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Re: [PATCH v3 1/1] virtio: fix the condition for iommu_platform not supp


From: Cornelia Huck
Subject: Re: [PATCH v3 1/1] virtio: fix the condition for iommu_platform not supported
Date: Tue, 01 Feb 2022 17:47:24 +0100
User-agent: Notmuch/0.34 (https://notmuchmail.org)

On Tue, Feb 01 2022, Halil Pasic <pasic@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

s/for, which //

> form the vhost device that does not need it, because on the vhost

s/form/from/

> 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 <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
>
> ---
>
> v2->v3:
> * Caught a bug: I tired to check if vdev has the feature
>    ACCESS_PLATFORM after we have forced it. Moved the check
>    to a better place
> 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..34f5a0a664 100644
> --- a/hw/virtio/virtio-bus.c
> +++ b/hw/virtio/virtio-bus.c
> @@ -48,6 +48,7 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error 
> **errp)
>      VirtioBusClass *klass = VIRTIO_BUS_GET_CLASS(bus);
>      VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
>      bool has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
> +    bool vdev_has_iommu = false;

Isn't vdev_has_iommu set unconditionally before you try to use it?

>      Error *local_err = NULL;
>  
>      DPRINTF("%s: plug device.\n", qbus->name);
> @@ -69,11 +70,6 @@ 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);
>      }
> @@ -82,9 +78,14 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error 
> **errp)
>          return;
>      }
>  
> +    vdev_has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
>      if (klass->get_dma_as != NULL && has_iommu) {
>          virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
>          vdev->dma_as = klass->get_dma_as(qbus->parent);
> +        if (!vdev_has_iommu && vdev->dma_as != &address_space_memory) {
> +            error_setg(errp,
> +                       "iommu_platform=true is not supported by the device");
> +        }
>      } else {

I agree that a short comment would be nice here, but this is preexisting
code anyway...

>          vdev->dma_as = &address_space_memory;
>      }
>
> base-commit: 6621441db50d5bae7e34dbd04bf3c57a27a71b32

...so (with or without fixing the nits):

Acked-by: Cornelia Huck <cohuck@redhat.com>

(i.e. looks sane, but I didn't follow all the paths)




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