linux-kernel - Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted

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Date:   Wed, 14 Aug 2019 00:24:39 +1000
From:   David Gibson <david@...son.dropbear.id.au>
To:     Christoph Hellwig <hch@....de>
Cc:     Ram Pai <linuxram@...ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        linuxppc-devel@...ts.ozlabs.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        Alexey Kardashevskiy <aik@...ux.ibm.com>,
        Paul Mackerras <paulus@...abs.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted

On Tue, Aug 13, 2019 at 03:26:17PM +0200, Christoph Hellwig wrote:
> On Mon, Aug 12, 2019 at 07:51:56PM +1000, David Gibson wrote:
> > AFAICT we already kind of abuse this for the VIRTIO_F_IOMMU_PLATFORM,
> > because to handle for cases where it *is* a device limitation, we
> > assume that if the hypervisor presents VIRTIO_F_IOMMU_PLATFORM then
> > the guest *must* select it.
> > 
> > What we actually need here is for the hypervisor to present
> > VIRTIO_F_IOMMU_PLATFORM as available, but not required.  Then we need
> > a way for the platform core code to communicate to the virtio driver
> > that *it* requires the IOMMU to be used, so that the driver can select
> > or not the feature bit on that basis.
> 
> I agree with the above, but that just brings us back to the original
> issue - the whole bypass of the DMA OPS should be an option that the
> device can offer, not the other way around.  And we really need to
> fix that root cause instead of doctoring around it.

I'm not exactly sure what you mean by "device" in this context.  Do
you mean the hypervisor (qemu) side implementation?

You're right that this was the wrong way around to begin with, but as
well as being hard to change now, I don't see how it really addresses
the current problem.  The device could default to IOMMU and allow
bypass, but the driver would still need to get information from the
platform to know that it *can't* accept that option in the case of a
secure VM.  Reversed sense, but the same basic problem.

The hypervisor does not, and can not be aware of the secure VM
restrictions - only the guest side platform code knows that.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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