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@xxxxxxxxxxxxx> >>> --- >>> 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@xxxxxxxxxx> Thanks! -- Thanks, David / dhildenb
