Re: [PATCH v2 1/9] docs: Document IO Address Space ID (IOASID) APIs

[Jean-Philippe Brucker]

On Fri, Aug 21, 2020 at 09:35:10PM -0700, Jacob Pan wrote:
> IOASID is used to identify address spaces that can be targeted by device
> DMA. It is a system-wide resource that is essential to its many users.
> This document is an attempt to help developers from all vendors navigate
> the APIs. At this time, ARM SMMU and Intel’s Scalable IO Virtualization
> (SIOV) enabled platforms are the primary users of IOASID. Examples of
> how SIOV components interact with IOASID APIs are provided in that many
> APIs are driven by the requirements from SIOV.
> 
> Signed-off-by: Liu Yi L <yi.l.liu@xxxxxxxxx>
> Signed-off-by: Wu Hao <hao.wu@xxxxxxxxx>
> Signed-off-by: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
> ---
>  Documentation/ioasid.rst | 618 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 618 insertions(+)
>  create mode 100644 Documentation/ioasid.rst
> 
> diff --git a/Documentation/ioasid.rst b/Documentation/ioasid.rst

Thanks for writing this up. Should it go to Documentation/driver-api/, or
Documentation/driver-api/iommu/? I think this also needs to Cc
linux-doc@xxxxxxxxxxxxxxx and corbet@xxxxxxx

> new file mode 100644
> index 000000000000..b6a8cdc885ff
> --- /dev/null
> +++ b/Documentation/ioasid.rst
> @@ -0,0 +1,618 @@
> +.. ioasid:
> +
> +=====================================
> +IO Address Space ID
> +=====================================
> +
> +IOASID is a generic name for PCIe Process Address ID (PASID) or ARM
> +SMMU sub-stream ID. An IOASID identifies an address space that DMA

"SubstreamID"

> +requests can target.
> +
> +The primary use cases for IOASID are Shared Virtual Address (SVA) and
> +IO Virtual Address (IOVA). However, the requirements for IOASID

IOVA alone isn't a use case, maybe "multiple IOVA spaces per device"?

> +management can vary among hardware architectures.
> +
> +This document covers the generic features supported by IOASID
> +APIs. Vendor-specific use cases are also illustrated with Intel's VT-d
> +based platforms as the first example.
> +
> +.. contents:: :local:
> +
> +Glossary
> +========
> +PASID - Process Address Space ID
> +
> +IOASID - IO Address Space ID (generic term for PCIe PASID and
> +sub-stream ID in SMMU)

"SubstreamID"

> +
> +SVA/SVM - Shared Virtual Addressing/Memory
> +
> +ENQCMD - New Intel X86 ISA for efficient workqueue submission [1]

Maybe drop the "New", to keep the documentation perennial. It might be
good to add internal links here to the specifications URLs at the bottom.

> +
> +DSA - Intel Data Streaming Accelerator [2]
> +
> +VDCM - Virtual device composition module [3]
> +
> +SIOV - Intel Scalable IO Virtualization
> +
> +
> +Key Concepts
> +============
> +
> +IOASID Set
> +-----------
> +An IOASID set is a group of IOASIDs allocated from the system-wide
> +IOASID pool. An IOASID set is created and can be identified by a
> +token of u64. Refer to IOASID set APIs for more details.

Identified either by an u64 or an mm_struct, right?  Maybe just drop the
second sentence if it's detailed in the IOASID set section below.

> +
> +IOASID set is particularly useful for guest SVA where each guest could
> +have its own IOASID set for security and efficiency reasons.
> +
> +IOASID Set Private ID (SPID)
> +----------------------------
> +SPIDs are introduced as IOASIDs within its set. Each SPID maps to a
> +system-wide IOASID but the namespace of SPID is within its IOASID
> +set.

The intro isn't super clear. Perhaps this is simpler:
"Each IOASID set has a private namespace of SPIDs. An SPID maps to a
single system-wide IOASID."

