Re: [PATCH v5 10/29] iommufd: Abstract iopt_pin_pages and iopt_unpin_pages helpers

Jason Gunthorpe
2025-05-28T17:20:35Z

On Sat, May 17, 2025 at 08:21:27PM -0700, Nicolin Chen wrote:
> The new HW QUEUE object will be added for HW to access the guest queue for
> HW-accelerated virtualization feature. Some of HW QUEUEs are designed in a
> way of accessing the guest queue via a host physical address without doing
> a translation using the nesting parent IO page table, while others can use
> the guest physical address. For the former case, kernel working with a VMM
> needs to pin the physical pages backing the guest memory to lock them when
> HW QUEUE is accessing, and to ensure those physical pages to be contiguous
> in the physical address space.
>
> This is very like the existing iommufd_access_pin_pages() that outputs the
> pinned page list for the caller to test its contiguity.
>
> Move those code from iommufd_access_pin/unpin_pages() and related function
> for a pair of iopt helpers that can be shared with the HW QUEUE allocator.
>
> Rename check_area_prot() to align with the existing iopt_area helpers, and
> inline it to the header since iommufd_access_rw() still uses it.
>
> Reviewed-by: Pranjal Shrivastava <praan@google.com>
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> ---
> drivers/iommu/iommufd/io_pagetable.h | 8 ++
> drivers/iommu/iommufd/iommufd_private.h | 6 ++
> drivers/iommu/iommufd/device.c | 119 ++----------------------
> drivers/iommu/iommufd/io_pagetable.c | 97 +++++++++++++++++++
> 4 files changed, 119 insertions(+), 111 deletions(-)

And if you do what was suggested do we need this patch at all? Just
use the normal access sequence:

iommufd_access_create(ops=NULL)
iommufd_access_attach(viommu->hwpt->ioas)
iommufd_access_pin_pages()

And store a viommu->access pointer to undo it all.

This avoids making it all special with different internal behavior
from a mdev. The only difference is we allowe ops=null for viommu but
not for mdev.

I don't think it is worth doing all these changes just to eliminate
the access memory allocation.. viommu allocation is not fast path.

Jason