[RFC PATCH] irqchip/gic-v3-its: enable dynamic MSI-X allocation

[Jinqian Yang]

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.

ITS_MSI_FLAGS_SUPPORTED lacks MSI_FLAG_PCI_MSIX_ALLOC_DYN, causing
pci_msix_can_alloc_dyn() to return false. VFIO then sets
has_dyn_msix=false and never clears VFIO_IRQ_INFO_NORESIZE for
MSI-X, keeping the old "disable and reallocate" behavior.

The essential prerequisite for enabling this flag is the fix to
msi_prepare() call timing (commit 1396e89e09f0 ("genirq/msi: Move
prepare() call to per-device allocation")): msi_prepare() is
now called once at per-device domain creation with hwsize, so ITS
creates an ITT with sufficient capacity for all MSI-X vectors.
Without this fix, msi_prepare() was called per-allocation with
semi-random nvec, maybe resulting in an ITT too small for dynamic
vector addition.

With this in place, dynamic MSI-X allocation works correctly:
msi_domain_alloc_irq_at() uses populate_alloc_info() to copy the
pre-prepared alloc_data without re-invoking msi_prepare(), so each
new vector simply gets a LPI entry in the already-allocated ITT,
without affecting existing vectors.

Signed-off-by: Jinqian Yang <yangjinqian1@xxxxxxxxxx>
---
 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