RE: [PATCH v3] iommufd/device: Enforce reserved IOVA also when attached to hwpt_nested

[Tian, Kevin]

> From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Sent: Wednesday, August 7, 2024 8:35 AM
> 
> Currently, device reserved regions are only enforced when the device is
> attached to an hwpt_paging. In other words, if the device gets attached
> to an hwpt_nested directly, the parent hwpt_paging of the hwpt_nested's
> would not enforce those reserved IOVAs. This works for most of reserved
> region types, but not for IOMMU_RESV_SW_MSI, which is a unique software
> defined window, required by a nesting case too to setup an MSI doorbell
> on the parent stage-2 hwpt/domain.
> 
> Kevin pointed out that:
> 1) there is no usage using up closely the entire IOVA space yet,
> 2) guest may change the viommu mode to switch between nested
>    and paging then VMM has to take all devices' reserved regions
>    into consideration anyway, when composing the GPA space.
> Link:
> https://lore.kernel.org/all/BN9PR11MB5276497781C96415272E6FED8CB12@
> BN9PR11MB5276.namprd11.prod.outlook.com/
> 
> So it would be actually convenient for us to also enforce reserved IOVA
> onto the parent hwpt_paging, when attaching a device to an hwpt_nested.
> 
> Repurpose the existing attach/replace_paging helpers to attach device's
> reserved IOVAs exclusively.
> 
> Add a new find_hwpt_paging helper, which is only used by these reserved
> IOVA functions, to allow an IOMMUFD_OBJ_HWPT_NESTED hwpt to redirect
> to
> its parent hwpt_paging. Return a NULL in these two helpers for any new
> HWPT type in the future.
> 
> Suggested-by: Tian, Kevin <kevin.tian@xxxxxxxxx>
> Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>

I'm not sure the name find_hwpt_paging() is good enough but can't
find a better alternative. So,

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>