> SPIDs can be used as guest IOASIDs where each guest could do
> +IOASID allocation from its own pool and map them to host physical
> +IOASIDs. SPIDs are particularly useful for supporting live migration
> +where decoupling guest and host physical resources are necessary.
> +
> +For example, two VMs can both allocate guest PASID/SPID #101 but map to
> +different host PASIDs #201 and #202 respectively as shown in the
> +diagram below.
> +::
> +
> + .------------------.    .------------------.
> + |   VM 1           |    |   VM 2           |
> + |                  |    |                  |
> + |------------------|    |------------------|
> + | GPASID/SPID 101  |    | GPASID/SPID 101  |
> + '------------------'    -------------------'     Guest
> + __________|______________________|______________________
> +           |                      |               Host
> +           v                      v
> + .------------------.    .------------------.
> + | Host IOASID 201  |    | Host IOASID 202  |
> + '------------------'    '------------------'
> + |   IOASID set 1   |    |   IOASID set 2   |
> + '------------------'    '------------------'
> +
> +Guest PASID is treated as IOASID set private ID (SPID) within an
> +IOASID set, mappings between guest and host IOASIDs are stored in the
> +set for inquiry.
> +
> +IOASID APIs
> +===========
> +To get the IOASID APIs, users must #include <linux/ioasid.h>. These APIs
> +serve the following functionalities:
> +
> +  - IOASID allocation/Free
> +  - Group management in the form of ioasid_set
> +  - Private data storage and lookup
> +  - Reference counting
> +  - Event notification in case of state change
> +
> +IOASID Set Level APIs
> +--------------------------
> +For use cases such as guest SVA it is necessary to manage IOASIDs at
> +a group level. For example, VMs may allocate multiple IOASIDs for
> +guest process address sharing (vSVA). It is imperative to enforce
> +VM-IOASID ownership such that malicious guest cannot target DMA

"a malicious guest"

> +traffic outside its own IOASIDs, or free an active IOASID belong to

"that belongs to"

> +another VM.
> +::
> +
> + struct ioasid_set *ioasid_alloc_set(void *token, ioasid_t quota, u32 type)
> +
> + int ioasid_adjust_set(struct ioasid_set *set, int quota);

These could be named "ioasid_set_alloc" and "ioasid_set_adjust" to be
consistent with the rest of the API.

> +
> + void ioasid_set_get(struct ioasid_set *set)
> +
> + void ioasid_set_put(struct ioasid_set *set)
> +
> + void ioasid_set_get_locked(struct ioasid_set *set)
> +
> + void ioasid_set_put_locked(struct ioasid_set *set)
> +
> + int ioasid_set_for_each_ioasid(struct ioasid_set *sdata,

Might be nicer to keep the same argument names within the API. Here "set"
rather than "sdata".

> +                                void (*fn)(ioasid_t id, void *data),
> +				void *data)

(alignment)

> +
> +
> +IOASID set concept is introduced to represent such IOASID groups. Each

Or just "IOASID sets represent such IOASID groups", but might be
redundant.

> +IOASID set is created with a token which can be one of the following
> +types:
> +
> + - IOASID_SET_TYPE_NULL (Arbitrary u64 value)
> + - IOASID_SET_TYPE_MM (Set token is a mm_struct)
> +
> +The explicit MM token type is useful when multiple users of an IOASID
> +set under the same process need to communicate about their shared IOASIDs.
> +E.g. An IOASID set created by VFIO for one guest can be associated
> +with the KVM instance for the same guest since they share a common mm_struct.
> +
> +The IOASID set APIs serve the following purposes:
> +
> + - Ownership/permission enforcement
> + - Take collective actions, e.g. free an entire set
> + - Event notifications within a set
> + - Look up a set based on token
> + - Quota enforcement

