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

Christian Borntraeger
2020-02-27T12:27:19Z

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.

> ---
> 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;
>