Re: [PATCH] [RFC] x86: kvm: remove KVM_SOFT_MAX_VCPUS

[Gleb Natapov <gleb@redhat.com>]

On Mon, Sep 16, 2013 at 10:22:09AM +0200, Andrew Jones wrote:
> > > [1] Actually, until 972fc544b6034a in uq/master is merged there won't be
> > >     any warnings either.
> > > 
> > > Signed-off-by: Andrew Jones <drjones@redhat.com>
> > > ---
> > >  arch/x86/include/asm/kvm_host.h | 1 -
> > >  arch/x86/kvm/x86.c              | 2 +-
> > >  2 files changed, 1 insertion(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > > index c76ff74a98f2e..9236c63315a9b 100644
> > > --- a/arch/x86/include/asm/kvm_host.h
> > > +++ b/arch/x86/include/asm/kvm_host.h
> > > @@ -32,7 +32,6 @@
> > >  #include <asm/asm.h>
> > >  
> > >  #define KVM_MAX_VCPUS 255
> > > -#define KVM_SOFT_MAX_VCPUS 160
> > >  #define KVM_USER_MEM_SLOTS 125
> > >  /* memory slots that are not exposed to userspace */
> > >  #define KVM_PRIVATE_MEM_SLOTS 3
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index e5ca72a5cdb6d..d9d3e2ed68ee9 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -2604,7 +2604,7 @@ int kvm_dev_ioctl_check_extension(long ext)
> > >  		r = !kvm_x86_ops->cpu_has_accelerated_tpr();
> > >  		break;
> > >  	case KVM_CAP_NR_VCPUS:
> > > -		r = KVM_SOFT_MAX_VCPUS;
> > > +		r = min(num_online_cpus(), KVM_MAX_VCPUS);
> > s/KVM_MAX_VCPUS/KVM_SOFT_MAX_VCPUS/.  Also what about hotplug cpus?
> 
> I'll send a v2 with this change.
> 
> I thought a bit about hotplug, and thus using num_possible_cpus()
> instead, but then decided it made more sense to stick to what's online now
> for the recommended number. It's just a recommendation anyway. So as long
> as KVM_MAX_VCPUS is >= num_possible_cpus(), then one can still configure
> more vcpus to count for all hotplugable cpus, if they wish.
> 
It is just recommended, but we do warn about it, so it is user visible.
Well, the whole point of it existence is to be user visible ;). If user
creates a guest with max cpus greater than current number if online
cpus, taking into account feature grows, he will get a warning, but we
should not warn about it.
 
--
			Gleb.