linux-kernel - Re: [PATCH v5 10/29] iommufd: Abstract iopt_pin_pages and iopt_unpin

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Message-ID: <20250528171754.GY61950@nvidia.com>
Date: Wed, 28 May 2025 14:17:54 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: kevin.tian@...el.com, corbet@....net, will@...nel.org,
	bagasdotme@...il.com, robin.murphy@....com, joro@...tes.org,
	thierry.reding@...il.com, vdumpa@...dia.com, jonathanh@...dia.com,
	shuah@...nel.org, jsnitsel@...hat.com, nathan@...nel.org,
	peterz@...radead.org, yi.l.liu@...el.com, mshavit@...gle.com,
	praan@...gle.com, zhangzekun11@...wei.com, iommu@...ts.linux.dev,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-tegra@...r.kernel.org,
	linux-kselftest@...r.kernel.org, patches@...ts.linux.dev,
	mochs@...dia.com, alok.a.tiwari@...cle.com, vasant.hegde@....com,
	dwmw2@...radead.org, baolu.lu@...ux.intel.com
Subject: Re: [PATCH v5 10/29] iommufd: Abstract iopt_pin_pages and
 iopt_unpin_pages helpers

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@...gle.com>
> Reviewed-by: Kevin Tian <kevin.tian@...el.com>
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Nicolin Chen <nicolinc@...dia.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