From: "Michael S. Tsirkin" <mst@redhat.com>
To: Halil Pasic <pasic@linux.ibm.com>
Cc: linux-s390@vger.kernel.org, Janosch Frank <frankja@linux.ibm.com>,
"Lendacky, Thomas" <Thomas.Lendacky@amd.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,
David Gibson <david@gibson.dropbear.id.au>,
Michael Mueller <mimu@linux.ibm.com>,
Viktor Mihajlovski <mihajlov@linux.ibm.com>,
Robin Murphy <robin.murphy@arm.com>,
Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 2/2] virtio: let virtio use DMA API when guest RAM is protected
Date: Thu, 20 Feb 2020 15:55:14 -0500 [thread overview]
Message-ID: <20200220154904-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20200220160606.53156-3-pasic@linux.ibm.com>
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@linux.ibm.com>
> Tested-by: Ram Pai <linuxram@us.ibm.com>
> Tested-by: Michael Mueller <mimu@linux.ibm.com>
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.
To summarize, the necessary conditions for a hack along these lines
(using DMA API without VIRTIO_F_ACCESS_PLATFORM) are that we detect that:
- secure guest mode is enabled - so we know that since we don't share
most memory regular virtio code won't
work, even though the buggy hypervisor didn't set VIRTIO_F_ACCESS_PLATFORM
- DMA API is giving us addresses that are actually also physical
addresses
- Hypervisor is buggy and didn't enable VIRTIO_F_ACCESS_PLATFORM
I don't see how this patch does this.
> ---
> drivers/virtio/virtio_ring.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 867c7ebd3f10..fafc8f924955 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -243,6 +243,9 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
> if (!virtio_has_iommu_quirk(vdev))
> return true;
>
> + if (force_dma_unencrypted(&vdev->dev))
> + return true;
> +
> /* Otherwise, we are left to guess. */
> /*
> * In theory, it's possible to have a buggy QEMU-supposed
> --
> 2.17.1
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next prev parent reply other threads:[~2020-02-20 20:55 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 [this message]
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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