diff mbox series

[01/11] bus/vmbus: stub for FreeBSD support

Message ID 20210927134231.11177-2-srikanth.k@oneconvergence.com (mailing list archive)
State New
Delegated to: Thomas Monjalon
Headers show
Series add FreeBSD support to VMBUS & NetVSC PMDs | expand

Checks

Context Check Description
ci/checkpatch warning coding style issues

Commit Message

Srikanth Kaka Sept. 27, 2021, 1:42 p.m. UTC
These files are a copy of their Linux equivalents.
They will be ported to FreeBSD.

Signed-off-by: Srikanth Kaka <srikanth.k@oneconvergence.com>
Signed-off-by: Vag Singh <vag.singh@oneconvergence.com>
Signed-off-by: Anand Thulasiram <avelu@juniper.net>
---
 drivers/bus/vmbus/freebsd/vmbus_bus.c | 376 +++++++++++++++++++++
 drivers/bus/vmbus/freebsd/vmbus_uio.c | 453 ++++++++++++++++++++++++++
 2 files changed, 829 insertions(+)
 create mode 100644 drivers/bus/vmbus/freebsd/vmbus_bus.c
 create mode 100644 drivers/bus/vmbus/freebsd/vmbus_uio.c
diff mbox series

Patch

diff --git a/drivers/bus/vmbus/freebsd/vmbus_bus.c b/drivers/bus/vmbus/freebsd/vmbus_bus.c
new file mode 100644
index 0000000000..3c924eee14
--- /dev/null
+++ b/drivers/bus/vmbus/freebsd/vmbus_bus.c
@@ -0,0 +1,376 @@ 
+/* SPDX-License-Identifier: BSD-3-Clause
+ * Copyright (c) 2018, Microsoft Corporation.
+ * All Rights Reserved.
+ */
+
+#include <string.h>
+#include <unistd.h>
+#include <dirent.h>
+#include <fcntl.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+
+#include <rte_eal.h>
+#include <rte_uuid.h>
+#include <rte_tailq.h>
+#include <rte_log.h>
+#include <rte_devargs.h>
+#include <rte_memory.h>
+#include <rte_malloc.h>
+#include <rte_bus_vmbus.h>
+
+#include "eal_filesystem.h"
+#include "private.h"
+
+/** Pathname of VMBUS devices directory. */
+#define SYSFS_VMBUS_DEVICES "/sys/bus/vmbus/devices"
+
+/*
+ * GUID associated with network devices
+ * {f8615163-df3e-46c5-913f-f2d2f965ed0e}
+ */
+static const rte_uuid_t vmbus_nic_uuid = {
+	0xf8, 0x61, 0x51, 0x63,
+	0xdf, 0x3e,
+	0x46, 0xc5,
+	0x91, 0x3f,
+	0xf2, 0xd2, 0xf9, 0x65, 0xed, 0xe
+};
+
+extern struct rte_vmbus_bus rte_vmbus_bus;
+
+/* Read sysfs file to get UUID */
+static int
+parse_sysfs_uuid(const char *filename, rte_uuid_t uu)
+{
+	char buf[BUFSIZ];
+	char *cp, *in = buf;
+	FILE *f;
+
+	f = fopen(filename, "r");
+	if (f == NULL) {
+		VMBUS_LOG(ERR, "cannot open sysfs value %s: %s",
+			  filename, strerror(errno));
+		return -1;
+	}
+
+	if (fgets(buf, sizeof(buf), f) == NULL) {
+		VMBUS_LOG(ERR, "cannot read sysfs value %s",
+				filename);
+		fclose(f);
+		return -1;
+	}
+	fclose(f);
+
+	cp = strchr(buf, '\n');
+	if (cp)
+		*cp = '\0';
+
+	/* strip { } notation */
+	if (buf[0] == '{') {
+		in = buf + 1;
+		cp = strchr(in, '}');
+		if (cp)
+			*cp = '\0';
+	}
+
+	if (rte_uuid_parse(in, uu) < 0) {
+		VMBUS_LOG(ERR, "%s %s not a valid UUID",
+			filename, buf);
+		return -1;
+	}
+
+	return 0;
+}
+
+static int
+get_sysfs_string(const char *filename, char *buf, size_t buflen)
+{
+	char *cp;
+	FILE *f;
+
+	f = fopen(filename, "r");
