linux-kernel - Re: [PATCH v5 2/2] kvm: introduce KVM_MAX_VCPU

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Date:	Tue, 3 May 2016 10:56:31 +0200
From:	Greg Kurz <gkurz@...ux.vnet.ibm.com>
To:	Cornelia Huck <cornelia.huck@...ibm.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>, james.hogan@...tec.com,
	mingo@...hat.com, linux-mips@...ux-mips.org, kvm@...r.kernel.org,
	rkrcmar@...hat.com, linux-kernel@...r.kernel.org,
	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	qemu-ppc@...gnu.org, Paul Mackerras <paulus@...ba.org>,
	David Gibson <david@...son.dropbear.id.au>
Subject: Re: [PATCH v5 2/2] kvm: introduce KVM_MAX_VCPU_ID

On Tue, 3 May 2016 08:49:12 +0200
Cornelia Huck <cornelia.huck@...ibm.com> wrote:

> On Tue, 03 May 2016 06:52:02 +0200
> Greg Kurz <gkurz@...ux.vnet.ibm.com> wrote:
> 
> > The KVM_MAX_VCPUS define provides the maximum number of vCPUs per guest, and
> > also the upper limit for vCPU ids. This is okay for all archs except PowerPC
> > which can have higher ids, depending on the cpu/core/thread topology. In the
> > worst case (single threaded guest, host with 8 threads per core), it limits
> > the maximum number of vCPUS to KVM_MAX_VCPUS / 8.
> > 
> > This patch separates the vCPU numbering from the total number of vCPUs, with
> > the introduction of KVM_MAX_VCPU_ID, as the maximal valid value for vCPU ids
> > plus one.
> > 
> > The corresponding KVM_CAP_MAX_VCPU_ID allows userspace to validate vCPU ids
> > before passing them to KVM_CREATE_VCPU.
> > 
> > Only PowerPC gets unlimited vCPU ids for the moment. This patch doesn't
> > change anything for other archs.
> > 
> > Suggested-by: Radim Krcmar <rkrcmar@...hat.com>
> > Signed-off-by: Greg Kurz <gkurz@...ux.vnet.ibm.com>
> > ---
> >  Documentation/virtual/kvm/api.txt   |   10 ++++++++--
> >  arch/powerpc/include/asm/kvm_host.h |    2 ++
> >  arch/powerpc/kvm/powerpc.c          |    3 +++
> >  include/linux/kvm_host.h            |    4 ++++
> >  include/uapi/linux/kvm.h            |    1 +
> >  virt/kvm/kvm_main.c                 |    2 +-
> >  6 files changed, 19 insertions(+), 3 deletions(-)
> >   
> 
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 23bfe1bd159c..3b4efa1c088c 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -35,6 +35,10 @@
> > 
> >  #include <asm/kvm_host.h>
> > 
> > +#ifndef KVM_MAX_VCPU_ID
> > +#define KVM_MAX_VCPU_ID KVM_MAX_VCPUS  
> 
> As you are defining KVM_MAX_VCPU_ID in any case, would it make sense to
> provide the cap in generic code? power will simply override the value.
> 

I'll do that in v6. Thanks !

> > +#endif
> > +
> >  /*
> >   * The bit 16 ~ bit 31 of kvm_memory_region::flags are internally used
> >   * in kvm, other bits are visible for userspace which are defined in