Subject: Re: [PATCH v8 3/8] thunderbolt: Communication with the ICM (firmware)

From: Mika Westerberg <[email-masked]>
Date: Mon, 19 Dec 2016 14:24:07 +0200

On Thu, Dec 01, 2016 at 05:21:01PM -0800, Andy Lutomirski wrote:
> On 09/28/2016 07:44 AM, Amir Levy wrote:
> > This patch provides the communication protocol between the
> > Intel Connection Manager(ICM) firmware that is operational in the
> > Thunderbolt controller in non-Apple hardware.
> > The ICM firmware-based controller is used for establishing and maintaining
> > the Thunderbolt Networking connection - we need to be able to communicate
> > with it.
> 
> I'm a bit late to the party, but here goes.  I have two big questions:
>
> 1. Why is this using netlink at all?  A system has zero or more Thunderbolt
> controllers, they're probed just like any other PCI devices (by nhi_probe()
> if I'm understanding correctly), they'll have nodes in sysfs, etc.
> Shouldn't there be a simple char device per Thunderbolt controller that a
> daemon can connect to?  This will clean up lots of things:
> 
> a) You can actually enforce one-daemon-at-a-time in a very natural way. Your
> current code seems to try, but it's rather buggy.  Your subscription count
> is a guess, your unsubscribe is entirely unchecked, and you are entirely
> unable to detect if a daemon crashes AFAICT.
> 
> b) You won't need all of the complexity that's currently there to figure out
> *which* Thunderbolt device a daemon is talking to.
> 
> c) You can use regular ioctl passing *structs* instead of netlink attrs.
> There's nothing wrong with netlink attrs, except that your driver seems to
> have a whole lot of boilerplate that just converts back and forth to regular
> structures.
> 
> d) The userspace code that does stuff like "send message, wait 150ms,
> receive reply, complain if no reply" goes away because ioctl is synchronous.
> (Or you can use read and write, but it's still simpler.)
> 
> e) You could have one daemon per Thunderbolt device if you were so inclined.
> 
> f) You get privilege separation in userspace.  Creating a netlink socket and
> dropping privilege is busted^Winteresting.  Opening a device node and
> dropping privilege works quite nicely.

I agree with your points. Using a char device here instead seems to be
the right way to go forward.

There is small problem, though. On non-Apple systems the host controller
only appears when something is connected to thunderbolt ports. So the
char device would not be there all the time. However, I think we can
still notify the userspace by sending an extra uevent when we detect
there is a PCIe device or inter-domain connection plugged in.

> 2. Why do you need a daemon anyway.  Functionally, what exactly does it do?
> (Okay, I get that it seems to talk to a giant pile of code running in SMM,
> and I get that Intel, for some bizarre reason, wants everyone except Apple
> to use this code in SMM, and that Apple (for entirely understandable
> reasons) turned it off, but that's beside the point. What does the user code
> do that's useful and that the kernel can't do all by itself?  The only
> really interesting bit I can see is the part that approves PCI devices.

As far as I can tell it is used to notify user (via dbus, I guess) that
there is a new PCIe device or inter-domain connection (networking)
available and it needs to be approved before it can be used.

I don't think anything prevents the kernel to do all this (Amir, Michael
can correct me if I'm mistaken).

In fact we could provide a simple "tbtadm" tool, built on top of the
char device that can be used to list and approve devices from shell
command line. That could also allow user to turn on "auto-approve" mode
or similar where the kernel approves all connected devices automatically
(if such functionality is wanted).

The daemon can still be useful for listening uevents generated by the
driver and forwarding approval requests to user.

> I'm not going to review this in detail, but here's a tiny bit:
> 
> > +static int nhi_genl_unsubscribe(__always_unused struct sk_buff *u_skb,
> > +				__always_unused struct genl_info *info)
> > +{
> > +	atomic_dec_if_positive(&subscribers);
> > +
> > +	return 0;
> > +}
> > +
> 
> This, for example, is really quite buggy.

