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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'.
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