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

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 27 Feb 2020 13:27:10 +0100
Christian Borntraeger <borntraeger@de.ibm.com> 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@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.

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@redhat.com>