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

[Marc Zyngier]

On Wed, 24 Jun 2026 03:53:45 +0100,
Jinqian Yang <yangjinqian1@xxxxxxxxxx> wrote:
> 
> 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.

Well, that's what you get for not exposing differentiated functions.
Eventually, you face the reality that this is a poor design.

> 
> 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.

How is this paragraph relevant? The kernel has had this fix for over a
year, and backporting this series is not something I plan to ever do.

>
> 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)
>  {

What has this been tested with? In which conditions?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.