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

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Message-ID: <20180415121157.GA17726@infradead.org>
Date:   Sun, 15 Apr 2018 05:11:57 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        "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 06:37:18PM +1000, Benjamin Herrenschmidt wrote:
> > > 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.
> 
> Fine with using a dma API always on the Linux side, but we do want to
> special case virtio still at the arch and qemu side to have a "direct
> mapping" mode. Not sure how (special flags on PCI devices) to avoid
> actually going through an emulated IOMMU on the qemu side, because that
> slows things down, esp. with vhost.
> 
> IE, we can't I think just treat it the same as a physical device.

We should have treated it like a physical device from the start, but
that device has unfortunately sailed.

But yes, we'll need a per-device quirk that says 'don't attach an
iommu'.