+	if (f == NULL) {
+		VMBUS_LOG(ERR, "cannot open sysfs value %s:%s",
+			  filename, strerror(errno));
+		return -1;
+	}
+
+	if (fgets(buf, buflen, f) == NULL) {
+		VMBUS_LOG(ERR, "cannot read sysfs value %s",
+				filename);
+		fclose(f);
+		return -1;
+	}
+	fclose(f);
+
+	/* remove trailing newline */
+	cp = memchr(buf, '\n', buflen);
+	if (cp)
+		*cp = '\0';
+
+	return 0;
+}
+
+static int
+vmbus_get_uio_dev(const struct rte_vmbus_device *dev,
+		  char *dstbuf, size_t buflen)
+{
+	char dirname[PATH_MAX];
+	unsigned int uio_num;
+	struct dirent *e;
+	DIR *dir;
+
+	/* Assume recent kernel where uio is in uio/uioX */
+	snprintf(dirname, sizeof(dirname),
+		 SYSFS_VMBUS_DEVICES "/%s/uio", dev->device.name);
+
+	dir = opendir(dirname);
+	if (dir == NULL)
+		return -1; /* Not a UIO device */
+
+	/* take the first file starting with "uio" */
+	while ((e = readdir(dir)) != NULL) {
+		const int prefix_len = 3;
+		char *endptr;
+
+		if (strncmp(e->d_name, "uio", prefix_len) != 0)
+			continue;
+
+		/* try uio%d */
+		errno = 0;
+		uio_num = strtoull(e->d_name + prefix_len, &endptr, 10);
+		if (errno == 0 && endptr != (e->d_name + prefix_len)) {
+			snprintf(dstbuf, buflen, "%s/uio%u", dirname, uio_num);
+			break;
+		}
+	}
+	closedir(dir);
+
+	if (e == NULL)
+		return -1;
+
+	return uio_num;
+}
+
+/* Check map names with kernel names */
+static const char *map_names[VMBUS_MAX_RESOURCE] = {
+	[HV_TXRX_RING_MAP] = "txrx_rings",
+	[HV_INT_PAGE_MAP]  = "int_page",
+	[HV_MON_PAGE_MAP]  = "monitor_page",
+	[HV_RECV_BUF_MAP]  = "recv:",
+	[HV_SEND_BUF_MAP]  = "send:",
+};
+
+
+/* map the resources of a vmbus device in virtual memory */
+int
+rte_vmbus_map_device(struct rte_vmbus_device *dev)
+{
+	char uioname[PATH_MAX], filename[PATH_MAX];
+	char dirname[PATH_MAX], mapname[64];
+	int i;
+
+	dev->uio_num = vmbus_get_uio_dev(dev, uioname, sizeof(uioname));
+	if (dev->uio_num < 0) {
+		VMBUS_LOG(DEBUG, "Not managed by UIO driver, skipped");
+		return 1;
+	}
+
+	/* Extract resource value */
+	for (i = 0; i < VMBUS_MAX_RESOURCE; i++) {
+		struct rte_mem_resource *res = &dev->resource[i];
+		unsigned long len, gpad = 0;
+		char *cp;
+
+		snprintf(dirname, sizeof(dirname),
+			 "%s/maps/map%d", uioname, i);
+
+		snprintf(filename, sizeof(filename),
+			 "%s/name", dirname);
+
+		if (get_sysfs_string(filename, mapname, sizeof(mapname)) < 0) {
+			VMBUS_LOG(ERR, "could not read %s", filename);
+			return -1;
+		}
+
+		if (strncmp(map_names[i], mapname, strlen(map_names[i])) != 0) {
+			VMBUS_LOG(ERR,
+				"unexpected resource %s (expected %s)",
+				mapname, map_names[i]);
+			return -1;
+		}
+
+		snprintf(filename, sizeof(filename),
+			 "%s/size", dirname);
+		if (eal_parse_sysfs_value(filename, &len) < 0) {
+			VMBUS_LOG(ERR,
+				"could not read %s", filename);
+			return -1;
+		}
+		res->len = len;
+
+		/* both send and receive buffers have gpad in name */
+		cp = memchr(mapname, ':', sizeof(mapname));
+		if (cp)
+			gpad = strtoul(cp+1, NULL, 0);
+
+		/* put the GPAD value in physical address */
