diff options
Diffstat (limited to 'Documentation/x86/sva.rst')
| -rw-r--r-- | Documentation/x86/sva.rst | 286 |
1 files changed, 286 insertions, 0 deletions
diff --git a/Documentation/x86/sva.rst b/Documentation/x86/sva.rst new file mode 100644 index 000000000..2e9b8b0f9 --- /dev/null +++ b/Documentation/x86/sva.rst @@ -0,0 +1,286 @@ +.. 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 the PCIe +specification Chapter 10: ATS Specification. + +Use of SVA requires IOMMU support in the platform. IOMMU is also +required to support the PCIe features ATS and PRI. ATS allows devices +to cache translations for virtual addresses. The IOMMU driver uses the +mmu_notifier() support to keep the device TLB cache and the CPU cache in +sync. When an ATS lookup fails for a virtual address, the device should +use the PRI in order to request the virtual address to be paged into the +CPU page tables. The device must use ATS again in order the fetch the +translation before use. + +Shared Hardware Workqueues +========================== + +Unlike Single Root I/O Virtualization (SR-IOV), 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 20-bit 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 should be provided. + +ENQCMD is the glue that ensures applications can directly submit commands +to the hardware and also permits hardware to be aware of application context +to perform I/O operations via use of PASID. + +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 register) in the + PASID context 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 invalidated, + the IOMMU propagates the invalidation to the 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 which will use +ENQCMD 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 transaction to +perform address translation. The IOMMU APIs setup the corresponding PASID +entry in IOMMU with the process address used by the CPU (e.g. %cr3 register in +x86). + +The MSR must be configured on each logical CPU before any application +thread can interact with a device. Threads that belong to the same +process share the same page tables, thus the same MSR value. + +PASID Life Cycle Management +=========================== + +PASID is initialized as INVALID_IOASID (-1) when a process is created. + +Only processes that access SVA-capable devices need to have a PASID +allocated. This allocation happens when a process opens/binds an SVA-capable +device but finds no PASID for this process. Subsequent binds 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. It's loaded to the +IA32_PASID MSR 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 has +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 PASID value might not be allocated yet or +the kernel does not know whether this thread is going to access the device +and the cleared IA32_PASID MSR reduces context switch overhead by xstate +init optimization. Since #GP faults have to be handled on any threads that +were created before the PASID was assigned to the mm of the process, newly +created threads might as well be treated in a consistent way. + +Due to complexity of freeing the PASID and clearing all IA32_PASID MSRs in +all threads in unbind, free the PASID lazily only on mm exit. + +If a process does a close(2) of the device file descriptor and munmap(2) +of the device MMIO portal, then the driver will unbind the device. The +PASID is still marked VALID in the PASID_MSR for any threads in the +process that accessed the device. But this is harmless as without the +MMIO portal they cannot submit new work to the device. + +Relationships +============= + + * Each process has many threads, but only one PASID. + * Devices have a limited number (~10's to 1000's) of hardware workqueues. + The device driver manages allocating hardware workqueues. + * A single mmap() maps a single hardware workqueue as a "portal" and + each portal maps down to a single workqueue. + * 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 + 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, i.e., to share it. Some call it Shared +Virtual Memory (SVM), but Linux community wanted to avoid confusing 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 Transaction Layer Packet +(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 workqueues different? + +Traditionally, in order for userspace applications to interact with hardware, +there is a separate hardware 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 it 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. + +A user should check Deferrable Memory Write (DMWr) capability on the device +and only submits ENQCMD when the device supports it. 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 atomic operations 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 helps the device scale to a large number of processes. + +* Is this the same as a user space device driver? + +Communicating with the device via the shared workqueue 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, it's required to be +almost fully functional interface to software supporting the traditional +BARs, 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 bus/device/function 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 SR-IOV type Virtual Functions (VF) is expensive. VFs require +duplicated hardware for PCI config space and interrupts such as MSI-X. +Resources such as interrupts have to be hard partitioned between VFs at +creation time, and cannot scale dynamically on demand. The VFs are not +completely independent from the Physical Function (PF). Most VFs require +some communication and assistance from the PF driver. SIOV, in contrast, +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 the 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 requests again 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 entry that might have been cached before removing the mappings from +the OS. + +References +========== + +VT-D: +https://01.org/blogs/ashokraj/2018/recent-enhancements-intel-virtualization-technology-directed-i/o-intel-vt-d + +SIOV: +https://01.org/blogs/2019/assignable-interfaces-intel-scalable-i/o-virtualization-linux + +ENQCMD in ISE: +https://software.intel.com/sites/default/files/managed/c5/15/architecture-instruction-set-extensions-programming-reference.pdf + +DSA spec: +https://software.intel.com/sites/default/files/341204-intel-data-streaming-accelerator-spec.pdf |