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Date:   Wed, 30 Jun 2021 08:05:40 +0100
From:   Christoph Hellwig <hch@infradead.org>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Jason Wang <jasowang@redhat.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        David Woodhouse <dwmw2@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        Kirti Wankhede <kwankhede@nvidia.com>,
        Robin Murphy <robin.murphy@arm.com>,
        David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [RFC] /dev/ioasid uAPI proposal
Message-ID: <YNwXxAEIRz5t631E@infradead.org>
References: <20210602173510.GE1002214@nvidia.com>
 <20210602120111.5e5bcf93.alex.williamson@redhat.com>
 <20210602180925.GH1002214@nvidia.com>
 <20210602130053.615db578.alex.williamson@redhat.com>
 <20210602195404.GI1002214@nvidia.com>
 <20210602143734.72fb4fa4.alex.williamson@redhat.com>
 <6a9426d7-ed55-e006-9c4c-6b7c78142e39@redhat.com>
 <20210603130927.GZ1002214@nvidia.com>
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 <20210604115805.GG1002214@nvidia.com>
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On Fri, Jun 04, 2021 at 08:58:05AM -0300, Jason Gunthorpe wrote:
> On Fri, Jun 04, 2021 at 09:11:03AM +0800, Jason Wang wrote:
> > > nor do any virtio drivers implement the required platform specific
> > > cache flushing to make no-snoop TLPs work.
> > 
> > I don't get why virtio drivers needs to do that. I think DMA API should hide
> > those arch/platform specific stuffs from us.
> 
> It is not arch/platform stuff. If the device uses no-snoop then a
> very platform specific recovery is required in the device driver.

Well, the proper way to support NO_SNOOP DMA would be to force the
DMA API into supporting the device as if the platform was not DMA
coherent, probably on a per-call basis.  It is just that no one bothered
to actually do the work an people just kept piling hacks over hacks.