Re: [PATCH] IOMMU and ATS not supported by vhost-user filesystem.

[Laszlo Ersek]

On 01/27/21 12:19, Stefan Hajnoczi wrote:
> On Tue, Jan 26, 2021 at 03:23:38PM -0300, lagarcia@linux.ibm.com wrote:
>> From: Leonardo Garcia <lagarcia@br.ibm.com>
>>
>> Currently, as IOMMU and ATS are not supported, if a user mistakenly set
>> any of them and tries to mount the vhost-user filesystem inside the
>> guest, whenever the user tries to access the mount point, the system
>> will hang forever.
>>
>> Signed-off-by: Leonardo Garcia <lagarcia@br.ibm.com>
>> ---
>>  hw/virtio/vhost-user-fs-pci.c | 7 +++++++
>>  hw/virtio/vhost-user-fs.c     | 5 +++++
>>  2 files changed, 12 insertions(+)
>>
>> diff --git a/hw/virtio/vhost-user-fs-pci.c b/hw/virtio/vhost-user-fs-pci.c
>> index 2ed8492b3f..564d1fd108 100644
>> --- a/hw/virtio/vhost-user-fs-pci.c
>> +++ b/hw/virtio/vhost-user-fs-pci.c
>> @@ -11,6 +11,8 @@
>>   * top-level directory.
>>   */
>>  
>> +#include "qemu/osdep.h"
>> +#include "qapi/error.h"
>>  #include "qemu/osdep.h"
>>  #include "hw/qdev-properties.h"
>>  #include "hw/virtio/vhost-user-fs.h"
>> @@ -45,6 +47,11 @@ static void vhost_user_fs_pci_realize(VirtIOPCIProxy 
>> *vpci_dev, Error **errp)
>>          vpci_dev->nvectors = dev->vdev.conf.num_request_queues + 2;
>>      }
>>  
>> +    if (vpci_dev->flags & VIRTIO_PCI_FLAG_ATS) {
>> +        error_setg(errp, "ATS is currently not supported with 
>> vhost-user-fs-pci");
>> +        return;
>> +    }
> 
> Why is this check needed in addition to VIRTIO_F_IOMMU_PLATFORM?
> 
> What needs to be added to support ATS?
> 
>> +
>>      qdev_realize(vdev, BUS(&vpci_dev->bus), errp);
>>  }
>>  
>> diff --git a/hw/virtio/vhost-user-fs.c b/hw/virtio/vhost-user-fs.c
>> index ac4fc34b36..914d68b3ee 100644
>> --- a/hw/virtio/vhost-user-fs.c
>> +++ b/hw/virtio/vhost-user-fs.c
>> @@ -203,6 +203,11 @@ static void vuf_device_realize(DeviceState *dev, Error 
>> **errp)
>>          return;
>>      }
>>  
>> +    if (virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
>> +        error_setg(errp, "IOMMU is currently not supported with 
>> vhost-user-fs");
>> +        return;
>> +    }
>> +
>>      if (!vhost_user_init(&fs->vhost_user, &fs->conf.chardev, errp)) {
> 
> I thought IOMMU support depends on the vhost-user device backend (e.g.
> virtiofsd), so the vhost-user backend should participate in advertising
> this feature.

... I had the same thought when a few weeks earlier I tried to use
virtio-fs with an SEV guest (just OVMF), and virtiofsd crashed,
apparently :)

(I didn't report it because, well, SEV wants to prevent sharing between
host and guest, and virtio-fs works precisely in the opposite direction;
so the use case may not have much merit.)

Thanks
Laszlo

> 
> Perhaps the check should be:
> 
>     ret = vhost_dev_init(&fs->vhost_dev, &fs->vhost_user,
>                          VHOST_BACKEND_TYPE_USER, 0);
>     if (ret < 0) {
>         error_setg_errno(errp, -ret, "vhost_dev_init failed");
>         goto err_virtio;
>     }
> +
> +   if (virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM) &&
> +       !(fs->vhost_dev.hdev_features & (1ull << VIRTIO_F_IOMMU_PLATFORM))) {
> +       error_setg(errp, "IOMMU is not supported by the vhost-user device 
> backend");
> +       goto err_iommu_needed;
> +   }
> 
> Also, can this logic be made generic for all vhost-user devices? It's
> not really specific to vhost-user-fs.
> 
> Stefan
>