+		res->phys_addr = gpad;
+	}
+
+	return vmbus_uio_map_resource(dev);
+}
+
+void
+rte_vmbus_unmap_device(struct rte_vmbus_device *dev)
+{
+	vmbus_uio_unmap_resource(dev);
+}
+
+/* Scan one vmbus sysfs entry, and fill the devices list from it. */
+static int
+vmbus_scan_one(const char *name)
+{
+	struct rte_vmbus_device *dev, *dev2;
+	char filename[PATH_MAX];
+	char dirname[PATH_MAX];
+	unsigned long tmp;
+
+	dev = calloc(1, sizeof(*dev));
+	if (dev == NULL)
+		return -1;
+
+	dev->device.bus = &rte_vmbus_bus.bus;
+	dev->device.name = strdup(name);
+	if (!dev->device.name)
+		goto error;
+
+	/* sysfs base directory
+	 *   /sys/bus/vmbus/devices/7a08391f-f5a0-4ac0-9802-d13fd964f8df
+	 * or on older kernel
+	 *   /sys/bus/vmbus/devices/vmbus_1
+	 */
+	snprintf(dirname, sizeof(dirname), "%s/%s",
+		 SYSFS_VMBUS_DEVICES, name);
+
+	/* get device class  */
+	snprintf(filename, sizeof(filename), "%s/class_id", dirname);
+	if (parse_sysfs_uuid(filename, dev->class_id) < 0)
+		goto error;
+
+	/* skip non-network devices */
+	if (rte_uuid_compare(dev->class_id, vmbus_nic_uuid) != 0) {
+		free(dev);
+		return 0;
+	}
+
+	/* get device id */
+	snprintf(filename, sizeof(filename), "%s/device_id", dirname);
+	if (parse_sysfs_uuid(filename, dev->device_id) < 0)
+		goto error;
+
+	/* get relid */
+	snprintf(filename, sizeof(filename), "%s/id", dirname);
+	if (eal_parse_sysfs_value(filename, &tmp) < 0)
+		goto error;
+	dev->relid = tmp;
+
+	/* get monitor id */
+	snprintf(filename, sizeof(filename), "%s/monitor_id", dirname);
+	if (eal_parse_sysfs_value(filename, &tmp) < 0)
+		goto error;
+	dev->monitor_id = tmp;
+
+	/* get numa node (if present) */
+	snprintf(filename, sizeof(filename), "%s/numa_node",
+		 dirname);
+
+	if (access(filename, R_OK) == 0) {
+		if (eal_parse_sysfs_value(filename, &tmp) < 0)
+			goto error;
+		dev->device.numa_node = tmp;
+	} else {
+		/* if no NUMA support, set default to 0 */
+		dev->device.numa_node = SOCKET_ID_ANY;
+	}
+
+	dev->device.devargs = vmbus_devargs_lookup(dev);
+
+	/* device is valid, add in list (sorted) */
+	VMBUS_LOG(DEBUG, "Adding vmbus device %s", name);
+
+	TAILQ_FOREACH(dev2, &rte_vmbus_bus.device_list, next) {
+		int ret;
+
+		ret = rte_uuid_compare(dev->device_id, dev2->device_id);
+		if (ret > 0)
+			continue;
+
+		if (ret < 0) {
+			vmbus_insert_device(dev2, dev);
+		} else { /* already registered */
+			VMBUS_LOG(NOTICE,
+				"%s already registered", name);
+			free(dev);
+		}
+		return 0;
+	}
+
+	vmbus_add_device(dev);
+	return 0;
+error:
+	VMBUS_LOG(DEBUG, "failed");
+
+	free(dev);
+	return -1;
+}
+
+/*
+ * Scan the content of the vmbus, and the devices in the devices list
+ */
+int
+rte_vmbus_scan(void)
+{
+	struct dirent *e;
+	DIR *dir;
+
+	dir = opendir(SYSFS_VMBUS_DEVICES);
+	if (dir == NULL) {
+		if (errno == ENOENT)
+			return 0;
+
+		VMBUS_LOG(ERR, "opendir %s failed: %s",
+			  SYSFS_VMBUS_DEVICES, strerror(errno));
+		return -1;
+	}
+
+	while ((e = readdir(dir)) != NULL) {
+		if (e->d_name[0] == '.')
