On Thu, 27 Feb 2020 13:27:10 +0100 Christian Borntraeger <borntraeger@xxxxxxxxxx> 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> > > 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. use_gisa vs disable_gisa is more a personal preference; I don't mind keeping it as use_gisa. > > 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. I don't think you should try to overload a debug knob like that; it's now simple enough, adding more code also adds to the potential for errors. > > > > --- > > 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."); Especially as the description explicitly says "if the host supports it" -- that's good enough for a new knob. > > + > > /* > > * 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; > > > Reviewed-by: Cornelia Huck <cohuck@xxxxxxxxxx>
