On Fri, 2020-01-31 at 14:27 -0600, Bjorn Helgaas wrote:
> On Fri, Jan 31, 2020 at 06:19:48PM +0000, Sergei Miroshnichenko
> wrote:
> > On Thu, 2020-01-30 at 17:52 -0600, Bjorn Helgaas wrote:
> > > On Wed, Jan 29, 2020 at 06:29:27PM +0300, Sergei Miroshnichenko
> > > wrote:
> > > > BARs and bridge windows are only allowed to be assigned to
> > > > their
> > > > parent bus's bridge windows, going up to the root complex's
> > > > resources.
> > > > So additional limitations on BAR address are not needed, and
> > > > the
> > > > PCIBIOS_MIN_MEM can be ignored.
> > >
> > > This is theoretically true, but I don't think we have reliable
> > > information about the host bridge windows in all cases, so
> > > PCIBIOS_MIN_MEM/_IO is something of an approximation.
> > >
> > > > Besides, the value of PCIBIOS_MIN_MEM reported by the BIOS 1.3
> > > > on
> > > > Supermicro H11SSL-i via e820__setup_pci_gap():
> > > >
> > > > [mem 0xebff1000-0xfe9fffff] available for PCI devices
> > > >
> > > > is only suitable for a single RC out of four:
> > > >
> > > > pci_bus 0000:00: root bus resource [mem 0xec000000-0xefffffff
> > > > window]
> > > > pci_bus 0000:20: root bus resource [mem 0xeb800000-0xebefffff
> > > > window]
> > > > pci_bus 0000:40: root bus resource [mem 0xeb200000-0xeb5fffff
> > > > window]
> > > > pci_bus 0000:60: root bus resource [mem 0xe8b00000-0xeaffffff
> > > > window]
> > > >
> > > > , which makes the AMD EPYC 7251 unable to boot with this
> > > > movable
> > > > BARs
> > > > patchset.
> > >
> > > Something's wrong if this system booted before this patch set but
> > > not
> > > after. We shouldn't be doing *anything* with the BARs until we
> > > need
> > > to, i.e., until we hot-add a device where we have to move things
> > > to
> > > find space for it.
> >
> > The one breaking boot on this system initially was 17/26 of this
> > patchset: "PCI: hotplug: Ignore the MEM BAR offsets from
> > BIOS/bootloader"
>
> I don't think that patch is a good idea. I think we should read the
> current BARs and windows at boot-time and leave them alone unless we
> *must* change them. I don't think we should change things
> preemptively to make future hotplug events easier.
>
> > Before it the kernel just took BARs pre-assigned by BIOS. In the
> > same
> > time, the same BIOS reports 0xebff1000-0xfe9fffff as available for
> > PCI
> > devices, but the real root bridge windows are 0xe8b00000-0xefffffff
> > in
> > total (and also 64-bit windows) - which are also reported by the
> > same
> > BIOS. So the kernel was only able to handle the 0xec000000-
> > 0xefffffff
> > root bus.
> >
> > With that patch reverted the kernel was able to boot, but unable to
> > rescan - to reassign BARs actually.
> >
> > > (And we don't want a bisection hole where this system can't boot
> > > until
> > > this patch is applied, but I assume that's obvious.)
> > >
> > > > Signed-off-by: Sergei Miroshnichenko <
> > > > s.miroshnichenko@xxxxxxxxx>
> > > > ---
> > > > drivers/pci/setup-res.c | 5 +++--
> > > > 1 file changed, 3 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/pci/setup-res.c b/drivers/pci/setup-res.c
> > > > index a7d81816d1ea..4043aab021dd 100644
> > > > --- a/drivers/pci/setup-res.c
> > > > +++ b/drivers/pci/setup-res.c
> > > > @@ -246,12 +246,13 @@ static int __pci_assign_resource(struct
> > > > pci_bus *bus, struct pci_dev *dev,
> > > > int resno, resource_size_t size,
> > > > resource_size_t align)
> > > > {
> > > > struct resource *res = dev->resource + resno;
> > > > - resource_size_t min;
> > > > + resource_size_t min = 0;
> > > > int ret;
> > > > resource_size_t start = (resource_size_t)-1;
> > > > resource_size_t end = 0;
> > > >
> > > > - min = (res->flags & IORESOURCE_IO) ? PCIBIOS_MIN_IO :
> > > > PCIBIOS_MIN_MEM;
> > > > + if (!pci_can_move_bars)
> > > > + min = (res->flags & IORESOURCE_IO) ?
> > > > PCIBIOS_MIN_IO :
> > > > PCIBIOS_MIN_MEM;
> > >
> > > I don't understand the connection here. PCIBIOS_MIN_MEM and
> > > PCIBIOS_MIN_IO are basically ways to say "we can't put PCI
> > > resources
> > > below this address".
> > >
> > > On ACPI systems, the devices in the ACPI namespace are supposed
> > > to
> > > tell the OS what resources they use, and obviously the OS should
> > > not
> > > assign those resources to anything else. If Linux handled all
> > > those
> > > ACPI resources correctly and in the absence of firmware defects,
> > > we
> > > shouldn't need PCIBIOS_MIN_MEM/_IO at all. But neither of those
> > > is
> > > currently true.
> > >
> > > It's true that we should be smarter about PCIBIOS_MIN_MEM/_IO,
> > > but I
> > > don't think that has anything to do with whether we support
> > > *moving*
> > > BARs. We have to avoid the address space that's already in use
> > > in
> > > *all* cases.
> >
> > This is connected to the approach of this feature: releasing,
> > recalculating and reassigning the BARs and bridge windows. If
> > movable
> > BARs are disabled, this bug doesn't reproduce. And the bug doesn't
> > let
> > the system boot when BARs are allowed to move. That's why I've tied
> > these together.
>
> My point is just that logically this has nothing to do with movable
> BARs.
>
> > This line setting the "min" to PCIBIOS_MIN_* is there untouched
> > since
> > the first kernel git commit in 2005 - could it be that all systems
> > are
> > just fine now, having their root bridge windows set up correctly?
>
> I don't understand the question, sorry.
>
I mean, every BAR assigned here can't reside outside of a host IO/MEM
bridge window, which is a bus->resource[n] set up by the platform code,
and their .start fields are seemed to be duplicated by the
PCIBIOS_MIN_* values - from the platform code as well. But the .start
fields are seem to be correct (aren't they?), and the PCIBIOS_MIN_*
values are sometimes definitely not.
What can be a reliable test to check if PCIBIOS_MIN_* are safe to
ignore unconditionally? Could it be a separate flag instead of the
pci_can_move_bars here?
Would it be fine for a start to ignore the PCIBIOS_MIN_* if it lies
completely outside of host bridge windows? So at least AMD EPYC can
obtain its hotplug power.
Best regards,
Serge
> > > > if (pci_can_move_bars && dev->subordinate && resno >=
> > > > PCI_BRIDGE_RESOURCES) {
> > > > struct pci_bus *child_bus = dev->subordinate;
> > > > --
> > > > 2.24.1
> > > >
