cachepc-linux

vas-api.rst (10949B)

.. SPDX-License-Identifier: GPL-2.0
.. _VAS-API:

===================================================
Virtual Accelerator Switchboard (VAS) userspace API
===================================================

Introduction
============

Power9 processor introduced Virtual Accelerator Switchboard (VAS) which
allows both userspace and kernel communicate to co-processor
(hardware accelerator) referred to as the Nest Accelerator (NX). The NX
unit comprises of one or more hardware engines or co-processor types
such as 842 compression, GZIP compression and encryption. On power9,
userspace applications will have access to only GZIP Compression engine
which supports ZLIB and GZIP compression algorithms in the hardware.

To communicate with NX, kernel has to establish a channel or window and
then requests can be submitted directly without kernel involvement.
Requests to the GZIP engine must be formatted as a co-processor Request
Block (CRB) and these CRBs must be submitted to the NX using COPY/PASTE
instructions to paste the CRB to hardware address that is associated with
the engine's request queue.

The GZIP engine provides two priority levels of requests: Normal and
High. Only Normal requests are supported from userspace right now.

This document explains userspace API that is used to interact with
kernel to setup channel / window which can be used to send compression
requests directly to NX accelerator.


Overview
========

Application access to the GZIP engine is provided through
/dev/crypto/nx-gzip device node implemented by the VAS/NX device driver.
An application must open the /dev/crypto/nx-gzip device to obtain a file
descriptor (fd). Then should issue VAS_TX_WIN_OPEN ioctl with this fd to
establish connection to the engine. It means send window is opened on GZIP
engine for this process. Once a connection is established, the application
should use the mmap() system call to map the hardware address of engine's
request queue into the application's virtual address space.

The application can then submit one or more requests to the engine by
using copy/paste instructions and pasting the CRBs to the virtual address
(aka paste_address) returned by mmap(). User space can close the
established connection or send window by closing the file descriptior
(close(fd)) or upon the process exit.

Note that applications can send several requests with the same window or
can establish multiple windows, but one window for each file descriptor.

Following sections provide additional details and references about the
individual steps.

NX-GZIP Device Node
===================

There is one /dev/crypto/nx-gzip node in the system and it provides
access to all GZIP engines in the system. The only valid operations on
/dev/crypto/nx-gzip are:

	* open() the device for read and write.
	* issue VAS_TX_WIN_OPEN ioctl
	* mmap() the engine's request queue into application's virtual
	  address space (i.e. get a paste_address for the co-processor
	  engine).
	* close the device node.

Other file operations on this device node are undefined.

Note that the copy and paste operations go directly to the hardware and
do not go through this device. Refer COPY/PASTE document for more
details.

Although a system may have several instances of the NX co-processor
engines (typically, one per P9 chip) there is just one
/dev/crypto/nx-gzip device node in the system. When the nx-gzip device
node is opened, Kernel opens send window on a suitable instance of NX
accelerator. It finds CPU on which the user process is executing and
determine the NX instance for the corresponding chip on which this CPU
belongs.

Applications may chose a specific instance of the NX co-processor using
the vas_id field in the VAS_TX_WIN_OPEN ioctl as detailed below.

A userspace library libnxz is available here but still in development:

	 https://github.com/abalib/power-gzip

Applications that use inflate / deflate calls can link with libnxz
instead of libz and use NX GZIP compression without any modification.

Open /dev/crypto/nx-gzip
========================

The nx-gzip device should be opened for read and write. No special
privileges are needed to open the device. Each window corresponds to one
file descriptor. So if the userspace process needs multiple windows,
several open calls have to be issued.

See open(2) system call man pages for other details such as return values,
error codes and restrictions.

VAS_TX_WIN_OPEN ioctl
=====================

Applications should use the VAS_TX_WIN_OPEN ioctl as follows to establish
a connection with NX co-processor engine:

	::

		struct vas_tx_win_open_attr {
			__u32   version;
			__s16   vas_id; /* specific instance of vas or -1
						for default */
			__u16   reserved1;
			__u64   flags;	/* For future use */
			__u64   reserved2[6];
		};

	version:
		The version field must be currently set to 1.
	vas_id:
		If '-1' is passed, kernel will make a best-effort attempt
		to assign an optimal instance of NX for the process. To
		select the specific VAS instance, refer
		"Discovery of available VAS engines" section below.

	flags, reserved1 and reserved2[6] fields are for future extension
	and must be set to 0.

	The attributes attr for the VAS_TX_WIN_OPEN ioctl are defined as
	follows::

		#define VAS_MAGIC 'v'
		#define VAS_TX_WIN_OPEN _IOW(VAS_MAGIC, 1,
						struct vas_tx_win_open_attr)

		struct vas_tx_win_open_attr attr;
		rc = ioctl(fd, VAS_TX_WIN_OPEN, &attr);

	The VAS_TX_WIN_OPEN ioctl returns 0 on success. On errors, it
	returns -1 and sets the errno variable to indicate the error.

