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

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Date:	Tue, 3 May 2016 08:49:12 +0200
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	Greg Kurz <gkurz@...ux.vnet.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, 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.

> +#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