RE: [PATCH v6 03/12] docs: x86: Add documentation for SVA (Shared Virtual Addressing)

Liu, Yi L
2020-07-14T03:25:28Z

> From: Fenghua Yu <fenghua.yu@intel.com>
> Sent: Tuesday, July 14, 2020 7:48 AM
>
> From: Ashok Raj <ashok.raj@intel.com>
>
> ENQCMD and Data Streaming Accelerator (DSA) and all of their associated features
> are a complicated stack with lots of interconnected pieces.
> This documentation provides a big picture overview for all of the features.
>
> Signed-off-by: Ashok Raj <ashok.raj@intel.com>
> Co-developed-by: Fenghua Yu <fenghua.yu@intel.com>
> Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
> Reviewed-by: Tony Luck <tony.luck@intel.com>
> ---
> v3:
> - Replace deprecated intel_svm_bind_mm() by iommu_sva_bind_mm() (Baolu)
> - Fix a couple of typos (Baolu)
>
> v2:
> - Fix the doc format and add the doc in toctree (Thomas)
> - Modify the doc for better description (Thomas, Tony, Dave)
>
> Documentation/x86/index.rst | 1 +
> Documentation/x86/sva.rst | 287 ++++++++++++++++++++++++++++++++++++
> 2 files changed, 288 insertions(+)
> create mode 100644 Documentation/x86/sva.rst
>
> diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst index
> 265d9e9a093b..e5d5ff096685 100644
> --- a/Documentation/x86/index.rst
> +++ b/Documentation/x86/index.rst
> @@ -30,3 +30,4 @@ x86-specific Documentation
> usb-legacy-support
> i386/index
> x86_64/index
> + sva
> diff --git a/Documentation/x86/sva.rst b/Documentation/x86/sva.rst new file mode
> 100644 index 000000000000..7242a84169ef
> --- /dev/null
> +++ b/Documentation/x86/sva.rst
> @@ -0,0 +1,287 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +===========================================
> +Shared Virtual Addressing (SVA) with ENQCMD
> +===========================================
> +
> +Background
> +==========
> +
> +Shared Virtual Addressing (SVA) allows the processor and device to use
> +the same virtual addresses avoiding the need for software to translate
> +virtual addresses to physical addresses. SVA is what PCIe calls Shared
> +Virtual Memory (SVM)
> +
> +In addition to the convenience of using application virtual addresses
> +by the device, it also doesn't require pinning pages for DMA.
> +PCIe Address Translation Services (ATS) along with Page Request
> +Interface
> +(PRI) allow devices to function much the same way as the CPU handling
> +application page-faults. For more information please refer to PCIe
> +specification Chapter 10: ATS Specification.
> +

nit: may be helpful to mention Chapter 10 of PCIe spec since 4.0. before that, ATS has its
own specification.

> +Use of SVA requires IOMMU support in the platform. IOMMU also is
> +required to support PCIe features ATS and PRI. ATS allows devices to
> +cache translations for the virtual address. IOMMU driver uses the
> +mmu_notifier() support to keep the device tlb cache and the CPU cache
> +in sync. PRI allows the device to request paging the virtual address
> +before using if they are not paged in the CPU page tables.
> +
> +
> +Shared Hardware Workqueues
> +==========================
> +
> +Unlike Single Root I/O Virtualization (SRIOV), Scalable IOV (SIOV)
> +permits the use of Shared Work Queues (SWQ) by both applications and
> +Virtual Machines (VM's). This allows better hardware utilization vs.
> +hard partitioning resources that could result in under utilization. In
> +order to allow the hardware to distinguish the context for which work
> +is being executed in the hardware by SWQ interface, SIOV uses Process
> +Address Space ID (PASID), which is a 20bit number defined by the PCIe SIG.
> +
> +PASID value is encoded in all transactions from the device. This allows
> +the IOMMU to track I/O on a per-PASID granularity in addition to using
> +the PCIe Resource Identifier (RID) which is the Bus/Device/Function.
> +
> +
> +ENQCMD
> +======
> +
> +ENQCMD is a new instruction on Intel platforms that atomically submits
> +a work descriptor to a device. The descriptor includes the operation to
> +be performed, virtual addresses of all parameters, virtual address of a
> +completion record, and the PASID (process address space ID) of the current process.
> +
> +ENQCMD works with non-posted semantics and carries a status back if the
> +command was accepted by hardware. This allows the submitter to know if
> +the submission needs to be retried or other device specific mechanisms
> +to implement fairness or ensure forward progress can be made.
> +
> +ENQCMD is the glue that ensures applications can directly submit
> +commands to the hardware and also permit hardware to be aware of
> +application context to perform I/O operations via use of PASID.
> +