This paragraph could be earlier in the section

> +
> +Individual IOASID APIs
> +----------------------
> +Once an ioasid_set is created, IOASIDs can be allocated from the set.
> +Within the IOASID set namespace, set private ID (SPID) is supported. In
> +the VM use case, SPID can be used for storing guest PASID.
> +
> +::
> +
> + ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t min, ioasid_t max,
> +                       void *private);
> +
> + int ioasid_get(struct ioasid_set *set, ioasid_t ioasid);
> +
> + void ioasid_put(struct ioasid_set *set, ioasid_t ioasid);
> +
> + int ioasid_get_locked(struct ioasid_set *set, ioasid_t ioasid);
> +
> + void ioasid_put_locked(struct ioasid_set *set, ioasid_t ioasid);
> +
> + void *ioasid_find(struct ioasid_set *set, ioasid_t ioasid,
> +                   bool (*getter)(void *));
> +
> + ioasid_t ioasid_find_by_spid(struct ioasid_set *set, ioasid_t spid)
> +
> + int ioasid_attach_data(struct ioasid_set *set, ioasid_t ioasid,
> +                        void *data);
> + int ioasid_attach_spid(struct ioasid_set *set, ioasid_t ioasid,
> +                        ioasid_t ssid);

s/ssid/spid

> +
> +
> +Notifications
> +-------------
> +An IOASID may have multiple users, each user may have hardware context
> +associated with an IOASID. When the status of an IOASID changes,
> +e.g. an IOASID is being freed, users need to be notified such that the
> +associated hardware context can be cleared, flushed, and drained.
> +
> +::
> +
> + int ioasid_register_notifier(struct ioasid_set *set, struct
> +                              notifier_block *nb)
> +
> + void ioasid_unregister_notifier(struct ioasid_set *set,
> +                                 struct notifier_block *nb)
> +
> + int ioasid_register_notifier_mm(struct mm_struct *mm, struct
> +                                 notifier_block *nb)
> +
> + void ioasid_unregister_notifier_mm(struct mm_struct *mm, struct
> +                                    notifier_block *nb)
> +
> + int ioasid_notify(ioasid_t ioasid, enum ioasid_notify_val cmd,
> +                   unsigned int flags)
> +
> +
> +Events
> +~~~~~~
> +Notification events are pertinent to individual IOASIDs, they can be
> +one of the following:
> +
> + - ALLOC
> + - FREE
> + - BIND
> + - UNBIND
> +
> +Ordering
> +~~~~~~~~
> +Ordering is supported by IOASID notification priorities as the
> +following (in ascending order):
> +
> +::
> +
> + enum ioasid_notifier_prios {
> +	IOASID_PRIO_LAST,
> +	IOASID_PRIO_IOMMU,
> +	IOASID_PRIO_DEVICE,
> +	IOASID_PRIO_CPU,
> + };
> +
> +The typical use case is when an IOASID is freed due to an exception, DMA
> +source should be quiesced before tearing down other hardware contexts
> +in the system. This will reduce the churn in handling faults. DMA work
> +submission is performed by the CPU which is granted higher priority than
> +devices.
> +
> +
> +Scopes
> +~~~~~~
> +There are two types of notifiers in IOASID core: system-wide and
> +ioasid_set-wide.
> +
> +System-wide notifier is catering for users that need to handle all
> +IOASIDs in the system. E.g. The IOMMU driver handles all IOASIDs.
> +
> +Per ioasid_set notifier can be used by VM specific components such as
> +KVM. After all, each KVM instance only cares about IOASIDs within its
> +own set.
> +
> +
> +Atomicity
> +~~~~~~~~~
> +IOASID notifiers are atomic due to spinlocks used inside the IOASID
> +core. For tasks cannot be completed in the notifier handler, async work

"tasks that cannot be"

> +can be submitted to complete the work later as long as there is no
> +ordering requirement.
> +
> +Reference counting
> +------------------
> +IOASID lifecycle management is based on reference counting. Users of
> +IOASID intend to align lifecycle with the IOASID need to hold

"who intend to"

> +reference of the IOASID. IOASID will not be returned to the pool for

"a reference to the IOASID. The IOASID"

