linux-kernel - Re: [PATCH v4 1/2] iommu/sva: Tighten SVA bind API with explicit flags

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Message-ID: <YJt3tGlzFK3b4E82@infradead.org>
Date:   Wed, 12 May 2021 07:37:40 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        iommu@...ts.linux-foundation.org, Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Christoph Hellwig <hch@...radead.org>,
        Yi Liu <yi.l.liu@...el.com>, Raj Ashok <ashok.raj@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Dave Jiang <dave.jiang@...el.com>, wangzhou1@...ilicon.com,
        zhangfei.gao@...aro.org, vkoul@...nel.org,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH v4 1/2] iommu/sva: Tighten SVA bind API with explicit
 flags

On Tue, May 11, 2021 at 04:47:26PM -0300, Jason Gunthorpe wrote:
> > Let me try to break down your concerns:
> > 1. portability - driver uses DMA APIs can function w/ and w/o IOMMU. is
> > that your concern? But PASID is intrinsically tied with IOMMU and if
> > the drivers are using a generic sva-lib API, why they are not portable?
> > SVA by its definition is to avoid map/unmap every time.
> 
> Kernel explicitly does not support this programming model. All DMA is
> explicit and the DMA API hides platform details like IOMMU and CPU
> cache coherences. Just because x86 doesn't care about this doesn't
> make any of it optional.

Exactly.

> If you want to do SVA PASID then it also must come with DMA APIs to
> manage the CPU cache coherence that are all NOP's on x86.

Yes.  And we have plenty of precende where an IOMMU is in "bypass" mode
to allow access to all memory and then uses the simple dma-direct case.