maybe a reader will ask about ENQCMDs after reading ENQCMD/S spec. :-)

> +Process Address Space Tagging
> +=============================
> +
> +A new thread scoped MSR (IA32_PASID) provides the connection between
> +user processes and the rest of the hardware. When an application first
> +accesses an SVA capable device this MSR is initialized with a newly
> +allocated PASID. The driver for the device calls an IOMMU specific api
> +that sets up the routing for DMA and page-requests.
> +
> +For example, the Intel Data Streaming Accelerator (DSA) uses
> +iommu_sva_bind_device(), which will do the following.
> +
> +- Allocate the PASID, and program the process page-table (cr3) in the
> +PASID
> + context entries.

nit: s/PASID context entries/PASID table entries/

> +- Register for mmu_notifier() to track any page-table invalidations to
> +keep
> + the device tlb in sync. For example, when a page-table entry is

not only device tlb. I guess iotlb is also included.

> +invalidated,
> + IOMMU propagates the invalidation to device tlb. This will force any
> + future access by the device to this virtual address to participate in
> + ATS. If the IOMMU responds with proper response that a page is not
> + present, the device would request the page to be paged in via the
> +PCIe PRI
> + protocol before performing I/O.
> +
> +This MSR is managed with the XSAVE feature set as "supervisor state" to
> +ensure the MSR is updated during context switch.
> +
> +PASID Management
> +================
> +
> +The kernel must allocate a PASID on behalf of each process and program
> +it into the new MSR to communicate the process identity to platform hardware.
> +ENQCMD uses the PASID stored in this MSR to tag requests from this process.
> +When a user submits a work descriptor to a device using the ENQCMD
> +instruction, the PASID field in the descriptor is auto-filled with the
> +value from MSR_IA32_PASID. Requests for DMA from the device are also
> +tagged with the same PASID. The platform IOMMU uses the PASID in the

not quite get " Requests for DMA from the device are also tagged with the same PASID"

> +transaction to perform address translation. The IOMMU api's setup the

s/api's/apis/ ?

> +corresponding PASID entry in IOMMU with the process address used by the CPU
> (for e.g cr3 in x86).

with the process page tables used by the CPU (e.g. the page tables pointed by cr3 in x86).

> +
> +The MSR must be configured on each logical CPU before any application

s/MSR/MSR_IA32_PASID/

> +thread can interact with a device. Threads that belong to the same
> +process share the same page tables, thus the same MSR value.

s/MSR/PASID/

> +
> +PASID is cleared when a process is created. The PASID allocation and

s/PASID/MSR_IA32_PASID/

> +MSR programming may occur long after a process and its threads have been
> created.
> +One thread must call bind() to allocate the PASID for the process. If a

s/bind()/iommu_sva_bind_device()/ or say "call iommu api to bind a process with
a device." :-)

> +thread uses ENQCMD without the MSR first being populated, it will cause #GP.
> +The kernel will fix up the #GP by writing the process-wide PASID into
> +the thread that took the #GP. A single process PASID can be used
> +simultaneously with multiple devices since they all share the same address space.

simultaneously with multiple devices if they all share the process address space.