+			continue;
+
+		if (vmbus_scan_one(e->d_name) < 0)
+			goto error;
+	}
+	closedir(dir);
+	return 0;
+
+error:
+	closedir(dir);
+	return -1;
+}
+
+void rte_vmbus_irq_mask(struct rte_vmbus_device *device)
+{
+	vmbus_uio_irq_control(device, 1);
+}
+
+void rte_vmbus_irq_unmask(struct rte_vmbus_device *device)
+{
+	vmbus_uio_irq_control(device, 0);
+}
+
+int rte_vmbus_irq_read(struct rte_vmbus_device *device)
+{
+	return vmbus_uio_irq_read(device);
+}
diff --git a/drivers/bus/vmbus/freebsd/vmbus_uio.c b/drivers/bus/vmbus/freebsd/vmbus_uio.c
new file mode 100644
index 0000000000..b52ca5bf1d
--- /dev/null
+++ b/drivers/bus/vmbus/freebsd/vmbus_uio.c
@@ -0,0 +1,453 @@ 
+/* SPDX-License-Identifier: BSD-3-Clause
+ * Copyright (c) 2018, Microsoft Corporation.
+ * All Rights Reserved.
+ */
+
+#include <string.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <dirent.h>
+#include <inttypes.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+
+#include <rte_log.h>
+#include <rte_bus.h>
+#include <rte_memory.h>
+#include <rte_common.h>
+#include <rte_malloc.h>
+#include <rte_bus_vmbus.h>
+#include <rte_string_fns.h>
+
+#include "private.h"
+
+/** Pathname of VMBUS devices directory. */
+#define SYSFS_VMBUS_DEVICES "/sys/bus/vmbus/devices"
+
+static void *vmbus_map_addr;
+
+/* Control interrupts */
+void vmbus_uio_irq_control(struct rte_vmbus_device *dev, int32_t onoff)
+{
+	if (write(dev->intr_handle.fd, &onoff, sizeof(onoff)) < 0) {
+		VMBUS_LOG(ERR, "cannot write to %d:%s",
+			dev->intr_handle.fd, strerror(errno));
+	}
+}
+
+int vmbus_uio_irq_read(struct rte_vmbus_device *dev)
+{
+	int32_t count;
+	int cc;
+
+	cc = read(dev->intr_handle.fd, &count, sizeof(count));
+	if (cc < (int)sizeof(count)) {
+		if (cc < 0) {
+			VMBUS_LOG(ERR, "IRQ read failed %s",
+				  strerror(errno));
+			return -errno;
+		}
+		VMBUS_LOG(ERR, "can't read IRQ count");
+		return -EINVAL;
+	}
+
+	return count;
+}
+
+void
+vmbus_uio_free_resource(struct rte_vmbus_device *dev,
+		struct mapped_vmbus_resource *uio_res)
+{
+	rte_free(uio_res);
+
+	if (dev->intr_handle.uio_cfg_fd >= 0) {
+		close(dev->intr_handle.uio_cfg_fd);
+		dev->intr_handle.uio_cfg_fd = -1;
+	}
+
+	if (dev->intr_handle.fd >= 0) {
+		close(dev->intr_handle.fd);
+		dev->intr_handle.fd = -1;
+		dev->intr_handle.type = RTE_INTR_HANDLE_UNKNOWN;
+	}
+}
+
+int
+vmbus_uio_alloc_resource(struct rte_vmbus_device *dev,
+			 struct mapped_vmbus_resource **uio_res)
+{
+	char devname[PATH_MAX]; /* contains the /dev/uioX */
+
+	/* save fd if in primary process */
+	snprintf(devname, sizeof(devname), "/dev/uio%u", dev->uio_num);
+	dev->intr_handle.fd = open(devname, O_RDWR);
+	if (dev->intr_handle.fd < 0) {
+		VMBUS_LOG(ERR, "Cannot open %s: %s",
+			devname, strerror(errno));
+		goto error;
+	}
+	dev->intr_handle.type = RTE_INTR_HANDLE_UIO_INTX;
+
