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

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Message-ID: <20220407163737.2a7ccd7a.alex.williamson@redhat.com>
Date:   Thu, 7 Apr 2022 16:37:37 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Christoph Hellwig <hch@....de>,
        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, 7 Apr 2022 12:23:31 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> 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..

I considered this option when implementing the current solution, but
ultimately I didn't have confidence in being able to prevent drivers
from using device specific means to effect the change anyway.  GPUs
especially have various back channels to config space.  Thanks,

Alex