vringh: host-side implementation of virtio rings.

Getting use of virtio rings correct is tricky, and a recent patch saw
an implementation of in-kernel rings (as separate from userspace).

This abstracts the business of dealing with the virtio ring layout
from the access (userspace or direct); to do this, we use function
pointers, which gcc inlines correctly.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Michael S. Tsirkin <mst@redhat.com>

drivers/Makefile

@@ -123,7 +123,7 @@ obj-$(CONFIG_PPC_PS3)		+= ps3/
 obj-$(CONFIG_OF)		+= of/
 obj-$(CONFIG_SSB)		+= ssb/
 obj-$(CONFIG_BCMA)		+= bcma/
-obj-$(CONFIG_VHOST_NET)		+= vhost/
+obj-$(CONFIG_VHOST_RING)	+= vhost/
 obj-$(CONFIG_VLYNQ)		+= vlynq/
 obj-$(CONFIG_STAGING)		+= staging/
 obj-y				+= platform/

drivers/vhost/Kconfig

@@ -1,6 +1,7 @@
 config VHOST_NET
 	tristate "Host kernel accelerator for virtio net"
 	depends on NET && EVENTFD && (TUN || !TUN) && (MACVTAP || !MACVTAP)
+	select VHOST_RING
 	---help---
 	  This kernel module can be loaded in host kernel to accelerate
 	  guest networking with virtio_net. Not to be confused with virtio_net
@@ -12,3 +13,10 @@ config VHOST_NET
 if STAGING
 source "drivers/vhost/Kconfig.tcm"
 endif
+
+config VHOST_RING
+	tristate
+	---help---
+	  This option is selected by any driver which needs to access
+	  the host side of a virtio ring.
+

drivers/vhost/Kconfig.tcm

@@ -1,6 +1,7 @@
 config TCM_VHOST
 	tristate "TCM_VHOST fabric module"
 	depends on TARGET_CORE && EVENTFD && m
+	select VHOST_RING
 	default n
 	---help---
 	Say M here to enable the TCM_VHOST fabric module for use with virtio-scsi guests

drivers/vhost/Makefile

@@ -2,3 +2,5 @@ obj-$(CONFIG_VHOST_NET) += vhost_net.o
 vhost_net-y := vhost.o net.o
 
