[RFC PATCH v3 01/14] iommu/riscv: Allocate MSI tables for second-stage domains

[Andrew Jones]

On Mon, Sep 28, 2026 at 04:31:00PM +0200, Andrew Jones wrote:
> IRQ bypass maps guest IMSIC addresses through a flat-mode MSI page
> table owned by the second-stage domain. Allocate the table lazily
> during device attachment when the attaching IOMMU supports MSI_FLAT,
> and free it when the domain is destroyed.
> 
> The required capacity depends on hypervisor support and VMM policy.
> Expose the runtime-writable riscv_iommu.nr_msi_ptes parameter to set the
> MSI table capacity of new second-stage domains. It defaults to 512
> entries (8 KiB), and zero disables MSI table allocation for new domains.
> Existing domains retain the capacity selected when they were created.
> 
> Signed-off-by: Andrew Jones <andrew.jones at oss.qualcomm.com>
> ---
>  drivers/iommu/riscv/iommu.c | 61 +++++++++++++++++++++++++++++++++++++
>  drivers/iommu/riscv/iommu.h |  7 +++++
>  2 files changed, 68 insertions(+)
> 
> diff --git a/drivers/iommu/riscv/iommu.c b/drivers/iommu/riscv/iommu.c
> index 0953aaf594cb..3bbb4d0d0a85 100644
> --- a/drivers/iommu/riscv/iommu.c
> +++ b/drivers/iommu/riscv/iommu.c
> @@ -22,6 +22,8 @@
>  #include <linux/iopoll.h>
>  #include <linux/irqchip/riscv-imsic.h>
>  #include <linux/kernel.h>
> +#include <linux/moduleparam.h>
> +#include <linux/mutex.h>
>  #include <linux/pci.h>
>  #include <linux/generic_pt/iommu.h>
>  
> @@ -30,6 +32,16 @@
>  #include "iommu-bits.h"
>  #include "iommu.h"
>  
> +#undef MODULE_PARAM_PREFIX
> +#define MODULE_PARAM_PREFIX "riscv_iommu."
> +
> +#define RISCV_IOMMU_DEFAULT_NR_MSI_PTES	512
> +
> +/* A zero value disables guest MSI table allocation. */
> +static unsigned int riscv_iommu_nr_msi_ptes = RISCV_IOMMU_DEFAULT_NR_MSI_PTES;
> +module_param_named(nr_msi_ptes, riscv_iommu_nr_msi_ptes, uint, 0644);
> +MODULE_PARM_DESC(nr_msi_ptes, "Number of PTEs for new second-stage domains (default: 512)");

I never liked the need for this parameter. I think I have a way to remove
it, ensure table size supports (and no more than supports) the guest's
IMSIC topology, and moves ownership of the MSI table to the hypervisor
(which, since the MSI table is only needed for VM's, is likely the better
owner). I'll experiment with that idea for the next version.

Thanks,
drew