> +
> +New threads could inherit the MSR value from the parent. But this would

s/MSR/MSR_IA32_PASID/

> +involve additional state management for those threads which may never
> +use ENQCMD. Clearing the MSR at thread creation permits all threads to
> +have a consistent behavior; the PASID is only programmed when the
> +thread calls the bind() api (iommu_sva_bind_device()()), or when a
> +thread calls ENQCMD for the first time.
> +
> +PASID Lifecycle Management
> +==========================
> +
> +Only processes that access SVA capable devices need to have a PASID
> +allocated. This allocation happens when a process first opens an SVA
> +capable device (subsequent opens of the same, or other devices will
> +share the same PASID).
> +
> +Although the PASID is allocated to the process by opening a device, it
> +is not active in any of the threads of that process. Activation is done
> +lazily when a thread tries to submit a work descriptor to a device
> +using the ENQCMD.
> +
> +That first access will trigger a #GP fault because the IA32_PASID MSR
> +has not been initialized with the PASID value assigned to the process
> +when the device was opened. The Linux #GP handler notes that a PASID as
> +been allocated for the process, and so initializes the IA32_PASID MSR
> +and returns so that the ENQCMD instruction is re-executed.
> +
> +On fork(2) or exec(2) the PASID is removed from the process as it no
> +longer has the same address space that it had when the device was opened.
> +
> +On clone(2) the new task shares the same address space, so will be able
> +to use the PASID allocated to the process. The IA32_PASID is not
> +preemptively initialized as the kernel does not know whether this
> +thread is going to access the device.
> +
> +On exit(2) the PASID is freed. The device driver ensures that any
> +pending operations queued to the device are either completed or aborted
> +before allowing the PASID to be reallocated.
> +
> +Relationships
> +=============
> +
> + * Each process has many threads, but only one PASID
> + * Devices have a limited number (~10's to 1000's) of hardware
> + workqueues and each portal maps down to a single workqueue.
> + The device driver manages allocating hardware workqueues.
> + * A single mmap() maps a single hardware workqueue as a "portal"
> + * For each device with which a process interacts, there must be
> + one or more mmap()'d portals.
> + * Many threads within a process can share a single portal to access
> + a single device.
> + * Multiple processes can separately mmap() the same portal, in
> + which case they still share one device hardware workqueue.
> + * The single process-wide PASID is used by all threads to interact
> + with all devices. There is not, for instance, a PASID for each