+	/* allocate the mapping details for secondary processes*/
+	*uio_res = rte_zmalloc("UIO_RES", sizeof(**uio_res), 0);
+	if (*uio_res == NULL) {
+		VMBUS_LOG(ERR, "cannot store uio mmap details");
+		goto error;
+	}
+
+	strlcpy((*uio_res)->path, devname, PATH_MAX);
+	rte_uuid_copy((*uio_res)->id, dev->device_id);
+
+	return 0;
+
+error:
+	vmbus_uio_free_resource(dev, *uio_res);
+	return -1;
+}
+
+static int
+find_max_end_va(const struct rte_memseg_list *msl, void *arg)
+{
+	size_t sz = msl->memseg_arr.len * msl->page_sz;
+	void *end_va = RTE_PTR_ADD(msl->base_va, sz);
+	void **max_va = arg;
+
+	if (*max_va < end_va)
+		*max_va = end_va;
+	return 0;
+}
+
+/*
+ * TODO: this should be part of memseg api.
+ *       code is duplicated from PCI.
+ */
+static void *
+vmbus_find_max_end_va(void)
+{
+	void *va = NULL;
+
+	rte_memseg_list_walk(find_max_end_va, &va);
+	return va;
+}
+
+int
+vmbus_uio_map_resource_by_index(struct rte_vmbus_device *dev, int idx,
+				struct mapped_vmbus_resource *uio_res,
+				int flags)
+{
+	size_t size = dev->resource[idx].len;
+	struct vmbus_map *maps = uio_res->maps;
+	void *mapaddr;
+	off_t offset;
+	int fd;
+
+	/* devname for mmap  */
+	fd = open(uio_res->path, O_RDWR);
+	if (fd < 0) {
+		VMBUS_LOG(ERR, "Cannot open %s: %s",
+			  uio_res->path, strerror(errno));
+		return -1;
+	}
+
+	/* try mapping somewhere close to the end of hugepages */
+	if (vmbus_map_addr == NULL)
+		vmbus_map_addr = vmbus_find_max_end_va();
+
+	/* offset is special in uio it indicates which resource */
+	offset = idx * rte_mem_page_size();
+
+	mapaddr = vmbus_map_resource(vmbus_map_addr, fd, offset, size, flags);
+	close(fd);
+
+	if (mapaddr == MAP_FAILED)
+		return -1;
+
+	dev->resource[idx].addr = mapaddr;
+	vmbus_map_addr = RTE_PTR_ADD(mapaddr, size);
+
+	/* Record result of successful mapping for use by secondary */
+	maps[idx].addr = mapaddr;
+	maps[idx].size = size;
+
+	return 0;
+}
+
+static int vmbus_uio_map_primary(struct vmbus_channel *chan,
+				 void **ring_buf, uint32_t *ring_size)
+{
+	struct mapped_vmbus_resource *uio_res;
+
+	uio_res = vmbus_uio_find_resource(chan->device);
+	if (!uio_res) {
+		VMBUS_LOG(ERR, "can not find resources!");
+		return -ENOMEM;
+	}
+
+	if (uio_res->nb_maps < VMBUS_MAX_RESOURCE) {
+		VMBUS_LOG(ERR, "VMBUS: only %u resources found!",
+			  uio_res->nb_maps);
+		return -EINVAL;
+	}
+
+	*ring_size = uio_res->maps[HV_TXRX_RING_MAP].size / 2;
+	*ring_buf  = uio_res->maps[HV_TXRX_RING_MAP].addr;
+	return 0;
+}
+
+static int vmbus_uio_map_subchan(const struct rte_vmbus_device *dev,
+				 const struct vmbus_channel *chan,
+				 void **ring_buf, uint32_t *ring_size)
+{
+	char ring_path[PATH_MAX];
+	size_t file_size;
+	struct stat sb;
+	void *mapaddr;
+	int fd;
+
+	snprintf(ring_path, sizeof(ring_path),
+		 "%s/%s/channels/%u/ring",
+		 SYSFS_VMBUS_DEVICES, dev->device.name,
