netdev - Re: [PATCH 1/5] iommu: Replace uses of IOMMU_CAP_CACHE_COHERENCY with dev_is_dma

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Date:   Wed, 6 Apr 2022 16:48:25 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Jason Gunthorpe <jgg@...dia.com>, Christoph Hellwig <hch@....de>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Christian Benvenuti <benve@...co.com>,
        Cornelia Huck <cohuck@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        iommu@...ts.linux-foundation.org, Jason Wang <jasowang@...hat.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-s390@...r.kernel.org,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Nelson Escobar <neescoba@...co.com>, netdev@...r.kernel.org,
        Rob Clark <robdclark@...il.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        virtualization@...ts.linux-foundation.org,
        Will Deacon <will@...nel.org>,
        "Tian, Kevin" <kevin.tian@...el.com>
Subject: Re: [PATCH 1/5] iommu: Replace uses of IOMMU_CAP_CACHE_COHERENCY with
 dev_is_dma_coherent()

On 2022-04-06 15:14, Jason Gunthorpe wrote:
> On Wed, Apr 06, 2022 at 03:51:50PM +0200, Christoph Hellwig wrote:
>> On Wed, Apr 06, 2022 at 09:07:30AM -0300, Jason Gunthorpe wrote:
>>> Didn't see it
>>>
>>> I'll move dev_is_dma_coherent to device.h along with
>>> device_iommu_mapped() and others then
>>
>> No.  It it is internal for a reason.  It also doesn't actually work
>> outside of the dma core.  E.g. for non-swiotlb ARM configs it will
>> not actually work.
> 
> Really? It is the only condition that dma_info_to_prot() tests to
> decide of IOMMU_CACHE is used or not, so you are saying that there is
> a condition where a device can be attached to an iommu_domain and
> dev_is_dma_coherent() returns the wrong information? How does
> dma-iommu.c safely use it then?

The common iommu-dma layer happens to be part of the subset of the DMA 
core which *does* play the dev->dma_coherent game. 32-bit Arm has its 
own IOMMU DMA ops which do not. I don't know if the set of PowerPCs with 
CONFIG_NOT_COHERENT_CACHE intersects the set of PowerPCs that can do 
VFIO, but that would be another example if so.

> In any case I still need to do something about the places checking
> IOMMU_CAP_CACHE_COHERENCY and thinking that means IOMMU_CACHE
> works. Any idea?

Can we improve the IOMMU drivers such that that *can* be the case 
(within a reasonable margin of error)? That's kind of where I was hoping 
to head with device_iommu_capable(), e.g. [1].

Robin.

[1] 
https://gitlab.arm.com/linux-arm/linux-rm/-/commit/53390e9505b3791adedc0974e251e5c7360e402e