Re: [PATCH] vdpa: Add support for no-IOMMU mode

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 30, 2024 at 03:48:23PM +0530, Srujana Challa wrote:
> This commit introduces support for an UNSAFE, no-IOMMU mode in the
> vhost-vdpa driver. When enabled, this mode provides no device isolation,
> no DMA translation, no host kernel protection, and cannot be used for
> device assignment to virtual machines. It requires RAWIO permissions
> and will taint the kernel.
> This mode requires enabling the "enable_vhost_vdpa_unsafe_noiommu_mode"
> option on the vhost-vdpa driver. This mode would be useful to get
> better performance on specifice low end machines and can be leveraged
> by embedded platforms where applications run in controlled environment.
> 
> Signed-off-by: Srujana Challa <schalla@marvell.com>

Thought hard about that.
I think given vfio supports this, we can do that too, and
the extension is small.

However, it looks like setting this parameter will automatically
change the behaviour for existing userspace when IOMMU_DOMAIN_IDENTITY
is set.

I suggest a new domain type for use just for this purpose.  This way if
host has an iommu, then the same kernel can run both VMs with
isolation and unsafe embedded apps without.

> ---
>  drivers/vhost/vdpa.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> index bc4a51e4638b..d071c30125aa 100644
> --- a/drivers/vhost/vdpa.c
> +++ b/drivers/vhost/vdpa.c
> @@ -36,6 +36,11 @@ enum {
>  
>  #define VHOST_VDPA_IOTLB_BUCKETS 16
>  
> +bool vhost_vdpa_noiommu;
> +module_param_named(enable_vhost_vdpa_unsafe_noiommu_mode,
> +		   vhost_vdpa_noiommu, bool, 0644);
> +MODULE_PARM_DESC(enable_vhost_vdpa_unsafe_noiommu_mode, "Enable UNSAFE, no-IOMMU mode.  This mode provides no device isolation, no DMA translation, no host kernel protection, cannot be used for device assignment to virtual machines, requires RAWIO permissions, and will taint the kernel.  If you do not know what this is for, step away. (default: false)");
> +
>  struct vhost_vdpa_as {
>  	struct hlist_node hash_link;
>  	struct vhost_iotlb iotlb;
> @@ -60,6 +65,7 @@ struct vhost_vdpa {
>  	struct vdpa_iova_range range;
>  	u32 batch_asid;
>  	bool suspended;
> +	bool noiommu_en;
>  };
>  
>  static DEFINE_IDA(vhost_vdpa_ida);
> @@ -887,6 +893,10 @@ static void vhost_vdpa_general_unmap(struct vhost_vdpa *v,
>  {
>  	struct vdpa_device *vdpa = v->vdpa;
>  	const struct vdpa_config_ops *ops = vdpa->config;
> +
> +	if (v->noiommu_en)
> +		return;
> +
>  	if (ops->dma_map) {
>  		ops->dma_unmap(vdpa, asid, map->start, map->size);
>  	} else if (ops->set_map == NULL) {
> @@ -980,6 +990,9 @@ static int vhost_vdpa_map(struct vhost_vdpa *v, struct vhost_iotlb *iotlb,
>  	if (r)
>  		return r;
>  
> +	if (v->noiommu_en)
> +		goto skip_map;
> +
>  	if (ops->dma_map) {
>  		r = ops->dma_map(vdpa, asid, iova, size, pa, perm, opaque);
>  	} else if (ops->set_map) {
> @@ -995,6 +1008,7 @@ static int vhost_vdpa_map(struct vhost_vdpa *v, struct vhost_iotlb *iotlb,
>  		return r;
>  	}
>  
> +skip_map:
>  	if (!vdpa->use_va)
>  		atomic64_add(PFN_DOWN(size), &dev->mm->pinned_vm);
>  
> @@ -1298,6 +1312,7 @@ static int vhost_vdpa_alloc_domain(struct vhost_vdpa *v)
>  	struct vdpa_device *vdpa = v->vdpa;
>  	const struct vdpa_config_ops *ops = vdpa->config;
>  	struct device *dma_dev = vdpa_get_dma_dev(vdpa);
> +	struct iommu_domain *domain;
>  	const struct bus_type *bus;
>  	int ret;
>  
> @@ -1305,6 +1320,14 @@ static int vhost_vdpa_alloc_domain(struct vhost_vdpa *v)
>  	if (ops->set_map || ops->dma_map)
>  		return 0;
>  
> +	domain = iommu_get_domain_for_dev(dma_dev);
> +	if ((!domain || domain->type == IOMMU_DOMAIN_IDENTITY) &&
> +	    vhost_vdpa_noiommu && capable(CAP_SYS_RAWIO)) {

So if userspace does not have CAP_SYS_RAWIO instead of failing
with a permission error the functionality changes silently?
That's confusing, I think.


> +		add_taint(TAINT_USER, LOCKDEP_STILL_OK);
> +		dev_warn(&v->dev, "Adding kernel taint for noiommu on device\n");
> +		v->noiommu_en = true;
> +		return 0;
> +	}
>  	bus = dma_dev->bus;
>  	if (!bus)
>  		return -EFAULT;
> -- 
> 2.25.1