Re: [PATCH V4 0/6] PCI, x86: update MMCFG information when hot-plugging PCI host bridges

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On 2012-4-26 11:35, Don Dutile wrote:
On 04/25/2012 12:50 PM, Bjorn Helgaas wrote:
On Mon, Apr 23, 2012 at 12:50 PM, Don Dutile<ddutile@xxxxxxxxxx> wrote:
On 04/23/2012 01:41 PM, Bjorn Helgaas wrote:

On Mon, Apr 16, 2012 at 11:54 AM, Don Dutile<ddutile@xxxxxxxxxx> wrote:

On 04/16/2012 12:09 PM, Jiang Liu wrote:


Hi Don,
Thanks for your comments and please refer to inline comments
below.



Thanks for the info below; couple quick replies below.. - Don


On 04/16/2012 11:30 PM, Don Dutile wrote:


On 04/13/2012 10:33 AM, Jiang Liu wrote:


On 04/13/2012 06:48 PM, Kenji Kaneshige wrote:


(2012/04/12 9:06), Bjorn Helgaas wrote:


On Wed, Apr 11, 2012 at 9:34 AM, Jiang Liu<liuj97@xxxxxxxxx>
wrote:


On 04/11/2012 08:05 PM, Kenji Kaneshige wrote:


(2012/04/11 13:02), Bjorn Helgaas wrote:


On Tue, Apr 10, 2012 at 6:10 PM, Jiang Liu<liuj97@xxxxxxxxx>
wrote:


This patchset enhance pci_root driver to update MMCFG
information
when
hot-plugging PCI root bridges. It applies to Yinghai's
tree at


git://git.kernel.org/pub/scm/linux/kernel/git/yinghai/linux-yinghai.git

for-pci-root-bus-hotplug

The second patch is based on Taku Izumi work with some
enhancements to
correctly handle PCI host bridges without _CBA method.



I'm sorry I won't have time to really review these for a
couple
weeks.

It always seemed wrong to me that we parse MCFG and set
things up
before we even look at PNP0A03/PNP0A08 devices. It would make
more
sense to me to have something in acpi_pci_root_add() to set up
MMCONFIG using _CBA if available, and falling back to parsing
MCFG
if
it's not.



I think your idea could make the code (design) much cleaner.
Do you have any other reason why you think "It always seemed
wrong..."?



The current scheme is just an ugly design. Does I need more
reasons?
:)



Ok, I just wanted to know if I'm missing anything we need to
take into account when re-factoring the code.

By the way, the following code makes me think there could be
some hardwares that need a fixup using mmconfig access before
scanning the PCI tree. If this is the case, we would need
something to enable early mmconfig initialization for those
hardwares.

static __init int pci_arch_init(void)
{
...
if (!(pci_probe& PCI_PROBE_NOEARLY))
pci_mmcfg_early_init();


Regards,
Kenji Kaneshige



If MMCFG could be treated as an optional configuration space access
method,
we can refine the MMCFG code according to Bjorn's suggestion.
And as
Kenji
has mentioned, there may be some risks ahead. So need more
confirmation
from other PCI experts here.

I looked at the thread, but didn't know which suggestion of
Bjorn's you
were referring to.
But, mmcfg access to PCI config space is need for any cap structure
greater than 256 byte offset. A number of devices have cap
structures
in this upper PCI config space, esp. SRIOV devices.
So, if 'optional MMCFG' only means at the beginning of kernel
scanning
of
PCI (pass-0 scanning), that should be ok, but in-depth, pass-1
scanning
of PCIe devices may require MMCFG for full functional support.


For mainstream systems with support of ACPI and MMCFG, the booting
sequences are about:
1) Probe for legacy PCI configuration access mechanism, such as
CONF1,
CONF2, BIOS
2) Start ACPICA/ACPI subsystem with the legacy PCI configuration
access
mechanism
3) Enumerate PCI root bridges (PNP0A03/PNP0A08) in ACPI namespace and
bind
pci_root
driver to them
4) pci_root driver calls into arch code to add MMCFG information
for the
host bridge
5) pci_root driver calls PCI core to enumerate all PCI devices
under the
host bridge

The above flow should work for SRIOV case. But still need to check
following cases:
1) ACPICA/ACPI subsystem has no dependency on MMCFG
2) Systems implementing SFI instead of ACPI work as expected
3) ACPI has been disabled by user (Bjorn points out we could
ignore this
case)


Agreed. My least favorite bz: "I set boot param to noacpi and can't
scan
entire PCI space.... duh!


