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

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Message-Id: <70cb433f-a8f7-5199-3c96-a760c7624804@linux.vnet.ibm.com>
Date:   Fri, 6 Apr 2018 08:23:10 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     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 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.