Re: [PATCH RFC 1/4] PCI: hotplug: Add parameter to put devices to reset during rescan

[Sergey Miroshnichenko <s.miroshnichenko@yadro.com>]

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@yadro.com>
>> ---
>>  .../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:
 - 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.

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.

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
>>