Re: [PATCH] iommu/tegra241-cmdqv: Read SMMU IDR1.CMDQS instead of hardcoding

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Dec 18, 2024 at 09:14:21PM -0800, Nicolin Chen wrote:
> The hardware limitation "max=19" actually comes from SMMU Command Queue.
> So, it'd be more natural for tegra241-cmdqv driver to read it out rather
> than hardcoding it itself.
> 
> This is not an issue yet for a kernel on a baremetal system, but a guest
> kernel setting the queue base/size in form of IPA/gPA might result in a
> noncontiguous queue in the physical address space, if underlying physical
> pages backing up the guest RAM aren't contiguous entirely: e.g. 2MB-page
> backed guest RAM cannot guarantee a contiguous queue if it is 8MB (capped
> to VCMDQ_LOG2SIZE_MAX=19). This might lead to command errors when HW does
> linear-read from a noncontiguous queue memory.
> 
> Adding this extra IDR1.CMDQS cap (in the guest kernel) allows VMM to set
> SMMU's IDR1.CMDQS=17 for the case mentioned above, so a guest-level queue
> will be capped to maximum 2MB, ensuring a contiguous queue memory.
> 
> Fixes: a3799717b881 ("iommu/tegra241-cmdqv: Fix alignment failure at max_n_shift")
> Reported-by: Ian Kalinowski <ikalinowski@nvidia.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> ---
>  drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)

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

Jason