[BUG] PCIe bridge resource allocation creates invalid limit addresses after Secondary Bus Reset recovery

Shawn Jin
2026-03-11T22:00:42Z

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Hello,

I'm reporting a potential critical bug in the Linux kernel's PCIe resource allocation code that creates invalid bridge window limit addresses during hotplug re-enumeration after Secondary Bus Reset (SBR) recovery.

## AFFECTED KERNEL VERSIONS
- Confirmed: 5.15.0, 6.8.0 (Ubuntu 6.8.0-88-generic, 6.8.0-90-generic)
- Likely affected: All recent kernels including 6.19

## HARDWARE CONFIGURATION
Intel Ice Lake server with PCIe Gen5 switches and endpoints:

Topology 1:
Root Port 96:01.0 → 98:00.0 → 99:01.0 → 9b:00.0 (NVIDIA L20 GPU)

Kernel parameter: pci=realloc=on

## PROBLEM DESCRIPTION

After performing Secondary Bus Reset on a PCIe switch port and clearing the reset bit, the kernel re-enumerates devices and assigns bridge window resources. However, the assigned memory window limit addresses are INVALID according to PCIe specification.

### Evidence from dmesg (Topology 1):

**Before SBR (correct allocation):**
```
[ 6.636493] pci 0000:98:00.0: PCI bridge to [bus 99-9c]
[ 6.636539] pci 0000:98:00.0: bridge window [mem 0xe9600000-0xe96fffff]
[ 6.636645] pci 0000:98:00.0: bridge window [mem 0x13b000000000-0x13b7ffffffff 64bit pref]

[ 6.644429] pci 0000:99:01.0: PCI bridge to [bus 9b]
[ 6.644476] pci 0000:99:01.0: bridge window [mem 0xe9600000-0xe96fffff]
[ 6.644656] pci 0000:99:01.0: bridge window [mem 0x13b000000000-0x13b7ffffffff 64bit pref]

[ 6.654203] pci 0000:9b:00.0: [1e3e:0002] type 00 class 0x120000 PCIe Endpoint
[ 6.654652] pci 0000:9b:00.0: BAR 0 [mem 0x13b000000000-0x13b7ffffffff 64bit pref]
[ 6.654666] pci 0000:9b:00.0: BAR 2 [mem 0xe9600000-0xe963ffff]
```

**After SBR clear (INVALID allocation):**
```
[ 656.644184] pci 0000:98:00.0: bridge window [mem 0x13b000000000-0x13b7ffffffff 64bit pref]: assigned
[ 656.644186] pci 0000:98:00.0: bridge window [mem 0xe9600000-0xe96fffff]: assigned
[ 656.644188] pci 0000:99:01.0: bridge window [mem 0x13b000000000-0x13b7fffffffe 64bit pref]: assigned
[ 656.644189] pci 0000:99:01.0: bridge window [mem 0xe9600000-0xe96ffffe]: assigned

[ 656.644830] pci 0000:9b:00.0: BAR 0 [mem size 0x800000000 64bit pref]: can't assign; no space
[ 656.644831] pci 0000:9b:00.0: BAR 0 [mem size 0x800000000 64bit pref]: failed to assign
// BAR2 can still be assigned because the size is only 256KB, while the min window in the bridge is 1MB
[ 656.644832] pci 0000:9b:00.0: BAR 2 [mem 0xe9600000-0xe963ffff]: assigned

```

### Invalid Addresses Created by Kernel:
- `0x13b7ffffffff` (ends in 0xFFFE - **2 bytes short**)
- `0xe96ffffe` (ends in 0xFFFE - **2 bytes short**)

## IMPACT

1. **Device initialization failure**: Endpoints cannot allocate required BARs
```
[ 656.644830] pci 0000:9b:00.0: BAR 0 [mem size 0x800000000 64bit pref]: can't assign; no space
[ 656.644831] pci 0000:9b:00.0: BAR 0 [mem size 0x800000000 64bit pref]: failed to assign
```

2. **Consistent across multiple hierarchies**: Affects different PCIe topologies independently

## REPRODUCTION

The attached script test_rc_sbr.sh.txt issues a SBR to the root port.

## SUSPECTED ROOT CAUSE

The bug appears to be in `drivers/pci/setup-bus.c`, likely in:
- `pci_bus_distribute_available_resources()`
- `adjust_bridge_window()`
- `pci_assign_unassigned_bridge_resources()`

The resource end address calculation appears to perform multiple subtractions:
1. Initial calculation: `res->end = res->start + size - 1` (correct)
2. During redistribution: Another subtraction occurs, creating `res->end = ... - 2`

## WORKAROUND ATTEMPTS

- `pci=realloc=on`: Does NOT fix the issue
- Manual remove/rescan from root: Does NOT fix the issue
- Initial boot allocation: Works correctly (bug only occurs during hotplug re-enumeration)

## REQUEST

I want to track how the bridge windows are allocated. Is there a way to enable additional kernel messages to show the path? Please investigate if this is a real kernel bug.

Thank you,
Shawn

[-- Attachment #2: test_rc_sbr.sh.txt --]
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#!/bin/bash

# Function to display usage
usage() {
echo "Usage: $0 -rp <ROOT_PORT_BDF> -usp <USP_BDF>"
echo "Example: $0 -rp c6:01.0 -usp c7:00.0"
exit 1
}

# Initialize variables
ROOT_PORT_BDF=""
USP_BDF=""

# Parse command-line arguments
while [[ $# -gt 0 ]]; do
case $1 in
-rp)
ROOT_PORT_BDF="$2"
shift 2
;;
-usp)
USP_BDF="$2"
shift 2
;;
-h|--help)
usage
;;
*)
echo "Unknown option: $1"
usage
;;
esac
done

# Validate that both arguments are provided
if [ -z "$ROOT_PORT_BDF" ] || [ -z "$USP_BDF" ]; then
echo "Error: Both -rp and -usp arguments are required"
usage
fi

echo "Root Port BDF: $ROOT_PORT_BDF"
echo "USP BDF: $USP_BDF"
echo ""

# Remove the USP device
echo 1 | sudo tee /sys/bus/pci/devices/0000:${USP_BDF}/remove

# Trigger SBR via Bridge Control register
BRIDGE_CTL=$(sudo setpci -s ${ROOT_PORT_BDF} 0x3E.w)
BRIDGE_CTL_RESET=$(printf "0x%04x" $((0x$BRIDGE_CTL | 0x0040)))

echo "Asserting Secondary Bus Reset..."
sudo setpci -s ${ROOT_PORT_BDF} 0x3E.w=$BRIDGE_CTL_RESET
sleep 1

echo "De-asserting Secondary Bus Reset..."
sudo setpci -s ${ROOT_PORT_BDF} 0x3E.w=$BRIDGE_CTL
sleep 2

# Rescan PCI bus
echo 1 | sudo tee /sys/bus/pci/rescan