Re: [PATCH RFC 1/9] iommu: Introduce external attach domain type

[Jacob Pan <jacob.pan@linux.microsoft.com>]

Hi Mostafa,

On Wed, 21 Oct 2026 10:57:20 +0000
Mostafa Saleh <smostafa@google.com> wrote:

> On Fri, Sep 25, 2026 at 12:07:34PM -0700, Jacob Pan wrote:
> > Add IOMMU_DOMAIN_EXTERNAL for externally managed domains that move
> > or attach devices to another VM address space provisioned by an
> > external owner, such as a type 1 hypervisor.
> > 
> > This is not a paging domain and is not sysfs-selectable as a default
> > domain type. It requires external owner context, such as a vIOMMU.
> >   
> 
> Can't we use a IOMMU_DOMAIN_BLOCKED domain for that, AFAIU, the host
> never inserts any mapping?
Right, the host does not own or install any mappings.

The external domain type is nevertheless useful for two reasons:

1. It allows the pvIOMMU driver's domain ops to do the MSHV & Xen attach
and detach hypercalls 
2. It allows an externally managed device to leverage the normal IOMMUFD
device-to-HWPT attachment interface: VFIO_DEVICE_ATTACH_IOMMUFD_PT
No new ioctls.

It also makes the lifetime and ownership clear for IOMMUFD and
hypervisor, the device can be returned back to host iommufd owned
domains once detached.

In pKVM case, my understanding is that you have a emulated root SMMU
which does not do hypercalls obviously. Then the external attach
is done via an "out of tree uAPI through KVM/VFIO bridge IOCTLs that
proxies those hypercalls."

Therefore, if we were to park the device in blocking domain while doing
external attach (as you suggested), this requires a new PV only uAPI
that must support pkvm, MSHV, and Xen. So it does not fit in the flow
supported by PV IOMMU root driver.

I don't have a better solution for both pv and emulated root IOMMU
drivers. Perhaps others do?

> Thanks,
> Mostafa
> 
> > PV IOMMU drivers are expected to use it when moving a device to a
> > guest VM with an already-created S2 IOPT.
> > 
> > Assisted-by: GPT-5.6 Sol
> > Signed-off-by: Jacob Pan <jacob.pan@linux.microsoft.com>
> > ---
> >  drivers/iommu/iommu.c | 2 ++
> >  include/linux/iommu.h | 9 +++++++++
> >  2 files changed, 11 insertions(+)
> > 
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index cd1bca7ede9a..d0bd3ef00379 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -220,6 +220,8 @@ static const char
> > *iommu_domain_type_str(unsigned int t) return "Translated";
> >  	case IOMMU_DOMAIN_PLATFORM:
> >  		return "Platform";
> > +	case IOMMU_DOMAIN_EXTERNAL:
> > +		return "External";
> >  	default:
> >  		return "Unknown";
> >  	}
> > diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> > index ac43b8b93f14..ede77b90f0fd 100644
> > --- a/include/linux/iommu.h
> > +++ b/include/linux/iommu.h
> > @@ -188,6 +188,7 @@ enum iommu_domain_cookie_type {
> >  
> >  #define __IOMMU_DOMAIN_NESTED	(1U << 6)  /* User-managed
> > address space nested on a stage-2 translation        */
> > +#define __IOMMU_DOMAIN_EXTERNAL	(1U << 7)  /* Externally
> > managed translation */ 
> >  #define IOMMU_DOMAIN_ALLOC_FLAGS ~__IOMMU_DOMAIN_DMA_FQ
> >  /*
> > @@ -207,6 +208,13 @@ enum iommu_domain_cookie_type {
> >   *				  represented by mm_struct's.
> >   *	IOMMU_DOMAIN_PLATFORM	- Legacy domain for drivers
> > that do their own
> >   *				  dma_api stuff. Do not use in
> > new drivers.
> > + *	IOMMU_DOMAIN_EXTERNAL	- Externally managed domain
> > for moving/attaching
> > + *				  devices to another VM address
> > space
> > + *				  provisioned by the external
> > owner, such as
> > + *				  a type 1 hypervisor. This is
> > not a
> > + *				  sysfs-selectable default
> > domain type; it
> > + *				  requires external owner
> > context such as a
> > + *				  vIOMMU.
> >   */
> >  #define IOMMU_DOMAIN_BLOCKED	(0U)
> >  #define IOMMU_DOMAIN_IDENTITY	(__IOMMU_DOMAIN_PT)
> > @@ -219,6 +227,7 @@ enum iommu_domain_cookie_type {
> >  #define IOMMU_DOMAIN_SVA	(__IOMMU_DOMAIN_SVA)
> >  #define IOMMU_DOMAIN_PLATFORM	(__IOMMU_DOMAIN_PLATFORM)
> >  #define IOMMU_DOMAIN_NESTED	(__IOMMU_DOMAIN_NESTED)
> > +#define IOMMU_DOMAIN_EXTERNAL	(__IOMMU_DOMAIN_EXTERNAL)
> >  
> >  struct iommu_domain {
> >  	unsigned type;
> > -- 
> > 2.43.0
> > 
> >