 obj-$(CONFIG_TCM_VHOST) += tcm_vhost.o
+
+obj-$(CONFIG_VHOST_RING) += vringh.o

include/linux/vringh.h

@@ -0,0 +1,196 @@
+/*
+ * Linux host-side vring helpers; for when the kernel needs to access
+ * someone else's vring.
+ *
+ * Copyright IBM Corporation, 2013.
+ * Parts taken from drivers/vhost/vhost.c Copyright 2009 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ * Written by: Rusty Russell <rusty@rustcorp.com.au>
+ */
+#ifndef _LINUX_VRINGH_H
+#define _LINUX_VRINGH_H
+#include <uapi/linux/virtio_ring.h>
+#include <linux/uio.h>
+#include <linux/slab.h>
+#include <asm/barrier.h>
+
+/* virtio_ring with information needed for host access. */
+struct vringh {
+	/* Guest publishes used event idx (note: we always do). */
+	bool event_indices;
+
+	/* Can we get away with weak barriers? */
+	bool weak_barriers;
+
+	/* Last available index we saw (ie. where we're up to). */
+	u16 last_avail_idx;
+
+	/* Last index we used. */
+	u16 last_used_idx;
+
+	/* How many descriptors we've completed since last need_notify(). */
+	u32 completed;
+
+	/* The vring (note: it may contain user pointers!) */
+	struct vring vring;
+};
+
+/* The memory the vring can access, and what offset to apply. */
+struct vringh_range {
+	u64 start, end_incl;
+	u64 offset;
+};
+
+/**
+ * struct vringh_iov - iovec mangler.
+ *
+ * Mangles iovec in place, and restores it.
+ * Remaining data is iov + i, of used - i elements.
+ */
+struct vringh_iov {
+	struct iovec *iov;
+	size_t consumed; /* Within iov[i] */
+	unsigned i, used, max_num;
+};
+
+/**
+ * struct vringh_iov - kvec mangler.
+ *
+ * Mangles kvec in place, and restores it.
+ * Remaining data is iov + i, of used - i elements.
+ */
+struct vringh_kiov {
+	struct kvec *iov;
+	size_t consumed; /* Within iov[i] */
+	unsigned i, used, max_num;
+};
+
+/* Flag on max_num to indicate we're kmalloced. */
+#define VRINGH_IOV_ALLOCATED 0x8000000
+
+/* Helpers for userspace vrings. */
+int vringh_init_user(struct vringh *vrh, u32 features,
+		     unsigned int num, bool weak_barriers,
+		     struct vring_desc __user *desc,
+		     struct vring_avail __user *avail,
+		     struct vring_used __user *used);
+
+static inline void vringh_iov_init(struct vringh_iov *iov,
+				   struct iovec *iovec, unsigned num)
+{
+	iov->used = iov->i = 0;
+	iov->consumed = 0;
+	iov->max_num = num;
+	iov->iov = iovec;
+}
+
+static inline void vringh_iov_reset(struct vringh_iov *iov)
+{
+	iov->iov[iov->i].iov_len += iov->consumed;
+	iov->iov[iov->i].iov_base -= iov->consumed;
+	iov->consumed = 0;
+	iov->i = 0;
+}
+
+static inline void vringh_iov_cleanup(struct vringh_iov *iov)
+{
+	if (iov->max_num & VRINGH_IOV_ALLOCATED)
+		kfree(iov->iov);
+	iov->max_num = iov->used = iov->i = iov->consumed = 0;
+	iov->iov = NULL;
+}
+
+/* Convert a descriptor into iovecs. */
+int vringh_getdesc_user(struct vringh *vrh,
+			struct vringh_iov *riov,
+			struct vringh_iov *wiov,
+			bool (*getrange)(struct vringh *vrh,
+					 u64 addr, struct vringh_range *r),
+			u16 *head);
+
+/* Copy bytes from readable vsg, consuming it (and incrementing wiov->i). */
+ssize_t vringh_iov_pull_user(struct vringh_iov *riov, void *dst, size_t len);
+
+/* Copy bytes into writable vsg, consuming it (and incrementing wiov->i). */
+ssize_t vringh_iov_push_user(struct vringh_iov *wiov,
+			     const void *src, size_t len);
+
+/* Mark a descriptor as used. */
+int vringh_complete_user(struct vringh *vrh, u16 head, u32 len);
+int vringh_complete_multi_user(struct vringh *vrh,
+			       const struct vring_used_elem used[],
+			       unsigned num_used);
+
+/* Pretend we've never seen descriptor (for easy error handling). */
+void vringh_abandon_user(struct vringh *vrh, unsigned int num);
+
+/* Do we need to fire the eventfd to notify the other side? */
+int vringh_need_notify_user(struct vringh *vrh);
+
+bool vringh_notify_enable_user(struct vringh *vrh);
+void vringh_notify_disable_user(struct vringh *vrh);
+
+/* Helpers for kernelspace vrings. */
+int vringh_init_kern(struct vringh *vrh, u32 features,
+		     unsigned int num, bool weak_barriers,
+		     struct vring_desc *desc,
+		     struct vring_avail *avail,
+		     struct vring_used *used);
+
+static inline void vringh_kiov_init(struct vringh_kiov *kiov,
+				    struct kvec *kvec, unsigned num)
+{
+	kiov->used = kiov->i = 0;
+	kiov->consumed = 0;
+	kiov->max_num = num;
+	kiov->iov = kvec;
+}
+
+static inline void vringh_kiov_reset(struct vringh_kiov *kiov)
+{
+	kiov->iov[kiov->i].iov_len += kiov->consumed;
+	kiov->iov[kiov->i].iov_base -= kiov->consumed;
+	kiov->consumed = 0;
+	kiov->i = 0;
+}
+
+static inline void vringh_kiov_cleanup(struct vringh_kiov *kiov)
+{
+	if (kiov->max_num & VRINGH_IOV_ALLOCATED)
+		kfree(kiov->iov);
+	kiov->max_num = kiov->used = kiov->i = kiov->consumed = 0;
+	kiov->iov = NULL;
+}
+
+int vringh_getdesc_kern(struct vringh *vrh,
+			struct vringh_kiov *riov,
+			struct vringh_kiov *wiov,
+			u16 *head,
+			gfp_t gfp);
+
+ssize_t vringh_iov_pull_kern(struct vringh_kiov *riov, void *dst, size_t len);
+ssize_t vringh_iov_push_kern(struct vringh_kiov *wiov,
+			     const void *src, size_t len);
+void vringh_abandon_kern(struct vringh *vrh, unsigned int num);
+int vringh_complete_kern(struct vringh *vrh, u16 head, u32 len);
+
+bool vringh_notify_enable_kern(struct vringh *vrh);
+void vringh_notify_disable_kern(struct vringh *vrh);
+
+int vringh_need_notify_kern(struct vringh *vrh);
+
+#endif /* _LINUX_VRINGH_H */