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

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Message-ID: <YNwU801EDIJsfTqV@infradead.org>
Date:   Wed, 30 Jun 2021 07:53:39 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Jason Gunthorpe <jgg@...dia.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>,
        Kirti Wankhede <kwankhede@...dia.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Robin Murphy <robin.murphy@....com>,
        David Gibson <david@...son.dropbear.id.au>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

On Tue, Jun 08, 2021 at 09:20:29AM +0800, Jason Wang wrote:
> "
> 
> 6.2.17 _CCA (Cache Coherency Attribute) The _CCA object returns whether or
> not a bus-master device supports hardware managed cache coherency. Expected
> values are 0 to indicate it is not supported, and 1 to indicate that it is
> supported. All other values are reserved.
> 
> ...
> 
> On Intel platforms, if the _CCA object is not supplied, the OSPM will assume
> the devices are hardware cache coherent.
> 
> "

_CCA is mostly used on arm/arm64 platforms to figure out if a device
needs non-coherent DMA handling in the DMA API or not.  It is not
related to the NoSnoop TLPs that override the setting for an otherwise
coherent device.