+		 chan->relid);
+
+	fd = open(ring_path, O_RDWR);
+	if (fd < 0) {
+		VMBUS_LOG(ERR, "Cannot open %s: %s",
+			  ring_path, strerror(errno));
+		return -errno;
+	}
+
+	if (fstat(fd, &sb) < 0) {
+		VMBUS_LOG(ERR, "Cannot state %s: %s",
+			  ring_path, strerror(errno));
+		close(fd);
+		return -errno;
+	}
+	file_size = sb.st_size;
+
+	if (file_size == 0 || (file_size & (rte_mem_page_size() - 1))) {
+		VMBUS_LOG(ERR, "incorrect size %s: %zu",
+			  ring_path, file_size);
+
+		close(fd);
+		return -EINVAL;
+	}
+
+	mapaddr = vmbus_map_resource(vmbus_map_addr, fd,
+				     0, file_size, 0);
+	close(fd);
+
+	if (mapaddr == MAP_FAILED)
+		return -EIO;
+
+	*ring_size = file_size / 2;
+	*ring_buf = mapaddr;
+
+	vmbus_map_addr = RTE_PTR_ADD(mapaddr, file_size);
+	return 0;
+}
+
+int
+vmbus_uio_map_secondary_subchan(const struct rte_vmbus_device *dev,
+				const struct vmbus_channel *chan)
+{
+	const struct vmbus_br *br = &chan->txbr;
+	char ring_path[PATH_MAX];
+	void *mapaddr, *ring_buf;
+	uint32_t ring_size;
+	int fd;
+
+	snprintf(ring_path, sizeof(ring_path),
+		 "%s/%s/channels/%u/ring",
+		 SYSFS_VMBUS_DEVICES, dev->device.name,
+		 chan->relid);
+
+	ring_buf = br->vbr;
+	ring_size = br->dsize + sizeof(struct vmbus_bufring);
+	VMBUS_LOG(INFO, "secondary ring_buf %p size %u",
+		  ring_buf, ring_size);
+
+	fd = open(ring_path, O_RDWR);
+	if (fd < 0) {
+		VMBUS_LOG(ERR, "Cannot open %s: %s",
+			  ring_path, strerror(errno));
+		return -errno;
+	}
+
+	mapaddr = vmbus_map_resource(ring_buf, fd, 0, 2 * ring_size, 0);
+	close(fd);
+
+	if (mapaddr == ring_buf)
+		return 0;
+
+	if (mapaddr == MAP_FAILED)
+		VMBUS_LOG(ERR,
+			  "mmap subchan %u in secondary failed", chan->relid);
+	else {
+		VMBUS_LOG(ERR,
+			  "mmap subchan %u in secondary address mismatch",
+			  chan->relid);
+		vmbus_unmap_resource(mapaddr, 2 * ring_size);
+	}
+	return -1;
+}
+
+int vmbus_uio_map_rings(struct vmbus_channel *chan)
+{
+	const struct rte_vmbus_device *dev = chan->device;
+	uint32_t ring_size;
+	void *ring_buf;
+	int ret;
+
+	/* Primary channel */
+	if (chan->subchannel_id == 0)
+		ret = vmbus_uio_map_primary(chan, &ring_buf, &ring_size);
+	else
+		ret = vmbus_uio_map_subchan(dev, chan, &ring_buf, &ring_size);
+
+	if (ret)
+		return ret;
+
+	vmbus_br_setup(&chan->txbr, ring_buf, ring_size);
+	vmbus_br_setup(&chan->rxbr, (char *)ring_buf + ring_size, ring_size);
+	return 0;
+}
+
+static int vmbus_uio_sysfs_read(const char *dir, const char *name,
+				unsigned long *val, unsigned long max_range)
+{
+	char path[PATH_MAX];
+	FILE *f;
+	int ret;
+
+	snprintf(path, sizeof(path), "%s/%s", dir, name);
+	f = fopen(path, "r");
+	if (!f) {
+		VMBUS_LOG(ERR, "can't open %s:%s",
+			  path, strerror(errno));
+		return -errno;
+	}
+
+	if (fscanf(f, "%lu", val) != 1)
+		ret = -EIO;
+	else if (*val > max_range)
