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

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

On 27.02.20 13:27, Christian Borntraeger wrote:
> 
> 
> On 27.02.20 10:10, Michael Mueller 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
>>
>> 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>
> 
> Looks good to me. Regarding the other comments, I think allowing for dynamic changes
> and keeping use_gisa vs disable_gisa makes sense. So I would think that the patch
> as is makes sense.
> 
> The only question is: shall we set use_gisa to 0 when the machine does not support
> it (e.g. VSIE?) and then also forbid setting it to 1? Could be overkill.

Then I would rename the parameter to "try_to_use_gisa" instead. (a joke 
;) )

In that case we exit gisa_init() because of the missing AIV facility.

void kvm_s390_gisa_init(struct kvm *kvm)
{
         struct kvm_s390_gisa_interrupt *gi = &kvm->arch.gisa_int;

-->	if (!css_general_characteristics.aiv)
                 return;
         gi->origin = &kvm->arch.sie_page2->gisa;
         gi->alert.mask = 0;
	...
}


Thanks,
Michael

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

-- 
Mit freundlichen Grüßen / Kind regards
Michael Müller

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