> +allocation until all references are dropped. Calling ioasid_free()
> +will mark the IOASID as FREE_PENDING if the IOASID has outstanding
> +reference. ioasid_get() is not allowed once an IOASID is in the
> +FREE_PENDING state.
> +
> +Event notifications are used to inform users of IOASID status change.
> +IOASID_FREE event prompts users to drop their references after
> +clearing its context.
> +
> +For example, on VT-d platform when an IOASID is freed, teardown
> +actions are performed on KVM, device driver, and IOMMU driver.
> +KVM shall register notifier block with::
> +
> + static struct notifier_block pasid_nb_kvm = {
> +	.notifier_call = pasid_status_change_kvm,
> +	.priority      = IOASID_PRIO_CPU,
> + };
> +
> +VDCM driver shall register notifier block with::
> +
> + static struct notifier_block pasid_nb_vdcm = {
> +	.notifier_call = pasid_status_change_vdcm,
> +	.priority      = IOASID_PRIO_DEVICE,
> + };
> +
> +In both cases, notifier blocks shall be registered on the IOASID set
> +such that *only* events from the matching VM is received.
> +
> +If KVM attempts to register notifier block before the IOASID set is
> +created for the MM token, the notifier block will be placed on a
> +pending list inside IOASID core. Once the token matching IOASID set
> +is created, IOASID will register the notifier block automatically.
> +IOASID core does not replay events for the existing IOASIDs in the
> +set. For IOASID set of MM type, notification blocks can be registered
> +on empty sets only. This is to avoid lost events.
> +
> +IOMMU driver shall register notifier block on global chain::
> +
> + static struct notifier_block pasid_nb_vtd = {
> +	.notifier_call = pasid_status_change_vtd,
> +	.priority      = IOASID_PRIO_IOMMU,
> + };
> +
> +Custom allocator APIs
> +---------------------
> +
> +::
> +
> + int ioasid_register_allocator(struct ioasid_allocator_ops *allocator);
> +
> + void ioasid_unregister_allocator(struct ioasid_allocator_ops *allocator);
> +
> +Allocator Choices
> +~~~~~~~~~~~~~~~~~
> +IOASIDs are allocated for both host and guest SVA/IOVA usage. However,
> +allocators can be different. For example, on VT-d guest PASID
> +allocation must be performed via a virtual command interface which is
> +emulated by VMM.
> +
> +IOASID core has the notion of "custom allocator" such that guest can
> +register virtual command allocator that precedes the default one.
> +
> +Namespaces
> +~~~~~~~~~~
> +IOASIDs are limited system resources that default to 20 bits in
> +size. Since each device has its own table, theoretically the namespace
> +can be per device also. However, for security reasons sharing PASID
> +tables among devices are not good for isolation. Therefore, IOASID
> +namespace is system-wide.

I don't follow this development. Having per-device PASID table would work
fine for isolation (assuming no hardware bug necessitating IOMMU groups).
If I remember correctly IOASID space was chosen to be OS-wide because it
simplifies the management code (single PASID per task), and it is
system-wide across VMs only in the case of VT-d scalable mode.

> +
> +There are also other reasons to have this simpler system-wide
> +namespace. Take VT-d as an example, VT-d supports shared workqueue
> +and ENQCMD[1] where one IOASID could be used to submit work on

Maybe use the Sphinx glossary syntax rather than "[1]"
https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#glossary-directive

