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On Mon, 17 Jan 2022 13:02:38 +0100
Halil Pasic <pa...@linux.ibm.com> wrote:
> 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
> support _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, so that, after the feature has been
> negotiated, the guest grants access to the portions of memory the
> device needs to see. So in for confidential guests, generally,
> _F_ACCESS_PLATFORM is about the restricted access to memory, but not
> about the addresses used being something else than guest physical
> addresses.
>
> This is the very reason for which commit f7ef7e6e3b ("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 f7ef7e6e3b ("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 <pa...@linux.ibm.com>
> Reported-by: Jakob Naucke <jakob.nau...@ibm.com>
> Fixes: 04ceb61a40 ("virtio: Fail if iommu_platform is requested, but
> unsupported")
> Cc: Kevin Wolf <kw...@redhat.com>
> Cc: qemu-sta...@nongnu.org
>
> ---
>
> v1->v2:
> * Commit message tweaks. Most notably fixed commit SHA (Michael)
>
> ---
> 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: 6621441db50d5bae7e34dbd04bf3c57a27a71b32
