On 9/18/18 2:35 AM, Oliver wrote:
> On Tue, Sep 18, 2018 at 6:55 AM, Sergey Miroshnichenko
> <s.miroshnichenko@xxxxxxxxx> wrote:
>> Hello Sam,
>>
>> On 9/17/18 8:28 AM, Sam Bobroff wrote:
>>> Hi Sergey,
>>>
>>> On Fri, Sep 14, 2018 at 07:14:01PM +0300, Sergey Miroshnichenko wrote:
>>>> Introduce a new command line option "pci=pcie_movable_bars" that indicates
>>>> support of PCIe hotplug without prior reservation of memory regions by
>>>> BIOS/bootloader.
>>>>
>>>> If a new PCIe device has been hot-plugged between two active ones, which
>>>> have no (or not big enough) gap between their BARs, allocating new BARs
>>>> requires to move BARs of the following working devices:
>>>>
>>>> 1) dev 4
>>>> |
>>>> v
>>>> .. | dev 3 | dev 3 | dev 5 | dev 7 |
>>>> .. | BAR 0 | BAR 1 | BAR 0 | BAR 0 |
>>>>
>>>> 2) dev 4
>>>> |
>>>> v
>>>> .. | dev 3 | dev 3 | --> --> | dev 5 | dev 7 |
>>>> .. | BAR 0 | BAR 1 | --> --> | BAR 0 | BAR 0 |
>>>>
>>>> 3)
>>>>
>>>> .. | dev 3 | dev 3 | dev 4 | dev 4 | dev 5 | dev 7 |
>>>> .. | BAR 0 | BAR 1 | BAR 0 | BAR 1 | BAR 0 | BAR 0 |
>>>>
>>>> Not only BARs, but also bridge windows can be updated during a PCIe rescan,
>>>> threatening all memory transactions during this procedure, so the PCI
>>>> subsystem will instruct the drivers to pause by calling the reset_prepare()
>>>> and reset_done() callbacks.
>>>>
>>>> If a device may be affected by BAR movement, the BAR changes tracking must
>>>> be implemented in its driver.
>>>>
>>>> Signed-off-by: Sergey Miroshnichenko <s.miroshnichenko@xxxxxxxxx>
>>>> ---
>>>> .../admin-guide/kernel-parameters.txt | 6 +++
>>>> drivers/pci/pci.c | 2 +
>>>> drivers/pci/probe.c | 43 +++++++++++++++++++
>>>> include/linux/pci.h | 1 +
>>>> 4 files changed, 52 insertions(+)
>>>>
>>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>>>> index 64a3bf54b974..f8132a709061 100644
>>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>>> @@ -3311,6 +3311,12 @@
>>>> bridges without forcing it upstream. Note:
>>>> this removes isolation between devices and
>>>> may put more devices in an IOMMU group.
>>>> + pcie_movable_bars Arrange a space at runtime for BARs of
>>>> + hotplugged PCIe devices - usable if bootloader
>>>> + doesn't reserve memory regions for them. Freeing
>>>> + a space may require moving BARs of active devices
>>>> + to higher addresses, so device drivers will be
>>>> + paused during rescan.
>>>>
>>>> pcie_aspm= [PCIE] Forcibly enable or disable PCIe Active State Power
>>>> Management.
>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>> index 1835f3a7aa8d..5f07a59b5924 100644
>>>> --- a/drivers/pci/pci.c
>>>> +++ b/drivers/pci/pci.c
>>>> @@ -6105,6 +6105,8 @@ static int __init pci_setup(char *str)
>>>> pci_add_flags(PCI_SCAN_ALL_PCIE_DEVS);
>>>> } else if (!strncmp(str, "disable_acs_redir=", 18)) {
>>>> disable_acs_redir_param = str + 18;
>>>> + } else if (!strncmp(str, "pcie_movable_bars", 17)) {
>>>> + pci_add_flags(PCI_MOVABLE_BARS);
>>>> } else {
>>>> printk(KERN_ERR "PCI: Unknown option `%s'\n",
>>>> str);
>>>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>>>> index 201f9e5ff55c..bdaafc48dc4c 100644
>>>> --- a/drivers/pci/probe.c
>>>> +++ b/drivers/pci/probe.c
>>>> @@ -3138,6 +3138,45 @@ unsigned int pci_rescan_bus_bridge_resize(struct pci_dev *bridge)
>>>> return max;
>>>> }
>>>>
>>>> +/*
>>>> + * Put all devices of the bus and its children to reset
>>>> + */
>>>> +static void pci_bus_reset_prepare(struct pci_bus *bus)
>>>> +{
>>>> + struct pci_dev *dev;
>>>> +
>>>> + list_for_each_entry(dev, &bus->devices, bus_list) {
>>>> + struct pci_bus *child = dev->subordinate;
>>>> +
>>>> + if (child) {
>>>> + pci_bus_reset_prepare(child);
>>>> + } else if (dev->driver &&
>>>> + dev->driver->err_handler &&
>>>> + dev->driver->err_handler->reset_prepare) {
>>>> + dev->driver->err_handler->reset_prepare(dev);
>>>> + }
>>>
>>> What about devices with drivers that don't have reset_prepare()? It
>>> looks like it will just reconfigure them anyway. Is that right?
