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SubjectRe: [PATCH v4] PCI: Fix disabling of bridge BARs when assigning bus resources

On 2020-01-07 5:41 p.m., Bjorn Helgaas wrote:
> On Tue, Jan 07, 2020 at 03:51:28PM -0700, Logan Gunthorpe wrote:
>> On 2020-01-07 2:13 p.m., Bjorn Helgaas wrote:
>>> On Tue, Jan 07, 2020 at 12:09:02PM -0700, Logan Gunthorpe wrote:
>>>> One odd quirk of PLX switches is that their upstream bridge port has
>>>> 256K of space allocated behind its BAR0 (most other bridge
>>>> implementations do not report any BAR space). The lspci for such device
>>>> looks like:
>>>> 04:00.0 PCI bridge: PLX Technology, Inc. PEX 8724 24-Lane, 6-Port PCI
>>>> Express Gen 3 (8 GT/s) Switch, 19 x 19mm FCBGA (rev ca)
>>>> (prog-if 00 [Normal decode])
>>>> Physical Slot: 1
>>>> Flags: bus master, fast devsel, latency 0, IRQ 30, NUMA node 0
>>>> Memory at 90a00000 (32-bit, non-prefetchable) [size=256K]
>>>> Bus: primary=04, secondary=05, subordinate=0a, sec-latency=0
>>>> I/O behind bridge: 00002000-00003fff
>>>> Memory behind bridge: 90000000-909fffff
>>>> Prefetchable memory behind bridge: 0000380000800000-0000380000bfffff
>>>> Kernel driver in use: pcieport
>>>> It's not clear what the purpose of the memory at 0x90a00000 is, and
>>>> currently the kernel never actually uses it for anything. In most cases,
>>>> it's safely ignored and does not cause a problem.
>>>> However, when the kernel assigns the resource addresses (with the
>>>> pci=realloc command line parameter, for example) it can inadvertently
>>>> disable the struct resource corresponding to the bar. When this happens,
>>>> lspci will report this memory as ignored:
>>>> Region 0: Memory at <ignored> (32-bit, non-prefetchable) [size=256K]
>>>> This is because the kernel reports a zero start address and zero flags
>>>> in the corresponding sysfs resource file and in /proc/bus/pci/devices.
>>>> Investigation with 'lspci -x', however shows the bios-assigned address
>>>> will still be programmed in the device's BAR registers.
>>>> It's clearly a bug that the kernel's view of the registers differs from
>>>> what's actually programmed in the BAR, but in most cases, this still
>>>> won't result in a visibile issue because nothing uses the memory,
>>>> so nothing is affected. However, a big problem shows up when an IOMMU
>>>> is in use: the IOMMU will not reserve this space in the IOVA because the
>>>> kernel no longer thinks the range is valid. (See
>>>> dmar_init_reserved_ranges() for the Intel implementation of this.)
>>>> Without the proper reserved range, we have a situation where a DMA
>>>> mapping may occasionally allocate an IOVA which the PCI bus will actually
>>>> route to a BAR in the PLX switch. This will result in some random DMA
>>>> writes not actually writing to the RAM they are supposed to, or random
>>>> DMA reads returning all FFs from the PLX BAR when it's supposed to have
>>>> read from RAM.
>>>> The problem is caused in pci_assign_unassigned_root_bus_resources().
>>>> When any resource from a bridge device fails to get assigned, the code
>>>> sets the resource's flags to zero. This makes sense for bridge resources,
>>>> as they will be re-enabled later, but for regular BARs, it disables them
>>>> permanently.
>>>> The code in question seems to indent to check if "dev->subordinate" is
>>>> zero to determine whether a device is a bridge, however this is not
>>>> likely valid as there might be a bridge without a subordinate bus due to
>>>> running out of bus numbers or other cases.
>>>> To fix these issues we instead check that the idx is in the
>>>> PCI_BRIDGE_RESOURCES range which are only used for bridge windows and