+		ret = -ERANGE;
+	else
+		ret = 0;
+	fclose(f);
+
+	return ret;
+}
+
+static bool vmbus_uio_ring_present(const struct rte_vmbus_device *dev,
+				   uint32_t relid)
+{
+	char ring_path[PATH_MAX];
+
+	/* Check if kernel has subchannel sysfs files */
+	snprintf(ring_path, sizeof(ring_path),
+		 "%s/%s/channels/%u/ring",
+		 SYSFS_VMBUS_DEVICES, dev->device.name, relid);
+
+	return access(ring_path, R_OK|W_OK) == 0;
+}
+
+bool vmbus_uio_subchannels_supported(const struct rte_vmbus_device *dev,
+				     const struct vmbus_channel *chan)
+{
+	return vmbus_uio_ring_present(dev, chan->relid);
+}
+
+static bool vmbus_isnew_subchannel(struct vmbus_channel *primary,
+				   unsigned long id)
+{
+	const struct vmbus_channel *c;
+
+	STAILQ_FOREACH(c, &primary->subchannel_list, next) {
+		if (c->relid == id)
+			return false;
+	}
+	return true;
+}
+
+int vmbus_uio_get_subchan(struct vmbus_channel *primary,
+			  struct vmbus_channel **subchan)
+{
+	const struct rte_vmbus_device *dev = primary->device;
+	char chan_path[PATH_MAX], subchan_path[PATH_MAX];
+	struct dirent *ent;
+	DIR *chan_dir;
+	int err;
+
+	snprintf(chan_path, sizeof(chan_path),
+		 "%s/%s/channels",
+		 SYSFS_VMBUS_DEVICES, dev->device.name);
+
+	chan_dir = opendir(chan_path);
+	if (!chan_dir) {
+		VMBUS_LOG(ERR, "cannot open %s: %s",
+			  chan_path, strerror(errno));
+		return -errno;
+	}
+
+	while ((ent = readdir(chan_dir))) {
+		unsigned long relid, subid, monid;
+		char *endp;
+
+		if (ent->d_name[0] == '.')
+			continue;
+
+		errno = 0;
+		relid = strtoul(ent->d_name, &endp, 0);
+		if (*endp || errno != 0 || relid > UINT16_MAX) {
+			VMBUS_LOG(NOTICE, "not a valid channel relid: %s",
+				  ent->d_name);
+			continue;
+		}
+
+		if (!vmbus_isnew_subchannel(primary, relid)) {
+			VMBUS_LOG(DEBUG, "skip already found channel: %lu",
+				  relid);
+			continue;
+		}
+
+		if (!vmbus_uio_ring_present(dev, relid)) {
+			VMBUS_LOG(DEBUG, "ring mmap not found (yet) for: %lu",
+				  relid);
+			continue;
+		}
+
+		snprintf(subchan_path, sizeof(subchan_path), "%s/%lu",
+			 chan_path, relid);
+		err = vmbus_uio_sysfs_read(subchan_path, "subchannel_id",
+					   &subid, UINT16_MAX);
+		if (err) {
+			VMBUS_LOG(NOTICE, "no subchannel_id in %s:%s",
+				  subchan_path, strerror(-err));
+			goto fail;
+		}
+
+		if (subid == 0)
+			continue;	/* skip primary channel */
+
+		err = vmbus_uio_sysfs_read(subchan_path, "monitor_id",
+					   &monid, UINT8_MAX);
+		if (err) {
+			VMBUS_LOG(NOTICE, "no monitor_id in %s:%s",
+				  subchan_path, strerror(-err));
+			goto fail;
+		}
+
+		err = vmbus_chan_create(dev, relid, subid, monid, subchan);
+		if (err) {
+			VMBUS_LOG(ERR, "subchannel setup failed");
+			goto fail;
+		}
+		break;
+	}
+	closedir(chan_dir);
+
+	return (ent == NULL) ? -ENOENT : 0;
+fail:
+	closedir(chan_dir);
+	return err;
+}