On ARM64 platforms with GICv3 ITS, VFIO PCI passthrough currently
cannot dynamically allocate MSI-X vectors after MSI-X has been
enabled. When QEMU needs to extend the vector range, it must
disable MSI-X, free all interrupts, then re-enable with a larger
allocation. This creates an interrupt loss window for already-active
vectors.

Consider HNS3 with RoCE: NIC and RDMA share one PCI device and
ITS DeviceID, with MSI-X vectors partitioned as NIC (lower range)
then RoCE (starting at base_vector = num_nic_msi). In VFIO
passthrough, loading hns_roce after hns3 forces QEMU to tear down
all interrupts before re-allocating the larger range. During this
process, NIC interrupts may be lost. Testing confirmed that this
occasionally occurs, causing the network port reset to fail. This
appears to be unavoidable, as it's a standard approach adopted by
all network card vendors.

On Hisilicon HIP09 (ARM64, GICv3/GICv4.1) with latest upstream kernel
and QEMU 8.2. VFIO passthrough of HNS3 NIC to VM: load both hns3 and
hns_roce_hw_v2 drivers and trigger FLR, this bug will occur occasionally.
After enabling dynamic MSIX allocation, this bug no longer occurs.

When set this flag, for the ITT, adding a new vector only inserts a new
entry, existing entries are untouched, so already-active vectors are
unaffected.

The ITT are sized to hwsize at MSI-X domain creation time. so there
is always sufficient space for any vector index that may be dynamiclly
allocated later. So the ITT never needs to be resized at runtime. The
dynamic allocation path(msi_domain_alloc_irq_at -> populate_alloc_info)
simply copies the pre-prepared alloc_data without re-invoking
msi_prepare(). Then picking a free EventID from the already-allocated
ITT and a free LPI form the already-reserved bitmap --- no existing
vector is affected.

Signed-off-by: Jinqian Yang <redacted>
Reviewed-by: Marc Zyngier <maz@kernel.org>
---
Changes in v2:
  - Updated the commit message to add test information.
Changes in v3:
  - Updated the commit message to add analysis.
  - Added Reviewed-by tags from Marc.

v1: https://lore.kernel.org/linux-arm-kernel/20260624025345.458387-1-yangjinqian1@huawei.com/ (local)
v2: https://lore.kernel.org/linux-arm-kernel/20260711022015.3049867-1-yangjinqian1@huawei.com/ (local)
---
 drivers/irqchip/irq-gic-its-msi-parent.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/irqchip/irq-gic-its-msi-parent.c b/drivers/irqchip/irq-gic-its-msi-parent.c
index b9257103a999..b2b9d2068bb1 100644
--- a/drivers/irqchip/irq-gic-its-msi-parent.c
+++ b/drivers/irqchip/irq-gic-its-msi-parent.c

@@ -18,7 +18,8 @@
 
 #define ITS_MSI_FLAGS_SUPPORTED (MSI_GENERIC_FLAGS_MASK |	\
 				 MSI_FLAG_PCI_MSIX      |	\
-				 MSI_FLAG_MULTI_PCI_MSI)
+				 MSI_FLAG_MULTI_PCI_MSI |	\
+				 MSI_FLAG_PCI_MSIX_ALLOC_DYN)
 
 static int its_translate_frame_address(struct fwnode_handle *msi_node, phys_addr_t *pa)
 {

-- 
2.33.0