Re: [PATCH v5 2/2] kvm: introduce KVM_MAX_VCPU_ID

From: Greg Kurz
Date: Tue May 03 2016 - 04:56:49 EST


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

> On Tue, 03 May 2016 06:52:02 +0200
> Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> > Signed-off-by: Greg Kurz <gkurz@xxxxxxxxxxxxxxxxxx>
> > ---
> > 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