> +multiple devices that are shared with other VMs. This requires IOASID
> +to be system-wide. This is also the reason why guests must use an
> +emulated virtual command interface to allocate IOASID from the host.
> +
> +
> +Life cycle
> +==========
> +This section covers IOASID lifecycle management for both bare-metal
> +and guest usages. In bare-metal SVA, MMU notifier is directly hooked
> +up with IOMMU driver, therefore the process address space (MM)
> +lifecycle is aligned with IOASID.
> +
> +However, guest MMU notifier is not available to host IOMMU driver,
> +when guest MM terminates unexpectedly, the events have to go through
> +VFIO and IOMMU UAPI to reach host IOMMU driver. There are also more
> +parties involved in guest SVA, e.g. on Intel VT-d platform, IOASIDs
> +are used by IOMMU driver, KVM, VDCM, and VFIO.
> +
> +Native IOASID Life Cycle (VT-d Example)
> +---------------------------------------
> +
> +The normal flow of native SVA code with Intel Data Streaming
> +Accelerator(DSA) [2] as example:
> +
> +1. Host user opens accelerator FD, e.g. DSA driver, or uacce;
> +2. DSA driver allocate WQ, do sva_bind_device();
> +3. IOMMU driver calls ioasid_alloc(), then bind PASID with device,
> +   mmu_notifier_get()
> +4. DMA starts by DSA driver userspace
> +5. DSA userspace close FD
> +6. DSA/uacce kernel driver handles FD.close()
> +7. DSA driver stops DMA
> +8. DSA driver calls sva_unbind_device();
> +9. IOMMU driver does unbind, clears PASID context in IOMMU, flush
> +   TLBs. mmu_notifier_put() called.
> +10. mmu_notifier.release() called, IOMMU SVA code calls ioasid_free()*
> +11. The IOASID is returned to the pool, reclaimed.
> +
> +::
> +

Use a footnote? https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#footnotes

> +   * With ENQCMD, PASID used on VT-d is not released in mmu_notifier() but
> +     mmdrop(). mmdrop comes after FD close. Should not matter.

"comes after FD close, which doesn't make a difference?"
The following might not be necessary since early process termination is
described later.

> +     If the user process dies unexpectedly, Step #10 may come before
> +     Step #5, in between, all DMA faults discarded. PRQ responded with

PRQ hasn't been defined in this document.

> +     code INVALID REQUEST.
> +
> +During the normal teardown, the following three steps would happen in
> +order:
> +
> +1. Device driver stops DMA request
> +2. IOMMU driver unbinds PASID and mm, flush all TLBs, drain in-flight
> +   requests.
> +3. IOASID freed
> +
> +Exception happens when process terminates *before* device driver stops
> +DMA and call IOMMU driver to unbind. The flow of process exists are as

"exits"

> +follows:
> +
> +::
> +
> +   do_exit() {
> +	exit_mm() {
> +		mm_put();
> +		exit_mmap() {
> +			intel_invalidate_range() //mmu notifier
> +			tlb_finish_mmu()
> +			mmu_notifier_release(mm) {
> +				intel_iommu_release() {
> +   [2]					intel_iommu_teardown_pasid();

