Re: [PATCH 0/2] virtio: decouple protected guest RAM form VIRTIO_F_IOMMU_PLATFORM

[David Gibson]

On Fri, Feb 21, 2020 at 03:56:02PM +0100, Halil Pasic wrote:
> On Fri, 21 Feb 2020 14:22:26 +0800
> Jason Wang <jasowang@xxxxxxxxxx> wrote:
> 
> > 
> > On 2020/2/21 äå12:06, Halil Pasic wrote:
> > > Currently if one intends to run a memory protection enabled VM with
> > > virtio devices and linux as the guest OS, one needs to specify the
> > > VIRTIO_F_IOMMU_PLATFORM flag for each virtio device to make the guest
> > > linux use the DMA API, which in turn handles the memory
> > > encryption/protection stuff if the guest decides to turn itself into
> > > a protected one. This however makes no sense due to multiple reasons:
> > > * The device is not changed by the fact that the guest RAM is
> > > protected. The so called IOMMU bypass quirk is not affected.
> > > * This usage is not congruent with  standardised semantics of
> > > VIRTIO_F_IOMMU_PLATFORM. Guest memory protected is an orthogonal reason
> > > for using DMA API in virtio (orthogonal with respect to what is
> > > expressed by VIRTIO_F_IOMMU_PLATFORM).
> > >
> > > This series aims to decouple 'have to use DMA API because my (guest) RAM
> > > is protected' and 'have to use DMA API because the device told me
> > > VIRTIO_F_IOMMU_PLATFORM'.
> > >
> > > Please find more detailed explanations about the conceptual aspects in
> > > the individual patches. There is however also a very practical problem
> > > that is addressed by this series.
> > >
> > > For vhost-net the feature VIRTIO_F_IOMMU_PLATFORM has the following side
> > > effect The vhost code assumes it the addresses on the virtio descriptor
> > > ring are not guest physical addresses but iova's, and insists on doing a
> > > translation of these regardless of what transport is used (e.g. whether
> > > we emulate a PCI or a CCW device). (For details see commit 6b1e6cc7855b
> > > "vhost: new device IOTLB API".) On s390 this results in severe
> > > performance degradation (c.a. factor 10).
> > 
> > 
> > Do you see a consistent degradation on the performance, or it only 
> > happen when for during the beginning of the test?
> > 
> 
> AFAIK the degradation is consistent.
> 
> > 
> > > BTW with ccw I/O there is
> > > (architecturally) no IOMMU, so the whole address translation makes no
> > > sense in the context of virtio-ccw.
> > 
> > 
> > I suspect we can do optimization in qemu side.
> > 
> > E.g send memtable entry via IOTLB API when vIOMMU is not enabled.
> > 
> > If this makes sense, I can draft patch to see if there's any difference.
> 
> Frankly I would prefer to avoid IOVAs on the descriptor ring (and the
> then necessary translation) for virtio-ccw altogether. But Michael
> voiced his opinion that we should mandate F_IOMMU_PLATFORM for devices
> that could be used with guests running in protected mode. I don't share
> his opinion, but that's an ongoing discussion.

I'm a bit confused by this.  For the ccw specific case,
F_ACCESS_PLATFORM shouldn't have any impact: for you, IOVA == GPA so
everything is easy.

> Should we end up having to do translation from IOVA in vhost, we are
> very interested in that translation being fast and efficient.
> 
> In that sense we would be very happy to test any optimization that aim
> into that direction.
> 
> Thank you very much for your input!
> 
> Regards,
> Halil
> 
> > 
> > Thanks
> > 
> > 
> > >
> > > Halil Pasic (2):
> > >    mm: move force_dma_unencrypted() to mem_encrypt.h
> > >    virtio: let virtio use DMA API when guest RAM is protected
> > >
> > >   drivers/virtio/virtio_ring.c |  3 +++
> > >   include/linux/dma-direct.h   |  9 ---------
> > >   include/linux/mem_encrypt.h  | 10 ++++++++++
> > >   3 files changed, 13 insertions(+), 9 deletions(-)
> > >
> > >
> > > base-commit: ca7e1fd1026c5af6a533b4b5447e1d2f153e28f2
> > 
> 

-- 
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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