		info->vm.max_guests = uv_info.max_num_sec_conf;
>> +		info->vm.max_guest_addr = uv_info.max_sec_stor_addr;
>> +		info->vm.feature_indication = uv_info.uv_feature_indications;
>> +
>> +		return 0;
>> +	}
>> +	default:
>> +		return -EINVAL;
>> +	}
>> +}
>> +
>>   static int kvm_s390_handle_pv(struct kvm *kvm, struct kvm_pv_cmd *cmd)
>>   {
>>   	int r = 0;
>> @@ -2353,6 +2381,25 @@ static int kvm_s390_handle_pv(struct kvm *kvm, struct kvm_pv_cmd *cmd)
>>   			     cmd->rc, cmd->rrc);
>>   		break;
>>   	}
>> +	case KVM_PV_INFO: {
>> +		struct kvm_s390_pv_info info = {};
>> +
>> +		if (copy_from_user(&info, argp, sizeof(info.header)))
>> +			return -EFAULT;
>> +
>> +		if (info.header.len < sizeof(info.header))
>> +			return -EINVAL;
>> +
>> +		r = kvm_s390_handle_pv_info(&info);
>> +		if (r)
>> +			return r;
>> +
>> +		r = copy_to_user(argp, &info, sizeof(info));
>> +
>> +		if (r)
>> +			return -EFAULT;
>> +		return 0;
>> +	}
>>   	default:
>>   		r = -ENOTTY;
>>   	}
>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>> index dbc550bbd9fa..96fceb204a92 100644
>> --- a/include/uapi/linux/kvm.h
>> +++ b/include/uapi/linux/kvm.h
>> @@ -1642,6 +1642,28 @@ struct kvm_s390_pv_unp {
>>   	__u64 tweak;
>>   };
>>   
>> +enum pv_cmd_info_id {
>> +	KVM_PV_INFO_VM,
>> +};
>> +
>> +struct kvm_s390_pv_info_vm {
>> +	__u64 inst_calls_list[4];
>> +	__u64 max_cpus;
>> +	__u64 max_guests;
>> +	__u64 max_guest_addr;
>> +	__u64 feature_indication;
>> +};
>> +
>> +struct kvm_s390_pv_info_header {
>> +	__u32 id;
>> +	__u32 len;
>> +};
>> +
>> +struct kvm_s390_pv_info {
>> +	struct kvm_s390_pv_info_header header;
>> +	struct kvm_s390_pv_info_vm vm;
>> +};
>> +
>>   enum pv_cmd_id {
>>   	KVM_PV_ENABLE,
>>   	KVM_PV_DISABLE,
>> @@ -1650,6 +1672,7 @@ enum pv_cmd_id {
>>   	KVM_PV_VERIFY,
>>   	KVM_PV_PREP_RESET,
>>   	KVM_PV_UNSHARE_ALL,
>> +	KVM_PV_INFO,
>>   };
>>   
>>   struct kvm_pv_cmd {
>