Re: [PATCH] KVM: s390: introduce module parameter kvm.use_gisa

[Michael Mueller <mimu@linux.ibm.com>]

On 27.02.20 10:47, Cornelia Huck wrote:
> On Thu, 27 Feb 2020 10:10:31 +0100
> Michael Mueller <mimu@linux.ibm.com> wrote:
> 
>> The boolean module parameter "kvm.use_gisa" controls if newly
>> created guests will use the GISA facility if provided by the
>> host system. The default is yes.
>>
>>    # cat /sys/module/kvm/parameters/use_gisa
>>    Y
>>
>> The parameter can be changed on the fly.
>>
>>    # echo N > /sys/module/kvm/parameters/use_gisa
>>
>> Already running guests are not affected by this change.
>>
>> The kvm s390 debug feature shows if a guest is running with GISA.
>>
>>    # grep gisa /sys/kernel/debug/s390dbf/kvm-$pid/sprintf
>>    00 01582725059:843303 3 - 08 00000000e119bc01  gisa 0x00000000c9ac2642 initialized
>>    00 01582725059:903840 3 - 11 000000004391ee22  00[0000000000000000-0000000000000000]: AIV gisa format-1 enabled for cpu 000
>>    ...
>>    00 01582725059:916847 3 - 08 0000000094fff572  gisa 0x00000000c9ac2642 cleared
> 
> Maybe log something as well if it is off due to this kernel parameter?

There is also no message when the host does not support it.

> 
>>
>> In general, that value should not be changed as the GISA facility
>> enhances interruption delivery performance.
>>
>> A reason to switch the GISA facility off might be a performance
>> comparison run or debugging.
>>
>> Signed-off-by: Michael Mueller <mimu@linux.ibm.com>
>> ---
>>   arch/s390/kvm/kvm-s390.c | 8 +++++++-
>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>> index d7ff30e45589..5c2081488024 100644
>> --- a/arch/s390/kvm/kvm-s390.c
>> +++ b/arch/s390/kvm/kvm-s390.c
>> @@ -184,6 +184,11 @@ static u8 halt_poll_max_steal = 10;
>>   module_param(halt_poll_max_steal, byte, 0644);
>>   MODULE_PARM_DESC(halt_poll_max_steal, "Maximum percentage of steal time to allow polling");
>>   
>> +/* if set to true, the GISA will be initialized and used if available */
>> +static bool use_gisa  = true;
>> +module_param(use_gisa, bool, 0644);
>> +MODULE_PARM_DESC(use_gisa, "Use the GISA if the host supports it.");
> 
> I probably would have inverted the logic (i.e. introduce a disable_gisa
> parameter that is off by default), as you want KVM to use the gisa,
> except in special circumstances.

Hm, in my opinion "use it := no" is more explicit and natural
than "don't use it := yes"

> 
>> +
>>   /*
>>    * For now we handle at most 16 double words as this is what the s390 base
>>    * kernel handles and stores in the prefix page. If we ever need to go beyond
>> @@ -2504,7 +2509,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>>   	kvm->arch.use_skf = sclp.has_skey;
>>   	spin_lock_init(&kvm->arch.start_stop_lock);
>>   	kvm_s390_vsie_init(kvm);
>> -	kvm_s390_gisa_init(kvm);
>> +	if (use_gisa)
>> +		kvm_s390_gisa_init(kvm);
> 
> I assume we're fine with no gisa but a gib (i.e. doesn't hurt?)

The GIB is a host entity and is required for guests with GISA that want 
to use AP with interruptions in guest.

> 
>>   	KVM_EVENT(3, "vm 0x%pK created by pid %u", kvm, current->pid);
>>   
>>   	return 0;
> 

Thanks a lot!
Michael