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Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices
From:   Benjamin Herrenschmidt <benh@kernel.crashing.org>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Will Deacon <will.deacon@arm.com>,
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Date:   Thu, 02 Aug 2018 11:01:26 -0500
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On Thu, 2018-08-02 at 18:41 +0300, Michael S. Tsirkin wrote:
> 
> > I don't completely agree:
> > 
> > 1 - VIRTIO_F_IOMMU_PLATFORM is a property of the "other side", ie qemu
> > for example. It indicates that the peer bypasses the normal platform
> > iommu. The platform code in the guest has no real way to know that this
> > is the case, this is a specific "feature" of the qemu implementation.
> > 
> > 2 - VIRTIO_F_PLATFORM_DMA (or whatever you want to call it), is a
> > property of the guest platform itself (not qemu), there's no way the
> > "peer" can advertize it via the virtio negociated flags. At least for
> > us. I don't know for sure whether that would be workable for the ARM
> > case. In our case, qemu has no idea at VM creation time that the VM
> > will turn itself into a secure VM and thus will require bounce
> > buffering for IOs (including virtio).
> > 
> > So unless we have another hook for the arch code to set
> > VIRTIO_F_PLATFORM_DMA on selected (or all) virtio devices from the
> > guest itself, I don't see that as a way to deal with it.
> > 
> > >  The other issue is VIRTIO_F_IO_BARRIER
> > > which is very vaguely defined, and which needs a better definition.
> > > And last but not least we'll need some text explaining the challenges
> > > of hardware devices - I think VIRTIO_F_PLATFORM_DMA + VIRTIO_F_IO_BARRIER
> > > is what would basically cover them, but a good description including
> > > an explanation of why these matter.
> > 
> > Ben.
> > 
> 
> So is it true that from qemu point of view there is nothing special
> going on?  You pass in a PA, host writes there.

Yes, qemu doesn't see a different. It's the guest that will bounce the
pages via a pool of "insecure" pages that qemu can access. Normal pages
in a secure VM come from PAs that qemu cannot physically access.

Cheers,
Ben.