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[Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options

From: Anthony Liguori
Subject: [Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options
Date: Fri, 26 Feb 2010 09:23:01 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20091209 Fedora/3.0-4.fc12 Lightning/1.0pre Thunderbird/3.0

On 02/26/2010 08:51 AM, Michael S. Tsirkin wrote:
On Thu, Feb 25, 2010 at 01:47:27PM -0600, Anthony Liguori wrote:
On 02/25/2010 12:28 PM, Michael S. Tsirkin wrote:
This adds vhost binary option to tap, to enable vhost net accelerator.
Default is off for now, we'll be able to make default on long term
when we know it's stable.

vhostfd option can be used by management, to pass in the fd. Assigning
vhostfd implies vhost=on.

Signed-off-by: Michael S. Tsirkin<address@hidden>

Since the thinking these days is that macvtap and tap is pretty much all
we'll ever need for vhost-net, perhaps we should revisit -net vhost vs.
-net tap,vhost=X?

I think -net vhost,fd=X makes a lot more sense than -net


Anthony Liguori
We'll have to duplicate all tap options.
I think long term we will just make vhost=on the default.

I don't think we can. vhost only works when using KVM and it doesn't support all of the features of userspace virtio. Since it's in upstream Linux without supporting all of the virtio-net features, it's something we're going to have to deal with for a long time.

Furthermore, vhost reduces a virtual machine's security. It offers an impressive performance boost (particularly when dealing with 10gbit+ networking) but for a user that doesn't have such strong networking performance requirements, I think it's reasonable for them to not want to make a security trade off.

One reason I like -net vhost is that it's a much less obscure syntax and it's the sort of thing that is easy to tell users that they should use. I understand you're argument for -net tap if you assume vhost=on will become the default because that means that users never really have to be aware of vhost once it becomes the default. But as I said above, I don't think it's reasonable to make it on by default with -net tap.

Users do not really care about vhost, it just makes tap
go fater. So promoting it to 1st class options is wrong IMO.

User's should care about vhost because it impacts the features supported by the virtual machine and it has security ramifications. It's a great feature and I think the most users will want to use it, but I do think it's something that users ought to be aware of.


Anthony Liguori

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