>>>> thus is sufficient for the "dev->subordinate" check and will also
>>>> prevent the bug above from clobbering PLX devices' regular BARs.
>>> s/bios/BIOS/
>>> s/bar/BAR/
>>> s/visibile/visible/
>>> s/indent/intend/
>>>> Reported-by: Kit Chow <>
>>>> Fixes: da7822e5ad71 ("PCI: update bridge resources to get more big ranges when allocating space (again)")
>>>> Signed-off-by: Logan Gunthorpe <>
>>>> Cc: Bjorn Helgaas <>
>>>> ---
>>>> drivers/pci/setup-bus.c | 6 +++++-
>>>> 1 file changed, 5 insertions(+), 1 deletion(-)
>>>> This patch was last submitted back in June as part of a series. I've
>>>> dropped the first patch in the series as a similar patch from Nicholas
>>>> takes care of the bug.
>>>> As a reminder, the previous discussion on this patch is here[1]. Per the
>>>> feedback, I've updated the patch to remove the check on
>>>> "dev->subordinate" entirely.
>>>> The patch is based on v5.5-rc5 and a git branch is available here:
>>>> pci_realloc_v4
>>>> [1]
>>>> diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
>>>> index f279826204eb..23f6c95f3fd7 100644
>>>> --- a/drivers/pci/setup-bus.c
>>>> +++ b/drivers/pci/setup-bus.c
>>>> @@ -1803,11 +1803,15 @@ void pci_assign_unassigned_root_bus_resources(struct pci_bus *bus)
>>>> /* Restore size and flags */
>>>> list_for_each_entry(fail_res, &fail_head, list) {
>>>> struct resource *res = fail_res->res;
>>>> + int idx;
>>>> res->start = fail_res->start;
>>>> res->end = fail_res->end;
>>>> res->flags = fail_res->flags;
>>>> - if (fail_res->dev->subordinate)
>>>> +
>>>> + idx = res - &fail_res->dev->resource[0];
>>>> + if (idx >= PCI_BRIDGE_RESOURCES &&
>>>> res->flags = 0;
>>> So I guess previously, for everything on the fail_head list, we
>>> restored flags/start/end *and* we cleared flags for every BAR and
>>> window of a bridge.
>>> Now we'll clear flags for only for bridge windows. I'm sure that was
>>> the original intent, but I don't see why we bother. The next thing we
>>> do is go back to "again", where we call __pci_bus_size_bridges(),
>>> where we immediately call pci_bridge_check_ranges(), which recomputes
>>> the flags.
>>> Is there actually any point in clearing res->flags, or could we just
>>> do this:
>> Hmm, well removing the check doesn't seem to cause any problems on my
>> test box. But I'm not very confident that it's not required for some
>> corner case. It was clearly added by someone for a reason that is not
>> clear based on the information I can find in git blame.
>> I don't agree that pci_bridge_check_ranges() recomputes the flags... it
>> only sets specific flags. So zeroing the flags may be intended to clear
>> it's not super clear to me how those are used either.
>> So I'd personally prefer to err on the side of caution here and not
>> introduce any new subtle bugs.
> OK, I hate maintaining this sort of black magic code, but that's a
> fair point, and we don't have to fix everything at once.

Yes, I can feel that pain. It's hard enough trying to fix bugs in it.
Seems like we need to get a unit testing suite for it built up so we can
at least have some way to know if changes are acceptable. I keep hearing
about bios bugs that are triggering other bugs in this code (some
fixable and some not) and they're hard to deal with because of the mess.
But that's a ton of work and I don't have the time to tackle it.

> pci_assign_unassigned_root_bus_resources() and
> pci_assign_unassigned_bridge_resources() both have this code fragment,
> and I *assume* both should be changed?

Oh, yes, that's probably true. I'll add that, fix up the nits above and
send a v5 later this week.


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