	Error conditions:

		======	================================================
		EINVAL	fd does not refer to a valid VAS device.
		EINVAL	Invalid vas ID
		EINVAL	version is not set with proper value
		EEXIST	Window is already opened for the given fd
		ENOMEM	Memory is not available to allocate window
		ENOSPC	System has too many active windows (connections)
			opened
		EINVAL	reserved fields are not set to 0.
		======	================================================

	See the ioctl(2) man page for more details, error codes and
	restrictions.

mmap() NX-GZIP device
=====================

The mmap() system call for a NX-GZIP device fd returns a paste_address
that the application can use to copy/paste its CRB to the hardware engines.

	::

		paste_addr = mmap(addr, size, prot, flags, fd, offset);

	Only restrictions on mmap for a NX-GZIP device fd are:

		* size should be PAGE_SIZE
		* offset parameter should be 0ULL

	Refer to mmap(2) man page for additional details/restrictions.
	In addition to the error conditions listed on the mmap(2) man
	page, can also fail with one of the following error codes:

		======	=============================================
		EINVAL	fd is not associated with an open window
			(i.e mmap() does not follow a successful call
			to the VAS_TX_WIN_OPEN ioctl).
		EINVAL	offset field is not 0ULL.
		======	=============================================

Discovery of available VAS engines
==================================

Each available VAS instance in the system will have a device tree node
like /proc/device-tree/vas@* or /proc/device-tree/xscom@*/vas@*.
Determine the chip or VAS instance and use the corresponding ibm,vas-id
property value in this node to select specific VAS instance.

Copy/Paste operations
=====================

Applications should use the copy and paste instructions to send CRB to NX.
Refer section 4.4 in PowerISA for Copy/Paste instructions:
https://openpowerfoundation.org/?resource_lib=power-isa-version-3-0

CRB Specification and use NX
============================

Applications should format requests to the co-processor using the
co-processor Request Block (CRBs). Refer NX-GZIP user's manual for the format
of CRB and use NX from userspace such as sending requests and checking
request status.

NX Fault handling
=================

Applications send requests to NX and wait for the status by polling on
co-processor Status Block (CSB) flags. NX updates status in CSB after each
request is processed. Refer NX-GZIP user's manual for the format of CSB and
status flags.

In case if NX encounters translation error (called NX page fault) on CSB
address or any request buffer, raises an interrupt on the CPU to handle the
fault. Page fault can happen if an application passes invalid addresses or
request buffers are not in memory. The operating system handles the fault by
updating CSB with the following data::

	csb.flags = CSB_V;
	csb.cc = CSB_CC_FAULT_ADDRESS;
	csb.ce = CSB_CE_TERMINATION;
	csb.address = fault_address;

When an application receives translation error, it can touch or access
the page that has a fault address so that this page will be in memory. Then
the application can resend this request to NX.

If the OS can not update CSB due to invalid CSB address, sends SEGV signal
to the process who opened the send window on which the original request was
issued. This signal returns with the following siginfo struct::

	siginfo.si_signo = SIGSEGV;
	siginfo.si_errno = EFAULT;
	siginfo.si_code = SEGV_MAPERR;
	siginfo.si_addr = CSB adress;

In the case of multi-thread applications, NX send windows can be shared
across all threads. For example, a child thread can open a send window,
but other threads can send requests to NX using this window. These
requests will be successful even in the case of OS handling faults as long
as CSB address is valid. If the NX request contains an invalid CSB address,
the signal will be sent to the child thread that opened the window. But if
the thread is exited without closing the window and the request is issued
using this window. the signal will be issued to the thread group leader
(tgid). It is up to the application whether to ignore or handle these
signals.

NX-GZIP User's Manual:
https://github.com/libnxz/power-gzip/blob/master/doc/power_nx_gzip_um.pdf

Simple example
==============

	::

		int use_nx_gzip()
		{
			int rc, fd;
			void *addr;
			struct vas_setup_attr txattr;

			fd = open("/dev/crypto/nx-gzip", O_RDWR);
			if (fd < 0) {
				fprintf(stderr, "open nx-gzip failed\n");
				return -1;
			}
			memset(&txattr, 0, sizeof(txattr));
			txattr.version = 1;
			txattr.vas_id = -1
			rc = ioctl(fd, VAS_TX_WIN_OPEN,
					(unsigned long)&txattr);
			if (rc < 0) {
				fprintf(stderr, "ioctl() n %d, error %d\n",
						rc, errno);
				return rc;
			}
			addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE,
					MAP_SHARED, fd, 0ULL);
			if (addr == MAP_FAILED) {
				fprintf(stderr, "mmap() failed, errno %d\n",
						errno);
				return -errno;
			}
			do {
				//Format CRB request with compression or
				//uncompression
				// Refer tests for vas_copy/vas_paste
				vas_copy((&crb, 0, 1);
				vas_paste(addr, 0, 1);
				// Poll on csb.flags with timeout
				// csb address is listed in CRB
			} while (true)
			close(fd) or window can be closed upon process exit
		}

	Refer https://github.com/libnxz/power-gzip for tests or more
	use cases.