Re: [dpdk-dev] PCIe Hot Insert/Remove Support

[Thomas Monjalon <thomas.monjalon@6wind.com>]

Hi Benjamin,

2016-10-24 18:16, Walker, Benjamin:
> Hi all,
> 
> My name is Ben Walker and I'm the technical lead for SPDK (it's like DPDK, but
> for storage devices). SPDK relies on DPDK only for the base functionality in the
> EAL - memory management, the rings, and the PCI scanning code. A key feature for
> storage devices is support for hot insert and remove, so we're currently working
> through how best to implement this for a user space driver. While doing this
> work, we've run into a few issues with the current DPDK PCI/device/driver
> framework that I'd like to discuss with this list. I'm not entirely ramped up on
> all of the current activity in this area or what the future plans are, so please
> educate me if something is coming that will address our current issues. I'm
> working off of the latest commit on the master branch as of today.
> 
> Today, there appears to be two lists - one of PCI devices and one of drivers. To
> update the list of PCI devices, you call rte_eal_pci_scan(), which scans the PCI
> bus. That call does not attempt to load any drivers. One scan is automatically
> performed when the eal is first initialized. To add or remove drivers from the
> driver list you call rte_eal_driver_register/unregister. To match drivers in the
> driver list to devices in the device list, you call rte_eal_pci_probe.
> 
> There are a few problems with how the code works for us. First,
> rte_eal_pci_scan's algorithm will not correctly detect devices that are in its
> internal list but weren't found by the most recent PCI bus scan (i.e. they were
> hot removed). DPDK's scan doesn't seem to comprehend hot remove in any way.
> Fortunately there is a public API to remove devices from the device list -
> rte_eal_pci_detach. That function will automatically unload any drivers
> associated with the device and then remove it from the list. There is a similar
> call for adding a device to the list - rte_eal_pci_probe_one, which will add a
> device to the device list and then automatically match it to drivers. I think if
> rte_eal_pci_scan is going to be a public interface (and it is), it needs to
> correctly comprehend the removal of PCI devices. Otherwise, make it a private
> API that is only called in response to rte_eal_init and only expose the public
> probe_one/detach calls for modifying the list of devices. My preference is for
> the former, not the latter.
> 
> Second, rte_eal_pci_probe will call the driver initialization functions each
> time a probe happens, even if the driver has already been successfully loaded.
> This tends to crash a lot of the PMDs. It seems to me like rte_eal_pci_probe is
> not safe to call more than once during the lifetime of the program, which is a
> real challenge when you have multiple users of the PCI framework. For instance,
> an application may manage both storage devices using the rte_eal_pci framework
> and NICs, and the initialization routine may go something like:
> 
> register NIC drivers
> rte_eal_probe()
> ...
> register SSD drivers
> rte_eal_probe()
> 
> This is almost certainly how any real code is going to function because the code
> dealing with NICs is unrelated and probably unaware of the code dealing with the
> SSDs. It should be fairly trivial to simply not call the probe() callback for a
> device if the driver has already been loaded. Is this a reasonable modification
> to make?

Yes it seems to be a reasonnable fix.
However we could improve this design a lot.

I'll try to describe the big picture around PCI hotplugging below.

PCI was too much mixed in DPDK code, so we are sorting out what is generic
to every buses and devices, and what is specific to PCI.
Then we want to manage PCI devices as any other device (real or virtual).

>From a generic perspective, we should be able to manage startup and hotplug
with few API functions:
        - scan a bus or all of them (called at startup)
note: scanning for vdev is an argument parsing
        - receive a notification (callback registered by the application)
note: whitelist/blacklist can be handled here at the application level
        - match the device with a driver
        - initialize the device in the matching driver
note: some drivers require binding with a kernel module, and that must
be implemented in EAL

The last missing part for a true hotplug is receiving hardware events
so that we don't need to manually scan anymore.

Hope it will answer your question.
We are waiting for contributions to progress in this direction. Thanks