Parentheses might be better than square brackets for step numbers

> +                                        intel_iommu_flush_tlbs();
> +				}
> +				// tlb_invalidate_range cb removed
> +			}
> +			unmap_vmas();
> +                        free_pgtables(); // IOMMU cannot walk PGT after this
> +		};
> +	}
> +	exit_files(tsk) {
> +		close_files() {
> +			dsa_close();
> +   [1]			dsa_stop_dma();
> +                        intel_svm_unbind_pasid(); //nothing to do
> +		}
> +	}
> +   }
> +
> +   mmdrop() /* some random time later, lazy mm user */ {
> +   	mm_free_pgd();
> +        destroy_context(mm); {
> +   [3]	        ioasid_free();
> +	}
> +   }
> +
> +As shown in the list above, step #2 could happen before
> +#1. Unrecoverable(UR) faults could happen between #2 and #1.
> +
> +Also notice that TLB invalidation occurs at mmu_notifier
> +invalidate_range callback as well as the release callback. The reason
> +is that release callback will delete IOMMU driver from the notifier
> +chain which may skip invalidate_range() calls during the exit path.
> +
> +To avoid unnecessary reporting of UR fault, IOMMU driver shall disable
> +fault reporting after free and before unbind.
> +
> +Guest IOASID Life Cycle (VT-d Example)
> +--------------------------------------
> +Guest IOASID life cycle starts with guest driver open(), this could be
> +uacce or individual accelerator driver such as DSA. At FD open,
> +sva_bind_device() is called which triggers a series of actions.
> +
> +The example below is an illustration of *normal* operations that
> +involves *all* the SW components in VT-d. The flow can be simpler if
> +no ENQCMD is supported.
> +
> +::
> +
> +     VFIO        IOMMU        KVM        VDCM        IOASID       Ref
> +   ..................................................................
> +   1             ioasid_register_notifier/_mm()
> +   2 ioasid_alloc()                                               1
> +   3 bind_gpasid()
> +   4             iommu_bind()->ioasid_get()                       2
> +   5             ioasid_notify(BIND)
> +   6                          -> ioasid_get()                     3
> +   7                          -> vmcs_update_atomic()
> +   8 mdev_write(gpasid)
> +   9                                    hpasid=
> +   10                                   find_by_spid(gpasid)      4
> +   11                                   vdev_write(hpasid)
> +   12 -------- GUEST STARTS DMA --------------------------
> +   13 -------- GUEST STOPS DMA --------------------------
> +   14 mdev_clear(gpasid)
> +   15                                   vdev_clear(hpasid)
> +   16                                   ioasid_put()               3
> +   17 unbind_gpasid()
> +   18            iommu_ubind()
> +   19            ioasid_notify(UNBIND)
> +   20                          -> vmcs_update_atomic()
> +   21                          -> ioasid_put()                     2
> +   22 ioasid_free()                                                1
> +   23            ioasid_put()                                      0
> +   24                                                 Reclaimed
> +   -------------- New Life Cycle Begin ----------------------------
> +   1  ioasid_alloc()                                  ->           1
> +
> +   Note: IOASID Notification Events: FREE, BIND, UNBIND
> +
> +Exception cases arise when a guest crashes or a malicious guest
> +attempts to cause disruption on the host system. The fault handling
> +rules are:
> +
> +1. IOASID free must *always* succeed.
> +2. An inactive period may be required before the freed IOASID is
> +   reclaimed. During this period, consumers of IOASID perform cleanup.
> +3. Malfunction is limited to the guest owned resources for all
> +   programming errors.
> +
> +The primary source of exception is when the following are out of
> +order:
> +
> +1. Start/Stop of DMA activity
> +   (Guest device driver, mdev via VFIO)
> +2. Setup/Teardown of IOMMU PASID context, IOTLB, DevTLB flushes
> +   (Host IOMMU driver bind/unbind)
> +3. Setup/Teardown of VMCS PASID translation table entries (KVM) in
> +   case of ENQCMD
> +4. Programming/Clearing host PASID in VDCM (Host VDCM driver)
> +5. IOASID alloc/free (Host IOASID)
> +
> +VFIO is the *only* user-kernel interface, which is ultimately
> +responsible for exception handlings.

"handling"

> +
> +#1 is processed the same way as the assigned device today based on
> +device file descriptors and events. There is no special handling.
> +
> +#3 is based on bind/unbind events emitted by #2.
> +
> +#4 is naturally aligned with IOASID life cycle in that an illegal
> +guest PASID programming would fail in obtaining reference of the
> +matching host IOASID.
> +
> +#5 is similar to #4. The fault will be reported to the user if PASID
> +used in the ENQCMD is not set up in VMCS PASID translation table.
> +
> +Therefore, the remaining out of order problem is between #2 and
> +#5. I.e. unbind vs. free. More specifically, free before unbind.
> +
> +IOASID notifier and refcounting are used to ensure order. Following
> +a publisher-subscriber pattern where:
> +
> +- Publishers: VFIO & IOMMU
> +- Subscribers: KVM, VDCM, IOMMU
> +
> +IOASID notifier is atomic which requires subscribers to do quick
> +handling of the event in the atomic context. Workqueue can be used for
> +any processing that requires thread context. IOASID reference must be
> +acquired before receiving the FREE event. The reference must be
> +dropped at the end of the processing in order to return the IOASID to
> +the pool.
> +
> +Let's examine the IOASID life cycle again when free happens *before*
> +unbind. This could be a result of misbehaving guests or crash. Assuming
> +VFIO cannot enforce unbind->free order. Notice that the setup part up
> +until step #12 is identical to the normal case, the flow below starts
> +with step 13.
> +
> +::
> +
> +     VFIO        IOMMU        KVM        VDCM        IOASID       Ref
> +   ..................................................................
> +   13 -------- GUEST STARTS DMA --------------------------
> +   14 -------- *GUEST MISBEHAVES!!!* ----------------
> +   15 ioasid_free()
> +   16                                             ioasid_notify(FREE)
> +   17                                             mark_ioasid_inactive[1]
> +   18                          kvm_nb_handler(FREE)
> +   19                          vmcs_update_atomic()
> +   20                          ioasid_put_locked()   ->           3
> +   21                                   vdcm_nb_handler(FREE)
> +   22            iomm_nb_handler(FREE)
> +   23 ioasid_free() returns[2]          schedule_work()           2
> +   24            schedule_work()        vdev_clear_wk(hpasid)
> +   25            teardown_pasid_wk()
> +   26                                   ioasid_put() ->           1
> +   27            ioasid_put()                                     0
> +   28                                                 Reclaimed
> +   29 unbind_gpasid()
> +   30            iommu_unbind()->ioasid_find() Fails[3]
> +   -------------- New Life Cycle Begin ----------------------------
> +
> +Note:
> +
> +1. By marking IOASID inactive at step #17, no new references can be

