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

[David Hildenbrand <david@redhat.com>]

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.

I assume initializing the gib is fine, and having some guests use it and
others not?

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.

-- 
Thanks,

David / dhildenb