4) Some host bridges are not reported by ACPI (Bjorn points out we
should
eventually
get rid of the blind probing logic)


And depend on BIOS-ACPI to be correct all the time?
....hahahahahaha ...
sorry.... you hit my funny bone! ;-)
Is blind probing problematic ?
Seems like a pci-fixup/quirk can be implemented under arch/<>/pci to
handle
these cases, and thus, depend on ACPI for host-bridge info... wait!
did I
just
say depend on ACPI?!?! :)


Hope your funny bone has stopped tingling by now :)

Not when ACPI's always there to bang it again! ;-)

When we probe blindly, we don't know what resources are available on
the bus (except for AMD systems). Therefore, we can't do reliable
assignment, and we have to rely on whatever the BIOS did.

Blind probing finds devices not exposed by the BIOS. This might be a
BIOS bug, or it might be a conscious decision to hide the devices from
the OS. Some OEMs hide devices to reduce the likelihood of users
messing things up with setpci.

It would be interesting and relatively easy to figure out whether
Windows ever discovers a device behind an unreported host bridge. My
guess is "no," but I haven't had time to verify this.

Well, call me crazy(again!), but why put a host-bridge device on a
system
and then (try to) hide it with software(BIOS/ACPI) ?
Sounds like a recipe for disaster if the OS tries to
reconfigure address space or bus-number space.....

Some OEMs do leave host bridges unreported. Here's an example, from
an HP DL380 G7:

I didn't doubt they tried to.... again, why!?!
Some OEMs may choose to hidden special PCI host bridges due to security
reasons.
For example, Intel NHM-EX processor has an embedded PCI host bridge,
which hosts all processor specific configuration information.
You could do memory physical address to memory device address
translation according to information exposed by PCI devices on those
embedded PCI host buses. (Once we have done that.) But later those
PCI host bridges are hidden from OS by BIOS due to security concerns.


ACPI: PCI Root Bridge [PCI0] (domain 0000 [bus 00-1a])
pci 0000:00:00.0: [8086:3406] type 0 class 0x000600
pci 0000:00:01.0: [8086:3408] type 1 class 0x000604
pci 0000:00:02.0: [8086:3409] type 1 class 0x000604
...
PCI: Discovered peer bus 3e
pci 0000:3e:00.0: [8086:2c70] type 0 class 0x000600
pci 0000:3e:00.1: [8086:2d81] type 0 class 0x000600
pci 0000:3e:02.0: [8086:2d90] type 0 class 0x000600
...
PCI: Discovered peer bus 3f
pci 0000:3f:00.0: [8086:2c70] type 0 class 0x000600
pci 0000:3f:00.1: [8086:2d81] type 0 class 0x000600
pci 0000:3f:02.0: [8086:2d90] type 0 class 0x000600

ACPI did not report host bridges leading to buses 3e and 3f; we found
those devices by probing blindly.

In this case, these are Intel CPU uncore devices, and they don't
consume MMIO or IO port resources. But you're absolutely right that
we can't safely reconfigure anything we find this way.

Hiding a PCI device is as simple as not loading its driver... :)

Sure, if it's the OS that wants to hide it. In this case, it's the
OEM that wants to hide it, and the only mechanism for doing that is to
refrain from telling the OS how to find it. I'm pretty confident that
the DL380 omission of those host bridges is an intentional choice by
the BIOS writers.

Bjorn

but this kind of hiding will break code paths/logic like pci=assign-busses,
b/c if the hidden bus nums aren't registered, and they are used by
the assign-bus code, then duplicate busses will have overlapping
sec-num/sub-num
ranges and then there are two bridges that will race (or as I
experienced, hang)
to reply with config cycles in the overlapping range.
so, scanning & registering (but not configuring) 'hidden' (non-ACPI-id'd)
bridges/busses sounds like something the pci code has to do.

.



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