[PATCH 1/1] virtio: fix the condition for iommu_platform not supported

[Halil Pasic]

The commit 04ceb61a40 ("virtio: Fail if iommu_platform is requested, but
unsupported") claims to fail the device hotplug when iommu_platform
is requested, but not supported by the (vhost) device. On the first
glance the condition for detecting that situation looks perfect, but
because a certain peculiarity of virtio_platform it ain't.

In fact the aforementioned commit introduces a regression. It breaks
virtio-fs support for Secure Execution, and most likely also for AMD SEV
or any other confidential guest scenario that relies encrypted guest
memory.  The same also applies to any other vhost device that does not
negotiate _F_ACCESS_PLATFORM.

The peculiarity is that iommu_platform and _F_ACCESS_PLATFORM collates
"device can not access all of the guest ram" and "iova != gpa, thus
device needs to translate iova".

Confidential guest technologies currently rely on the device/hypervisor
offering _F_ACCESS_PLATFORM to grant access to whatever the device needs
to see, because of the first. But, generally, they don't care for the
second.

This is the very reason for which commit 7ef7e6e3b ("vhost: correctly
turn on VIRTIO_F_IOMMU_PLATFORM") for, which fences _F_ACCESS_PLATFORM
form the vhost device that does not need it, because on the vhost
interface it only means "I/O address translation is needed".

This patch takes inspiration from 7ef7e6e3b ("vhost: correctly turn on
VIRTIO_F_IOMMU_PLATFORM"), and uses the same condition for detecting the
situation when _F_ACCESS_PLATFORM is requested, but no I/O translation
by the device, and thus no device capability is needed. In this
situation claiming that the device does not support iommu_plattform=on
is counter-productive. So let us stop doing that!

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Reported-by: Jakob Naucke <Jakob.Naucke@ibm.com>
Fixes: 04ceb61a40 ("virtio: Fail if iommu_platform is requested, but
unsupported")
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-stable@nongnu.org

---

@Kevin: Can you please verify, that I don't break your fix?
---
 hw/virtio/virtio-bus.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
index d23db98c56..c1578f3de2 100644
--- a/hw/virtio/virtio-bus.c
+++ b/hw/virtio/virtio-bus.c
@@ -69,11 +69,6 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error 
**errp)
         return;
     }
 
-    if (has_iommu && !virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
-        error_setg(errp, "iommu_platform=true is not supported by the device");
-        return;
-    }
-
     if (klass->device_plugged != NULL) {
         klass->device_plugged(qbus->parent, &local_err);
     }
@@ -88,6 +83,12 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error 
**errp)
     } else {
         vdev->dma_as = &address_space_memory;
     }
+
+    if (has_iommu && vdev->dma_as != &address_space_memory
+                  && !virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
+        error_setg(errp, "iommu_platform=true is not supported by the device");
+        return;
+    }
 }
 
 /* Reset the virtio_bus */

base-commit: f8d75e10d3e0033a0a29a7a7e4777a4fbc17a016
-- 
2.32.0