linux-kernel - Re: [RFC] /dev/ioasid uAPI proposal

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Message-ID: <YNwXxAEIRz5t631E@infradead.org>
Date:   Wed, 30 Jun 2021 08:05:40 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        David Woodhouse <dwmw2@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Kirti Wankhede <kwankhede@...dia.com>,
        Robin Murphy <robin.murphy@....com>,
        David Gibson <david@...son.dropbear.id.au>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

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.