From: David Gibson <david@gibson.dropbear.id.au>
To: Halil Pasic <pasic@linux.ibm.com>
Cc: linux-s390@vger.kernel.org, Janosch Frank <frankja@linux.ibm.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
Jason Wang <jasowang@redhat.com>,
Cornelia Huck <cohuck@redhat.com>, Ram Pai <linuxram@us.ibm.com>,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org,
Christian Borntraeger <borntraeger@de.ibm.com>,
iommu@lists.linux-foundation.org,
Michael Mueller <mimu@linux.ibm.com>,
"Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
Viktor Mihajlovski <mihajlov@linux.ibm.com>,
Robin Murphy <robin.murphy@arm.com>,
Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 0/2] virtio: decouple protected guest RAM form VIRTIO_F_IOMMU_PLATFORM
Date: Mon, 24 Feb 2020 14:38:53 +1100 [thread overview]
Message-ID: <20200224033853.GE1751@umbus.fritz.box> (raw)
In-Reply-To: <20200221155602.4de41fa7.pasic@linux.ibm.com>
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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@redhat.com> wrote:
>
> >
> > On 2020/2/21 上午12:06, 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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next prev parent reply other threads:[~2020-02-24 6:44 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-20 16:06 [PATCH 0/2] virtio: decouple protected guest RAM form VIRTIO_F_IOMMU_PLATFORM Halil Pasic
2020-02-20 16:06 ` [PATCH 1/2] mm: move force_dma_unencrypted() to mem_encrypt.h Halil Pasic
2020-02-20 16:11 ` Christoph Hellwig
2020-02-20 16:23 ` Christian Borntraeger
2020-02-20 16:31 ` Christoph Hellwig
2020-02-20 17:00 ` Christian Borntraeger
2020-02-21 3:27 ` David Gibson
2020-02-21 13:06 ` Halil Pasic
2020-02-21 15:48 ` Michael S. Tsirkin
2020-02-21 18:07 ` Halil Pasic
2020-02-24 3:33 ` David Gibson
2020-02-24 18:49 ` Halil Pasic
2020-02-25 18:08 ` Cornelia Huck
2020-02-28 0:23 ` David Gibson
2020-02-20 16:06 ` [PATCH 2/2] virtio: let virtio use DMA API when guest RAM is protected Halil Pasic
2020-02-20 16:13 ` Christoph Hellwig
2020-02-21 2:59 ` David Gibson
2020-02-21 3:41 ` Jason Wang
2020-02-21 13:31 ` Halil Pasic
2020-02-21 13:27 ` Halil Pasic
2020-02-21 16:36 ` Christoph Hellwig
2020-02-24 6:50 ` David Gibson
2020-02-24 18:59 ` Halil Pasic
2020-02-21 14:33 ` Halil Pasic
2020-02-21 16:39 ` Christoph Hellwig
2020-02-21 18:16 ` Halil Pasic
2020-02-22 19:07 ` Michael S. Tsirkin
2020-02-24 17:16 ` Christoph Hellwig
2020-10-28 14:24 ` Alexander Graf via iommu
2020-10-28 18:01 ` Michael S. Tsirkin
2020-02-20 20:55 ` Michael S. Tsirkin
2020-02-21 1:17 ` Ram Pai
2020-02-21 3:29 ` David Gibson
2020-02-21 13:12 ` Halil Pasic
2020-02-21 15:39 ` Tom Lendacky
2020-02-24 6:40 ` David Gibson
2020-02-21 15:56 ` Michael S. Tsirkin
2020-02-21 16:35 ` Christoph Hellwig
2020-02-21 18:03 ` Halil Pasic
2020-02-20 20:48 ` [PATCH 0/2] virtio: decouple protected guest RAM form VIRTIO_F_IOMMU_PLATFORM Michael S. Tsirkin
2020-02-20 21:29 ` Michael S. Tsirkin
2020-02-21 13:37 ` Halil Pasic
2020-02-20 21:33 ` Michael S. Tsirkin
2020-02-21 13:49 ` Halil Pasic
2020-02-21 16:41 ` Christoph Hellwig
2020-02-24 5:44 ` David Gibson
2020-02-21 6:22 ` Jason Wang
2020-02-21 14:56 ` Halil Pasic
2020-02-24 3:38 ` David Gibson [this message]
2020-02-24 4:01 ` Jason Wang
2020-02-24 6:06 ` Michael S. Tsirkin
2020-02-24 6:45 ` Jason Wang
2020-02-24 7:48 ` Michael S. Tsirkin
2020-02-24 9:26 ` Jason Wang
2020-02-24 13:40 ` Michael S. Tsirkin
2020-02-25 3:38 ` Jason Wang
2020-02-24 13:56 ` Halil Pasic
2020-02-25 3:30 ` Jason Wang
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