Is "inactive" FREE_PENDING?

> +   held. ioasid_get/find() will return -ENOENT;
> +2. After step #23, all events can go out of order. Shall not affect
> +   the outcome.
> +3. IOMMU driver fails to find private data for unbinding. If unbind is
> +   called after the same IOASID is allocated for the same guest again,
> +   this is a programming error. The damage is limited to the guest
> +   itself since unbind performs permission checking based on the
> +   IOASID set associated with the guest process.
> +
> +KVM PASID Translation Table Updates
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +Per VM PASID translation table is maintained by KVM in order to
> +support ENQCMD in the guest. The table contains host-guest PASID
> +translations to be consumed by CPU ucode. The synchronization of the
> +PASID states depends on VFIO/IOMMU driver, where IOCTL and atomic
> +notifiers are used. KVM must register IOASID notifier per VM instance
> +during launch time. The following events are handled:
> +
> +1. BIND/UNBIND
> +2. FREE
> +
> +Rules:
> +
> +1. Multiple devices can bind with the same PASID, this can be different PCI
> +   devices or mdevs within the same PCI device. However, only the
> +   *first* BIND and *last* UNBIND emit notifications.
> +2. IOASID code is responsible for ensuring the correctness of H-G
> +   PASID mapping. There is no need for KVM to validate the
> +   notification data.
> +3. When UNBIND happens *after* FREE, KVM will see error in
> +   ioasid_get() even when the reclaim is not done. IOMMU driver will
> +   also avoid sending UNBIND if the PASID is already FREE.
> +4. When KVM terminates *before* FREE & UNBIND, references will be
> +   dropped for all host PASIDs.
> +
> +VDCM PASID Programming
> +~~~~~~~~~~~~~~~~~~~~~~
> +VDCM composes virtual devices and exposes them to the guests. When
> +the guest allocates a PASID then program it to the virtual device, VDCM
> +intercepts the programming attempt then program the matching host

"programs"

Thanks,
Jean

> +PASID on to the hardware.
> +Conversely, when a device is going away, VDCM must be informed such
> +that PASID context on the hardware can be cleared. There could be
> +multiple mdevs assigned to different guests in the same VDCM. Since
> +the PASID table is shared at PCI device level, lazy clearing is not
> +secure. A malicious guest can attack by using newly freed PASIDs that
> +are allocated by another guest.
> +
> +By holding a reference of the PASID until VDCM cleans up the HW context,
> +it is guaranteed that PASID life cycles do not cross within the same
> +device.
> +
> +
> +Reference
> +====================================================
> +1. https://software.intel.com/sites/default/files/managed/c5/15/architecture-instruction-set-extensions-programming-reference.pdf
> +
> +2. https://01.org/blogs/2019/introducing-intel-data-streaming-accelerator
> +
> +3. https://software.intel.com/en-us/download/intel-data-streaming-accelerator-preliminary-architecture-specification
> -- 
> 2.7.4
>