Re: [Qemu-devel] [PATCH 0/2] usb-ccid device (v2)

[Robert Relyea]

On 10/14/2010 03:16 PM, Anthony Liguori wrote:

On 10/14/2010 05:03 PM, Robert Relyea wrote:

Remote device passthrough is just a special case of passthrough.  It's got interesting characteristics in that unlike local device passthrough, if you preserve the connection to the remove device, migration is still possible.

However, remote device *emulation* is the thing that I'm concerned about.  Having a device emulated outside of QEMU means that it's not possible to participate in many of QEMU's features (like live migration, tracing, debugging, etc.).  Device creation is extremely complicated because you have to launch the external daemon and somehow configure that.

There's always some emulation going on on the client side. The client side has the device drivers, so you are either emulating an actual device or you are emulating the abstraction you invent. Once you have the client side, you have to launch the external daemon anyway.

That's not a very convincing argument.

Neither is that;). My point is that no matter what you do, there is *always* some sort of client emulation going on and the client is the only one in this scenario that has access to the local drivers. This is all moot. The only client side qemu supports is vnc/X/xdesktop. All those devices have client side drivers that are emulating the overall protocol.

It's pretty simple really.  We don't want to split QEMU into a bunch of different daemons that all implement device emulation in slightly different ways.  The user complexity is enormous and the ability to manage the complexity because impossible because nothing is centralized.

I think you misunderstand me. In some sense I'm agreeing with you. I agree you don't want to split QEMU into a bunch of daemons, which is why I think you handle the smart card remotely only with spice. Currently QEMU doesn't have the infrastructure to handle lots of different client devices (pretty much only if there is some preexisting client like vnc that handles those things).

In some sense we seem to be talking cross purposes here. I agree that QEMU isn't the right place to handle client side devices. It's clearly not in the QEMU model, so it makes sense NOT to emulate on the client side for QEMU...

It seems to me that the best way to go is to provide the native host == client support like other devices and allow the passthru. If you really need client support, just run spice (which is a single client daemon that handles everything).

Let's not confuse passthrough with implementing device emulation outside of QEMU.  They are two very different things.

I think a remote passthrough protocol who's sole purpose is to allow external device emulation is a bad idea for QEMU.

The passthru is passthru. I'm not sure what you mean here.... I'm presuming the way forward is to have passthru and qemu implementing a local smart card emulation. I'm fine with just passthru and local emulation to a local smart card.

After talking to Alon in IRC, I think a better model for Spice would be to integrate the smart card emulation into QEMU and then develop a specific protocol for the smart card emulation to interface with the physical smart card.  This interface isn't really any different than the network interface or the block interface in QEMU today.

I seems to me that a second protocol is overkill. Having 2 protocols is a bit much to manage. We can do everything we need with the passthru.

How is external device emulation not overkill?  I don't see why two protocols are necessary.  You just need one.

If you have one for passthru and one for emulated cards, that's two protocols.

My worry about creating any thing else is we may not have the flexibility to handle future cards. Smart cards themselves are programmable, so the interface for new cards are pretty dynamic.

My worry is that we're creating an impossible situation to maintain in the long term because device emulation is happening in 10 different places.  If there's a bug in your smart card emulation, a guest can now break into a Spice client.  Part of the advantage of keeping everything contained in a single place (QEMU) is that we can restrict QEMU from a security perspective via sVirt and other mechanisms.  Once you split apart device emulation, you break that security model.

That's true whether I'm emulating on qemu or not. If there is a bug in the emulation code, you can break out. If there is a bug in the client side driver, you can break out. The risk is exactly the same.

But again, this is academic, I'm not advocating doing this in qemu. Clearly there needs to be more work before we can even talk about qemu client side devices where client != host.

bob

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