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

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

On Tue, Jul 23, 2024 at 07:10:52AM +0000, Srujana Challa wrote:
> > On Mon, Jul 22, 2024 at 03:22:22PM +0800, Jason Wang wrote:
> > > On Fri, Jul 19, 2024 at 11:40 PM Srujana Challa <schalla@marvell.com>
> > wrote:
> > > >
> > > > > 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.
> Our initial thought was to support only for no-iommu case, in which domain itself
> won't be exist.   So, we can modify the code as below to check for only presence of domain.
> I think,  only handling of no-iommu case wouldn't effect the existing userspace.
> +   if ((!domain) && vhost_vdpa_noiommu && capable(CAP_SYS_RAWIO)) {

I would prefer some explicit action.
Just not specifying a domain is something I'd like
to keep reserved for something of more wide usefulness.


> > > > >
> > > > > I suggest a new domain type for use just for this purpose.
> > >
> > > I'm not sure I get this, we want to bypass IOMMU, so it doesn't even
> > > have a doman.
> > 
> > yes, a fake one. or come up with some other flag that userspace will set.
> > 
> > > > This way if host has
> > > > > an iommu, then the same kernel can run both VMs with isolation and
> > > > > unsafe embedded apps without.
> > > > Could you provide further details on this concept? What criteria
> > > > would determine the configuration of the new domain type? Would this
> > > > require a boot parameter similar to IOMMU_DOMAIN_IDENTITY, such as
> > iommu.passthrough=1 or iommu.pt?
> > >
> > > Thanks
> > >
> > > > >
> > > > > > ---
> > > > > >  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.
> > > > Yes, you are correct. I will modify the code to return error when
> > > > vhost_vdpa_noiommu is set and CAP_SYS_RAWIO is not set.
> > > >
> > > > Thanks.
> > > > >
> > > > >
> > > > > > +           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
> > > >
>