linux-kernel - Re: [RFC] virtio: Use DMA MAP API for devices without an IOMMU

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Message-ID: <20180406071634.GA31108@infradead.org>
Date:   Fri, 6 Apr 2018 00:16:34 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        "Michael S. Tsirkin" <mst@...hat.com>, robh@...nel.org,
        aik@...abs.ru, jasowang@...hat.com, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, joe@...ches.com,
        david@...son.dropbear.id.au, linuxppc-dev@...ts.ozlabs.org,
        elfring@...rs.sourceforge.net
Subject: Re: [RFC] virtio: Use DMA MAP API for devices without an IOMMU

On Fri, Apr 06, 2018 at 08:23:10AM +0530, Anshuman Khandual wrote:
> On 04/06/2018 02:48 AM, Benjamin Herrenschmidt wrote:
> > On Thu, 2018-04-05 at 21:34 +0300, Michael S. Tsirkin wrote:
> >>> In this specific case, because that would make qemu expect an iommu,
> >>> and there isn't one.
> >>
> >>
> >> I think that you can set iommu_platform in qemu without an iommu.
> > 
> > No I mean the platform has one but it's not desirable for it to be used
> > due to the performance hit.
> 
> Also the only requirement is to bounce the I/O buffers through SWIOTLB
> implemented as DMA API which the virtio core understands. There is no
> need for an IOMMU to be involved for the device representation in this
> case IMHO.

This whole virtio translation issue is a mess.  I think we need to
switch it to the dma API, and then quirk the legacy case to always
use the direct mapping inside the dma API.