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

[Gleb Natapov <gleb@redhat.com>]

On Sat, Sep 14, 2013 at 02:16:51PM +0200, Andrew Jones wrote:
> This patch removes KVM_SOFT_MAX_VCPUS and uses num_online_cpus() for
> KVM_CAP_NR_VCPUS instead, as ARM does. While the API doc simply says
> KVM_CAP_NR_VCPUS should return the recommended maximum number of vcpus,
> it has been returning KVM_SOFT_MAX_VCPUS, which was defined as the
> maximum tested number of vcpus. As that concept could be
> distro-specific, this patch uses the other recommended maximum, the
> number of physical cpus, as we never recommend configuring a guest that
> has more vcpus than the host has pcpus. Of course a guest can always
> still be configured with up to KVM_CAP_MAX_VCPUS though anyway.
> 
> I've put RFC on this patch because I'm not sure if there are any gotchas
> lurking with this change. The change now means hosts no longer return
> the same number for KVM_CAP_NR_VCPUS, and that number is likely going to
> generally be quite a bit less than what KVM_SOFT_MAX_VCPUS was (160). I
> can't think of anything other than generating more warnings[1] from qemu
> with guests that configure more vcpus than pcpus though.
> 
Another gotcha is that on a host with more then 160 cpus recommended
value will grow which is not a good idea without appropriate testing.

> [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?

>  		break;
>  	case KVM_CAP_MAX_VCPUS:
>  		r = KVM_MAX_VCPUS;
> -- 
> 1.8.1.4

--
			Gleb.