Re: [PATCH] kvm: Increase NR_IOBUS_DEVS limit to 200

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Mar 30, 2010 at 04:48:25PM -0700, Sridhar Samudrala wrote:
> This patch increases the current hardcoded limit of NR_IOBUS_DEVS
> from 6 to 200. We are hitting this limit when creating a guest with more
> than 1 virtio-net device using vhost-net backend. Each virtio-net
> device requires 2 such devices to service notifications from rx/tx queues.
> 
> Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
> 

I tried this, but observed a measurable performance
degradation with vhost net and this patch. Did not
investigate yet.  Do you see this as well?

> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index a3fd0f9..7fb48d3 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -54,7 +54,7 @@ extern struct kmem_cache *kvm_vcpu_cache;
>   */
>  struct kvm_io_bus {
>  	int                   dev_count;
> -#define NR_IOBUS_DEVS 6
> +#define NR_IOBUS_DEVS 200 
>  	struct kvm_io_device *devs[NR_IOBUS_DEVS];
>  };
>  
> 
>