From: David Gibson <david@gibson.dropbear.id.au>
To: Cornelia Huck <cohuck@redhat.com>
Cc: linux-s390@vger.kernel.org, Janosch Frank <frankja@linux.ibm.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
Jason Wang <jasowang@redhat.com>, Ram Pai <linuxram@us.ibm.com>,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org,
Halil Pasic <pasic@linux.ibm.com>,
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 1/2] mm: move force_dma_unencrypted() to mem_encrypt.h
Date: Fri, 28 Feb 2020 11:23:53 +1100 [thread overview]
Message-ID: <20200228002353.GA1842@umbus.fritz.box> (raw)
In-Reply-To: <20200225190802.753cffef.cohuck@redhat.com>
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On Tue, Feb 25, 2020 at 07:08:02PM +0100, Cornelia Huck wrote:
> On Mon, 24 Feb 2020 19:49:53 +0100
> Halil Pasic <pasic@linux.ibm.com> wrote:
>
> > On Mon, 24 Feb 2020 14:33:14 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >
> > > On Fri, Feb 21, 2020 at 07:07:02PM +0100, Halil Pasic wrote:
> > > > On Fri, 21 Feb 2020 10:48:15 -0500
> > > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > >
> > > > > On Fri, Feb 21, 2020 at 02:06:39PM +0100, Halil Pasic wrote:
> > > > > > On Fri, 21 Feb 2020 14:27:27 +1100
> > > > > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > > > > >
> > > > > > > On Thu, Feb 20, 2020 at 05:31:35PM +0100, Christoph Hellwig wrote:
> > > > > > > > On Thu, Feb 20, 2020 at 05:23:20PM +0100, Christian Borntraeger wrote:
> > > > > > > > > >From a users perspective it makes absolutely perfect sense to use the
> > > > > > > > > bounce buffers when they are NEEDED.
> > > > > > > > > Forcing the user to specify iommu_platform just because you need bounce buffers
> > > > > > > > > really feels wrong. And obviously we have a severe performance issue
> > > > > > > > > because of the indirections.
> > > > > > > >
> > > > > > > > The point is that the user should not have to specify iommu_platform.
> > > > > > > > We need to make sure any new hypervisor (especially one that might require
> > > > > > > > bounce buffering) always sets it,
> > > > > > >
> > > > > > > So, I have draft qemu patches which enable iommu_platform by default.
> > > > > > > But that's really because of other problems with !iommu_platform, not
> > > > > > > anything to do with bounce buffering or secure VMs.
> > > > > > >
> > > > > > > The thing is that the hypervisor *doesn't* require bounce buffering.
> > > > > > > In the POWER (and maybe s390 as well) models for Secure VMs, it's the
> > > > > > > *guest*'s choice to enter secure mode, so the hypervisor has no reason
> > > > > > > to know whether the guest needs bounce buffering. As far as the
> > > > > > > hypervisor and qemu are concerned that's a guest internal detail, it
> > > > > > > just expects to get addresses it can access whether those are GPAs
> > > > > > > (iommu_platform=off) or IOVAs (iommu_platform=on).
> > > > > >
> > > > > > I very much agree!
> > > > > >
> > > > > > >
> > > > > > > > as was a rather bogus legacy hack
> > > > > > >
> > > > > > > It was certainly a bad idea, but it was a bad idea that went into a
> > > > > > > public spec and has been widely deployed for many years. We can't
> > > > > > > just pretend it didn't happen and move on.
> > > > > > >
> > > > > > > Turning iommu_platform=on by default breaks old guests, some of which
> > > > > > > we still care about. We can't (automatically) do it only for guests
> > > > > > > that need bounce buffering, because the hypervisor doesn't know that
> > > > > > > ahead of time.
>
> We could default to iommu_platform=on on s390 when the host has active
> support for protected virtualization... but that's just another kind of
> horrible, so let's just pretend I didn't suggest it.
Yeah, that would break migration between hosts with the feature and
hosts without - for everything, not just protected guests. In general
any kind of guest visible configuration change based on host
properties is incompatible with the qemu/KVM migration model.
--
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:[~2020-02-28 1:05 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 [this message]
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
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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