[+cc Russell, Ben, Oliver, linuxppc-dev]
On Mon, Sep 17, 2018 at 11:55:43PM +0300, Sergey Miroshnichenko 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:
> - 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" ?
I like the overall idea of this a lot.
- Why do we need a command line parameter to enable this? Can't we
do it unconditionally and automatically when it's possible? We
could have a chicken switch to *disable* it in case this breaks
something horribly, but I would like this functionality to be
always available without a special option.
- I'm not sure the existence of .reset_done() is evidence that a
driver is prepared for its BARs to move. I don't see any
documentation that refers to BAR movement, and I doubt it's been
tested. But I only see 5 implementations in the tree, so it'd be
easy to verify.
- I think your second proposal above sounds right: we should regard
any device whose driver lacks .reset_done() as immovable. We will
likely be able to move some devices but not others. Implementing
.reset_done() will increase flexibility but it shouldn't be a
requirement for all drivers.
> >> + }
> >> +}
> >> +
> >> +/*
> >> + * 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
> >>
