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

David Hildenbrand
2020-02-27T12:32:10Z

On 27.02.20 13:04, Michael Mueller wrote:
>
>
> On 27.02.20 10:26, David Hildenbrand 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>
>>> ---
>>> 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);
>>
>> Looks sane to me. gi->origin will remain NULL and act like
>> css_general_characteristics.aiv wouldn't be around.
>
> right
>
>>
>> I assume initializing the gib is fine, and having some guests use it and
>> others not?
>
> Is fine as well.
>
>>
>> I do wonder if it would be even clearer/cleaner to not allow to change
>> this property on the fly, and to also not init the gib if disabled.
>>
>> If you want to perform performance tests, simply unload+reload KVM.
>
> That would work if kvm is build as module but not in-kernel, then

Right, but AFAIK that's not an usual setup (at least in distros) - and
for testing purposes not an issue as well.

Anyhow, I'm fine with this

Reviewed-by: David Hildenbrand <david@redhat.com>

Thanks!

--
Thanks,

David / dhildenb