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

[Bjorn Helgaas]

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 <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.

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.

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

Bjorn