linux-kernel - Re: [PATCH 0/7] iommu cleanup and refactoring

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Message-ID: <7febcba4-f5bf-6bf6-6180-895b18d1b806@arm.com>
Date:   Tue, 25 Jan 2022 14:48:02 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Ben Skeggs <bskeggs@...hat.com>,
        "Raj, Ashok" <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/7] iommu cleanup and refactoring

On 2022-01-24 17:44, Jason Gunthorpe wrote:
> On Mon, Jan 24, 2022 at 09:46:26AM +0000, Tian, Kevin wrote:
>>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>>> Sent: Monday, January 24, 2022 3:11 PM
>>>
>>> Hi,
>>>
>>> The guest pasid and aux-domain related code are dead code in current
>>> iommu subtree. As we have reached a consensus that all these features
>>> should be based on the new iommufd framework (which is under active
>>> development), the first part of this series removes and cleanups all
>>> the dead code.
>>>
>>> The second part of this series refactors the iommu_domain by moving all
>>> domain-specific ops from iommu_ops to a new domain_ops. This makes an
>>> iommu_domain self-contained and represent the abstraction of an I/O
>>> translation table in the IOMMU subsystem. With different type of
>>> iommu_domain providing different set of ops, it's easier to support more
>>> types of I/O translation tables.
>>
>> You may want to give more background on this end goal. In general there
>> are four IOPT types in iommufd discussions:
>>
>> 1) The one currently tracked by iommu_domain, with a map/unmap semantics
>> 2) The one managed by mm and shared to iommu via sva_bind/unbind ops
>> 3) The one managed by userspace and bound to iommu via iommufd (require nesting)
>> 4) The one managed by KVM (e.g. EPT) and shared to iommu via a TBD interface
> 
> Yes, at least from an iommufd perspective I'd like to see one struct
> for all of these types, mainly so we can have a uniform alloc, attach
> and detatch flow for all io page table types.

Agreed, certainly an IOMMU_DOMAIN_SVA type that can both encapsulate the 
mm and effectively replace iommu_sva seems like a logical and fairly 
small next step. We already have the paradigm of different domain types 
supporting different ops, so initially an SVA domain would simply allow 
bind/unbind rather than attach/detach/map/unmap.

It might then further be possible to hide SVA bind/unbind behind the 
attach/detach interface, but AFAICS we'd still need distinct flows for 
attaching/binding the whole device vs. attaching/binding to a PASID, 
since they are fundamentally different things in their own right, and 
the ideal API should give us the orthogonality to also bind a device to 
an SVA domain without PASID (e.g. for KVM stage 2, or userspace 
assignment of simpler fault/stall-tolerant devices), or attach PASIDs to 
regular iommu_domains.

That distinction could of course be consolidated by flipping to the 
other approach of explicitly allocating the PASID first, then wrapping 
it in a struct device that could then be passed through the same 
attach/detach interfaces and distinguished internally, but although I 
still have a fondness for that approach I know I'm about the only one :)

Cheers,
Robin.

> If we want to use the iommu_domain, or make iommu_domain a sub-class
> of a new struct, can be determined as we go along.
> 
> Regardless, I think this cleanup stands on its own. Moving the ops and
> purging the dead code is clearly the right thing to do.
> 
> Thanks,
> Jason