Re: [PATCH 2/2] virtio: let virtio use DMA API when guest RAM is protected

[David Gibson]

On Thu, Feb 20, 2020 at 05:17:48PM -0800, Ram Pai wrote:
> On Thu, Feb 20, 2020 at 03:55:14PM -0500, Michael S. Tsirkin wrote:
> > On Thu, Feb 20, 2020 at 05:06:06PM +0100, Halil Pasic wrote:
> > > Currently the advanced guest memory protection technologies (AMD SEV,
> > > powerpc secure guest technology and s390 Protected VMs) abuse the
> > > VIRTIO_F_IOMMU_PLATFORM flag to make virtio core use the DMA API, which
> > > is in turn necessary, to make IO work with guest memory protection.
> > > 
> > > But VIRTIO_F_IOMMU_PLATFORM a.k.a. VIRTIO_F_ACCESS_PLATFORM is really a
> > > different beast: with virtio devices whose implementation runs on an SMP
> > > CPU we are still fine with doing all the usual optimizations, it is just
> > > that we need to make sure that the memory protection mechanism does not
> > > get in the way. The VIRTIO_F_ACCESS_PLATFORM mandates more work on the
> > > side of the guest (and possibly he host side as well) than we actually
> > > need.
> > > 
> > > An additional benefit of teaching the guest to make the right decision
> > > (and use DMA API) on it's own is: removing the need, to mandate special
> > > VM configuration for guests that may run with protection. This is
> > > especially interesting for s390 as VIRTIO_F_IOMMU_PLATFORM pushes all
> > > the virtio control structures into the first 2G of guest memory:
> > > something we don't necessarily want to do per-default.
> > > 
> > > Signed-off-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
> > > Tested-by: Ram Pai <linuxram@xxxxxxxxxx>
> > > Tested-by: Michael Mueller <mimu@xxxxxxxxxxxxx>
> > 
> > This might work for you but it's fragile, since without
> > VIRTIO_F_ACCESS_PLATFORM hypervisor assumes it gets
> > GPA's, not DMA addresses.
> > 
> > 
> > 
> > IOW this looks like another iteration of:
> > 
> > 	virtio: Support encrypted memory on powerpc secure guests
> > 
> > which I was under the impression was abandoned as unnecessary.
> 
> It has been abondoned on powerpc. We enabled VIRTIO_F_ACCESS_PLATFORM;
> by default, flag on powerpc.

Uh... we haven't yet, though we're working on it.

> We would like to enable secure guests on powerpc without this flag
> aswell enabled, but past experience has educated us that its not a easy
> path.  However if Halil makes some inroads in this path for s390, we
> will like to support him.
> 
> 
> RP
> 

-- 
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_!
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