Re: [PATCH v6 23/25] iommu/tegra241-cmdqv: Do not statically map LVCMDQs

On Sat, Jun 14, 2025 at 12:14:48AM -0700, Nicolin Chen wrote:

To simplify the mappings from global VCMDQs to VINTFs' LVCMDQs, the design
chose to do static allocations and mappings in the global reset function.

However, with the user-owned VINTF support, it exposes a security concern:
if user space VM only wants one LVCMDQ for a VINTF, statically mapping two
or more LVCMDQs creates a hidden VCMDQ that user space could DoS attack by
writing random stuff to overwhelm the kernel with unhandleable IRQs.

Thus, to support the user-owned VINTF feature, a LVCMDQ mapping has to be
done dynamically.

HW allows pre-assigning global VCMDQs in the CMDQ_ALLOC registers, without
finalizing the mappings by keeping CMDQV_CMDQ_ALLOCATED=0. So, add a pair
of map/unmap helper that simply sets/clears that bit.

For kernel-owned VINTF0, move LVCMDQ mappings to tegra241_vintf_hw_init(),
and the unmappings to tegra241_vintf_hw_deinit().

For user-owned VINTFs that will be added, the mappings/unmappings will be
on demand upon an LVCMDQ allocation from the user space.

However, the dynamic LVCMDQ mapping/unmapping can complicate the timing of
calling tegra241_vcmdq_hw_init/deinit(), which write LVCMDQ address space,
i.e. requiring LVCMDQ to be mapped. Highlight that with a note to the top
of either of them.

Acked-by: Pranjal Shrivastava <praan@google.com>
Signed-off-by: Nicolin Chen <redacted>
---
 .../iommu/arm/arm-smmu-v3/tegra241-cmdqv.c    | 37 +++++++++++++++++--
 1 file changed, 33 insertions(+), 4 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason