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

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Date:   Fri, 06 Apr 2018 07:18:33 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, robh@...nel.org, aik@...abs.ru,
        jasowang@...hat.com, joe@...ches.com,
        linuxppc-dev@...ts.ozlabs.org, elfring@...rs.sourceforge.net,
        david@...son.dropbear.id.au
Subject: Re: [RFC] virtio: Use DMA MAP API for devices without an IOMMU

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.

Cheers,
Ben.
> 
> > Anshuman, you need to provide more background here. I don't have time
> > right now it's late, but explain about the fact that this is for a
> > specific type of secure VM which has only a limited pool of (insecure)
> > memory that can be shared with qemu, so all IOs need to bounce via that
> > pool, which can be achieved by using swiotlb.
> > 
> > Note: this isn't urgent, we can discuss alternative approaches, this is
> > just to start the conversation.
> > 
> > Cheers,
> > Ben.