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

[Logan Gunthorpe]

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 <kchow@xxxxxxxxxx>
>> Fixes: da7822e5ad71 ("PCI: update bridge resources to get more big ranges when allocating space (again)")
>> Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
>> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
>> ---
>>  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:
>>
>> https://github.com/sbates130272/linux-p2pmem pci_realloc_v4
>>
>> [1] https://lore.kernel.org/linux-pci/20190617135307.GA13533@xxxxxxxxxx/
>>
>> 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 &&
>> +		    idx <= PCI_BRIDGE_RESOURCE_END)
>>  			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
other flags like IORESOURCE_STARTALIGN or IORESOURCE_SIZEALIGN; though
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.

Logan