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@xxxxxxxxxxxxx> 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; >