s/with all devices/with all devices manipulated by the process/

Regards,
Yi Liu

> + thread or each thread<->device pair.
> +
> +FAQ
> +===
> +
> +* What is SVA/SVM?
> +
> +Shared Virtual Addressing (SVA) permits I/O hardware and the processor
> +to work in the same address space. In short, sharing the address space.
> +Some call it Shared Virtual Memory (SVM), but Linux community wanted to
> +avoid it with Posix Shared Memory and Secure Virtual Machines which
> +were terms already in circulation.
> +
> +* What is a PASID?
> +
> +A Process Address Space ID (PASID) is a PCIe-defined TLP Prefix. A
> +PASID is a 20 bit number allocated and managed by the OS. PASID is
> +included in all transactions between the platform and the device.
> +
> +* How are shared work queues different?
> +
> +Traditionally to allow user space applications interact with hardware,
> +there is a separate instance required per process. For example,
> +consider doorbells as a mechanism of informing hardware about work to
> +process. Each doorbell is required to be spaced 4k (or page-size) apart
> +for process isolation. This requires hardware to provision that space
> +and reserve in MMIO. This doesn't scale as the number of threads
> +becomes quite large. The hardware also manages the queue depth for
> +Shared Work Queues (SWQ), and consumers don't need to track queue
> +depth. If there is no space to accept a command, the device will return
> +an error indicating retry. Also submitting a command to an MMIO address
> +that can't accept ENQCMD will return retry in response. In the new DMWr
> +PCIe terminology, devices need to support DMWr completer capability. In
> +addition it requires all switch ports to support DMWr routing and must
> +be enabled by the PCIe subsystem, much like how PCIe Atomics() are managed for
> instance.
> +
> +SWQ allows hardware to provision just a single address in the device.
> +When used with ENQCMD to submit work, the device can distinguish the
> +process submitting the work since it will include the PASID assigned to
> +that process. This decreases the pressure of hardware requiring to
> +support hardware to scale to a large number of processes.
> +
> +* Is this the same as a user space device driver?
> +
> +Communicating with the device via the shared work queue is much simpler
> +than a full blown user space driver. The kernel driver does all the
> +initialization of the hardware. User space only needs to worry about
> +submitting work and processing completions.
> +
> +* Is this the same as SR-IOV?
> +
> +Single Root I/O Virtualization (SR-IOV) focuses on providing
> +independent hardware interfaces for virtualizing hardware. Hence its
> +required to be almost fully functional interface to software supporting
> +the traditional BAR's, space for interrupts via MSI-x, its own register layout.
> +Virtual Functions (VFs) are assisted by the Physical Function (PF)
> +driver.
> +
> +Scalable I/O Virtualization builds on the PASID concept to create
> +device instances for virtualization. SIOV requires host software to
> +assist in creating virtual devices, each virtual device is represented
> +by a PASID along with the BDF of the device. This allows device
> +hardware to optimize device resource creation and can grow dynamically
> +on demand. SR-IOV creation and management is very static in nature.
> +Consult references below for more details.
> +
> +* Why not just create a virtual function for each app?
> +
> +Creating PCIe SRIOV type virtual functions (VF) are expensive. They
> +create duplicated hardware for PCI config space requirements,
> +Interrupts such as MSIx for instance. Resources such as interrupts have
> +to be hard partitioned between VF's at creation time, and cannot scale
> +dynamically on demand. The VF's are not completely independent from the
> +Physical function (PF). Most VF's require some communication and
> +assistance from the PF driver. SIOV creates a software defined device.
> +Where all the configuration and control aspects are mediated via the
> +slow path. The work submission and completion happen without any mediation.
> +
> +* Does this support virtualization?
> +
> +ENQCMD can be used from within a guest VM. In these cases the VMM helps
> +with setting up a translation table to translate from Guest PASID to
> +Host PASID. Please consult the ENQCMD instruction set reference for
> +more details.
> +
> +* Does memory need to be pinned?
> +
> +When devices support SVA, along with platform hardware such as IOMMU
> +supporting such devices, there is no need to pin memory for DMA purposes.
> +Devices that support SVA also support other PCIe features that remove
> +the pinning requirement for memory.
> +
> +Device TLB support - Device requests the IOMMU to lookup an address
> +before use via Address Translation Service (ATS) requests. If the
> +mapping exists but there is no page allocated by the OS, IOMMU hardware
> +returns that no mapping exists.
> +
> +Device requests that virtual address to be mapped via Page Request
> +Interface (PRI). Once the OS has successfully completed the mapping,
> +it returns the response back to the device. The device continues again
> +to request for a translation and continues.
> +
> +IOMMU works with the OS in managing consistency of page-tables with the
> +device. When removing pages, it interacts with the device to remove any
> +device-tlb that might have been cached before removing the mappings
> +from the OS.
> +
> +References
> +==========
> +
> +VT-D:
> +https://01.org/blogs/ashokraj/2018/recent-enhancements-intel-virtualiza
> +tion-technology-directed-i/o-intel-vt-d
> +
> +SIOV:
> +https://01.org/blogs/2019/assignable-interfaces-intel-scalable-i/o-virt
> +ualization-linux
> +
> +ENQCMD in ISE:
> +https://software.intel.com/sites/default/files/managed/c5/15/architectu
> +re-instruction-set-extensions-programming-reference.pdf
> +
> +DSA spec:
> +https://software.intel.com/sites/default/files/341204-intel-data-stream
> +ing-accelerator-spec.pdf
> --
> 2.19.1
>
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