>>
>> It is possible that unprepared driver without these hooks will get BARs
>> moved, I should put a warning message there. There three ways we can see
>> to make this safe:>
>
> It might make more sense to allow drivers to opt-in to this rather
> than assuming any driver with the error handlers supports re-assigning
> BARs on the fly. As far as I know we restore the previous BAR values a
> part of the EEH recovery process when there's a driver bound, but I
> could be wrong.
>
If I add the new two callbacks rescan_prepare() and rescan_done() to the
struct pci_driver, would they be a good indicator of driver's awareness
of movable BARs?
>> - add the reset_prepare()/reset_done() hooks to *every* PCIe driver;
>> - refuse BAR movement if at least one unprepared driver has been
>> encountered during rescan;
>> - reduce the number of drivers which can be affected to some
>> controllable value and prepare them on demand.
>
> Is there any reason we couldn't just unbind the unaware drivers and
> re-bind them afterwards? That might be useful when hotplugging NVMe
> racks since you wouldn't want a switch management driver (or similar)
> to prevent re-assignments if they're required.
>
We would like the hotplug events to be as much transparent for the rest
of the system as possible: it is ok if a device takes a pause for a few
seconds, but it is not allowed to disappear, even if it will return soon.
>> Applying the second proposal as a major restriction seems fairly
>> reasonable, but for our particular setups and use-cases it is probably
>> too stiff:
>> - we've noticed that devices connected directly to the root bridge
>> don't get moved BARs, and this covers our x86_64 servers: we only
>> insert/remove devices into "second-level" and "lower" bridges there, but
>> not root;
>>
>> - on PowerNV we have system devices (network interfaces, USB hub, etc.)
>> grouped into dedicated domain, with all other domains ready for hotplug,
>> and only these domains can be rescanned.
>
> Are you doing anything to enforce this or just relying on people not
> re-scanning the system device bus?
>
It was the latter. But then Bjorn has suggested how we can avoid such
prerequisites and assumptions: I'll change the implementation so the
arrangement of a free space for BARs of new devices may fail because of
memory fragmentation by "immovable" devices. In return, the whole
operation of rescan becomes safe.
Serge
>> With our scenarios currently reduced to these two, we can live with just
>> a few drivers "prepared" for now: NVME and few Ethernet adapters, this
>> gives us a possibility to use this feature before "converting" *all* the
>> drivers, and even have the NVidia cards running on a closed proprietary
>> driver.
>>
>> Should we make this behavior adjustable with something like
>> "pcie_movable_bars=safe" and "pcie_movable_bars=always" ?
>>
>> Thanks!
>>
>> Best regards,
>> Serge
>>
>>>> + }
>>>> +}
>>>> +
>>>> +/*
>>>> + * Complete the reset of all devices for the bus and its children
>>>> + */
>>>> +static void pci_bus_reset_done(struct pci_bus *bus)
>>>> +{
>>>> + struct pci_dev *dev;
>>>> +
>>>> + list_for_each_entry(dev, &bus->devices, bus_list) {
>>>> + struct pci_bus *child = dev->subordinate;
>>>> +
>>>> + if (child) {
>>>> + pci_bus_reset_done(child);
>>>> + } else if (dev->driver && dev->driver->err_handler &&
>>>> + dev->driver->err_handler->reset_done) {
>>>> + dev->driver->err_handler->reset_done(dev);
>>>> + }
>>>> + }
>>>> +}
>>>> +
>>>> /**
>>>> * pci_rescan_bus - Scan a PCI bus for devices
>>>> * @bus: PCI bus to scan
>>>> @@ -3151,8 +3190,12 @@ unsigned int pci_rescan_bus(struct pci_bus *bus)
>>>> {
>>>> unsigned int max;
>>>>
>>>> + if (pci_has_flag(PCI_MOVABLE_BARS))
>>>> + pci_bus_reset_prepare(bus);
>>>> max = pci_scan_child_bus(bus);
>>>> pci_assign_unassigned_bus_resources(bus);
>>>> + if (pci_has_flag(PCI_MOVABLE_BARS))
>>>> + pci_bus_reset_done(bus);
>>>> pci_bus_add_devices(bus);
>>>>
>>>> return max;
>>>> diff --git a/include/linux/pci.h b/include/linux/pci.h
>>>> index 6925828f9f25..a8cb1a367c34 100644
>>>> --- a/include/linux/pci.h
>>>> +++ b/include/linux/pci.h
>>>> @@ -847,6 +847,7 @@ enum {
>>>> PCI_ENABLE_PROC_DOMAINS = 0x00000010, /* Enable domains in /proc */
>>>> PCI_COMPAT_DOMAIN_0 = 0x00000020, /* ... except domain 0 */
>>>> PCI_SCAN_ALL_PCIE_DEVS = 0x00000040, /* Scan all, not just dev 0 */
>>>> + PCI_MOVABLE_BARS = 0x00000080, /* Runtime BAR reassign after hotplug */
>>>> };
>>>>
>>>> /* These external functions are only available when PCI support is enabled */
>>>> --
>>>> 2.17.1
>>>>
