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

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Date:	Wed, 4 May 2016 18:45:34 +0200
From:	Radim Krčmář <rkrcmar@...hat.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,
	linux-kernel@...r.kernel.org,
	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	qemu-ppc@...gnu.org, Cornelia Huck <cornelia.huck@...ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	David Gibson <david@...son.dropbear.id.au>
Subject: Re: [PATCH v5 2/2] kvm: introduce KVM_MAX_VCPU_ID

2016-05-03 06:52+0200, Greg Kurz:
> 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>
> ---
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> @@ -2272,7 +2272,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
>  	int r;
>  	struct kvm_vcpu *vcpu;
>  
> -	if (id >= KVM_MAX_VCPUS)
> +	if (id >= KVM_MAX_VCPU_ID)
>  		return -EINVAL;

book3s_hv will currently fail with vcpu_id above threads_per_subcore *
KVM_MAX_VCORES, so userspace cannot use KVM_CAP_MAX_VCPU_ID to limit
vcpu_id ... I thought the check for vcpu_id would move to arch-specific
code, like the previous version did, to simplify implementation of a
dynamic limit.

The dynamic limit was too complicated to be worth it?
(This version is ok too.)