Re: [RFC V2] virtio: Add platform specific DMA API translation for virito devices

On Thu, May 24, 2018 at 08:27:04AM +1000, Benjamin Herrenschmidt wrote:

On Wed, 2018-05-23 at 21:50 +0300, Michael S. Tsirkin wrote:

quoted

I re-read that discussion and I'm still unclear on the
original question, since I got several apparently
conflicting answers.

I asked:

	Why isn't setting VIRTIO_F_IOMMU_PLATFORM on the
	hypervisor side sufficient?

I thought I had replied to this...

There are a couple of reasons:

- First qemu doesn't know that the guest will switch to "secure mode"
in advance. There is no difference between a normal and a secure
partition until the partition does the magic UV call to "enter secure
mode" and qemu doesn't see any of it. So who can set the flag here ?

This seems weird to me.  As a rule HV calls should go through qemu -
or be allowed to go directly to KVM *by* qemu.  We generally reserve
the latter for hot path things.  Since this isn't a hot path, having
the call handled directly by the kernel seems wrong.

Unless a "UV call" is something different I don't know about.

- Second, when using VIRTIO_F_IOMMU_PLATFORM, we also make qemu (or
vhost) go through the emulated MMIO for every access to the guest,
which adds additional overhead.

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