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

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Date:   Thu, 7 Apr 2022 12:23:31 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Christoph Hellwig <hch@....de>,
        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" <iommu@...ts.linux-foundation.org>,
        Jason Wang <jasowang@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-s390@...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" <netdev@...r.kernel.org>,
        Rob Clark <robdclark@...il.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 1/5] iommu: Replace uses of IOMMU_CAP_CACHE_COHERENCY
 with dev_is_dma_coherent()

On Thu, Apr 07, 2022 at 04:17:11PM +0100, Robin Murphy wrote:

> For the specific case of overriding PCIe No Snoop (which is more problematic
> from an Arm SMMU PoV) when assigning to a VM, would that not be easier
> solved by just having vfio-pci clear the "Enable No Snoop" control bit in
> the endpoint's PCIe capability?

Ideally.

That was rediscussed recently, apparently there are non-compliant
devices and drivers that just ignore the bit. 

Presumably this is why x86 had to move to an IOMMU enforced feature..

> That seems a pretty good summary - I think they're basically all "firmware
> told Linux I'm coherent so I'd better act coherent" cases, but that still
> doesn't necessarily mean that they're *forced* to respect that. One of the
> things on my to-do list is to try adding a DMA_ATTR_NO_SNOOP that can force
> DMA cache maintenance for coherent devices, primarily to hook up in Panfrost
> (where there is a bit of a performance to claw back on the coherent AmLogic
> SoCs by leaving certain buffers non-cacheable).

It would be great to see that in a way that could bring in the few other
GPU drivers doing no-snoop to a formal DMA API instead of hacking
their own stuff with wbinvd calls or whatever.

Thanks,
Jason