netdev - Re: Unable to create more than 1 guest virtio-net device using vhost-net backend

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Message-ID: <20100321095544.GA6443@redhat.com>
Date:	Sun, 21 Mar 2010 11:55:44 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sridhar Samudrala <samudrala.sridhar@...il.com>
Cc:	netdev <netdev@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, avi@...hat.com,
	gleb@...hat.com
Subject: Re: Unable to create more than 1 guest virtio-net device using
	vhost-net backend

On Fri, Mar 19, 2010 at 03:19:27PM -0700, Sridhar Samudrala wrote:
> When creating a guest with 2 virtio-net interfaces, i am running
> into a issue causing the 2nd i/f falling back to userpace virtio
> even when vhost is enabled.
> 
> After some debugging, it turned out that KVM_IOEVENTFD ioctl() 
> call in qemu is failing with ENOSPC.
> This is because of the NR_IOBUS_DEVS(6) limit in kvm_io_bus_register_dev()
> routine in the host kernel.
> 
> I think we need to increase this limit if we want to support multiple
> network interfaces using vhost-net.
> Is there an alternate solution?
> 
> Thanks
> Sridhar

Nothing easy that I can see. Each device needs 2 of these.  Avi, Gleb,
any objections to increasing the limit to say 16?  That would give us
5 more devices to the limit of 6 per guest.

-- 
MST
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