OK.

> This entire function here:
> 
> > +static int nhi_genl_query_information(__always_unused struct sk_buff *u_skb,
> > +				      struct genl_info *info)
> > +{
> > +	struct tbt_nhi_ctxt *nhi_ctxt;
> > +	struct sk_buff *skb;
> > +	bool msg_too_long;
> > +	int res = -ENODEV;
> > +	u32 *msg_head;
> > +
> > +	if (!info || !info->userhdr)
> > +		return -EINVAL;
> > +
> > +	skb = genlmsg_new(NLMSG_ALIGN(nhi_genl_family.hdrsize) +
> > +			  nla_total_size(sizeof(DRV_VERSION)) +
> > +			  nla_total_size(sizeof(nhi_ctxt->nvm_ver_offset)) +
> > +			  nla_total_size(sizeof(nhi_ctxt->num_ports)) +
> > +			  nla_total_size(sizeof(nhi_ctxt->dma_port)) +
> > +			  nla_total_size(0),	/* nhi_ctxt->support_full_e2e */
> > +			  GFP_KERNEL);
> > +	if (!skb)
> > +		return -ENOMEM;
> > +
> > +	msg_head = genlmsg_put_reply(skb, info, &nhi_genl_family, 0,
> > +				     NHI_CMD_QUERY_INFORMATION);
> > +	if (!msg_head) {
> > +		res = -ENOMEM;
> > +		goto genl_put_reply_failure;
> > +	}
> > +
> > +	if (mutex_lock_interruptible(&controllers_list_mutex)) {
> > +		res = -ERESTART;
> > +		goto genl_put_reply_failure;
> > +	}
> > +
> > +	nhi_ctxt = nhi_search_ctxt(*(u32 *)info->userhdr);
> > +	if (nhi_ctxt && !nhi_ctxt->d0_exit) {
> > +		*msg_head = nhi_ctxt->id;
> > +
> > +		msg_too_long = !!nla_put_string(skb, NHI_ATTR_DRV_VERSION,
> > +						DRV_VERSION);
> > +
> > +		msg_too_long = msg_too_long ||
> > +			       nla_put_u16(skb, NHI_ATTR_NVM_VER_OFFSET,
> > +					   nhi_ctxt->nvm_ver_offset);
> > +
> > +		msg_too_long = msg_too_long ||
> > +			       nla_put_u8(skb, NHI_ATTR_NUM_PORTS,
> > +					  nhi_ctxt->num_ports);
> > +
> > +		msg_too_long = msg_too_long ||
> > +			       nla_put_u8(skb, NHI_ATTR_DMA_PORT,
> > +					  nhi_ctxt->dma_port);
> > +
> > +		if (msg_too_long) {
> > +			res = -EMSGSIZE;
> > +			goto release_ctl_list_lock;
> > +		}
> > +
> > +		if (nhi_ctxt->support_full_e2e &&
> > +		    nla_put_flag(skb, NHI_ATTR_SUPPORT_FULL_E2E)) {
> > +			res = -EMSGSIZE;
> > +			goto release_ctl_list_lock;
> > +		}
> > +		mutex_unlock(&controllers_list_mutex);
> > +
> > +		genlmsg_end(skb, msg_head);
> > +
> > +		return genlmsg_reply(skb, info);
> > +	}
> > +
> > +release_ctl_list_lock:
> > +	mutex_unlock(&controllers_list_mutex);
> > +	genlmsg_cancel(skb, msg_head);
> > +
> > +genl_put_reply_failure:
> > +	nlmsg_free(skb);
> > +
> > +	return res;
> > +}
> 
> would be about three lines of code if you used copy_to_user and a struct.

Understood.

Thanks Andy for your comments.

We will rework the driver to take your suggestions into account and
expose a char device instead of using netlink.

Meanwhile we will continue in the github to add new features and support
the new Thunderbolt HW generation.