Enable Qualcomm WiFi PCIe passthrough to VMs

This series enables Qualcomm ath11k/ath12k WiFi devices to work in VM passthrough scenarios, addressing a long-standing issue where these devices fail to initialize when passed through to VMs via VFIO.

Qualcomm ath11k/ath12k WiFi devices have been broken in VM passthrough since they were introduced. The devices use an embedded interrupt controller that requires physical host MSI addresses programmed to device registers, but in VMs the driver only sees virtualized guest addresses. This causes firmware initialization to fail with errors like "BHI offset: 0xffffffff is out of range".

Multiple attempts have been made to solve this over the past 2 years:

March 2024: Workaround using module parameters [1]
- Required manually copying MSI values from host to VM
- Rejected by Johannes Berg as too manual, suggested VMM solution

August 2024: QEMU + kernel solution by Alex Williamson [2]
- Kernel disables MSI virtualization, QEMU intercepts config writes
- Uses brute force pattern matching to infer MSI data writes
- Stuck as RFC - no progress in 8+ months

No action from Qualcomm: Despite multiple reports, no vendor solution

The solution in this series provides a clean, kernel-only solution that addresses the previous concerns.

Architecture:
1. qcom-vfio-pci variant driver caches physical host MSI values and exposes them via extended config space with magic signature "QMSI" In this way, device-specific hardware quirks belong only in kernel drivers, not userspace, independent of the tools used.
This follows the established VFIO variant driver pattern used by other drivers like mlx5-vfio-pci (netdev tree) and nvgrace-gpu (platform tree).
2. VM drivers (ath11k/ath12k) automatically discover and use cached values
3. PCIe link recovery handles VM timing variations

Thoroughly tested: 3 VMs across 2 WiFi generations, all devices successfully initialized and are ready for use.
- VM1: ath11k (WCN6855)
- VM2: ath11k (WCN6855)
- VM3: ath12k (WCN7850)

[1] https://lore.kernel.org/all/[email protected]/
[2] https://lore.kernel.org/kvm/[email protected]/
[3] Original problem report:

    https://lore.kernel.org/all/[email protected]/

Jose Ignacio Tornos Martinez (7):
PCI: Add pci_find_free_ext_cap_offset() helper
vfio: Add qcom_vfio.h header for MSI cache protocol vfio/pci: Add qcom-vfio-pci variant driver
ath11k: Add PCIe link recovery retry for VMs
ath11k: Use VFIO MSI cache when available
ath12k: Add PCIe link recovery retry for VMs
ath12k: Use VFIO MSI cache when available
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2.53.0