Re: [RFC PATCH 1/3] powerpc/pseries/iommu: Bring back userspace view for single level TCE tables

From: Shivaprasad G Bhat <sbhat@linux.ibm.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: tpearson@raptorengineering.com, alex.williamson@redhat.com,
	linuxppc-dev@lists.ozlabs.org, mpe@ellerman.id.au,
	npiggin@gmail.com, christophe.leroy@csgroup.eu,
	aneesh.kumar@kernel.org, naveen.n.rao@linux.ibm.com,
	gbatra@linux.vnet.ibm.com, brking@linux.vnet.ibm.com,
	aik@ozlabs.ru, robh@kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, aik@amd.com, msuchanek@suse.de,
	jroedel@suse.de, vaibhav@linux.ibm.com, svaidy@linux.ibm.com
Subject: Re: [RFC PATCH 1/3] powerpc/pseries/iommu: Bring back userspace view for single level TCE tables
Date: Wed, 20 Mar 2024 20:59:34 +0530	[thread overview]
Message-ID: <0514fb50-443d-427f-ac5b-a29679b6938b@linux.ibm.com> (raw)
In-Reply-To: <20240319143202.GA66976@ziepe.ca>

Hi Jason,

On 3/19/24 20:02, Jason Gunthorpe wrote:
> On Tue, Mar 12, 2024 at 01:14:20PM -0500, Shivaprasad G Bhat wrote:
>> The commit 090bad39b237a ("powerpc/powernv: Add indirect levels to
>> it_userspace") which implemented the tce indirect levels
>> support for PowerNV ended up removing the single level support
>> which existed by default(generic tce_iommu_userspace_view_alloc/free()
>> calls). On pSeries the TCEs are single level, and the allocation
>> of userspace view is lost with the removal of generic code.
> :( :(
>
> If this has been broken since 2018 and nobody cared till now can we
> please go in a direction of moving this code to the new iommu APIs
> instead of doubling down on more of this old stuff that apparently
> almost nobody cares about ??

We have existing software stack deployments using VFIO userspace
device assignment running on Power platform. We have to enable
similar software stack on newer generation Power10 platform and
also in a pSeries lpar environment. These distros rely on VFIO enabled
in kernel and currently have IOMMUFD disabled. This patch series is
a simpler low risk enablement that functionally get the software stack
working while we continue to enable and move to IOMMUFD in phases.
We have to fix the older APIs in order to stage the functional enablement
in small increments.

We are working on iommufd support for pSeries and looking forward
to Timothy's patches.

-Thanks

Shivaprasad

> Jason