Virtio helper routines for a descriptor ringbuffer implementation

These helper routines supply most of the virtqueue_ops for hypervisors
which want to use a ring for virtio.  Unlike the previous lguest
implementation:

1) The rings are variable sized (2^n-1 elements).
2) They have an unfortunate limit of 65535 bytes per sg element.
3) The page numbers are always 64 bit (PAE anyone?)
4) They no longer place used[] on a separate page, just a separate
   cacheline.
5) We do a modulo on a variable.  We could be tricky if we cared.
6) Interrupts and notifies are suppressed using flags within the rings.

Users need only get the ring pages and provide a notify hook (KVM
wants the guest to allocate the rings, lguest does it sanely).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Dor Laor <dor.laor@qumranet.com>

5 files changed, 439 insertions, 0 deletions

diff --git a/arch/x86/lguest/Kconfig b/arch/x86/lguest/Kconfig
index 0fabf87db998..44dccfd845f8 100644
--- a/arch/x86/lguest/Kconfig
+++ b/arch/x86/lguest/Kconfig
@@ -2,6 +2,7 @@ config LGUEST_GUEST
         bool "Lguest guest support"
         select PARAVIRT
         depends on !X86_PAE
+        select VIRTIO_RING
         help
           Lguest is a tiny in-kernel hypervisor.  Selecting this will
           allow your kernel to boot under lguest.  This option will increase
diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
index bce84b56a659..9e33fc4da875 100644
--- a/drivers/virtio/Kconfig
+++ b/drivers/virtio/Kconfig
@@ -1,3 +1,8 @@
 # Virtio always gets selected by whoever wants it.
 config VIRTIO
         bool
+
+# Similarly the virtio ring implementation.
+config VIRTIO_RING
+        bool
+        depends on VIRTIO
diff --git a/drivers/virtio/Makefile b/drivers/virtio/Makefile
index af0d57dad3eb..f70e40971dd9 100644
--- a/drivers/virtio/Makefile
+++ b/drivers/virtio/Makefile
@@ -1 +1,2 @@
 obj-$(CONFIG_VIRTIO) += virtio.o
+obj-$(CONFIG_VIRTIO_RING) += virtio_ring.o
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
new file mode 100644
index 000000000000..0e4baca21b8f
--- /dev/null
+++ b/drivers/virtio/virtio_ring.c
@@ -0,0 +1,313 @@
+/* Virtio ring implementation.
+ *
+ *  Copyright 2007 Rusty Russell IBM Corporation
+ *
+ *  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., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+#include <linux/virtio.h>
+#include <linux/virtio_ring.h>
+#include <linux/device.h>
+
+#ifdef DEBUG
+/* For development, we want to crash whenever the ring is screwed. */
+#define BAD_RING(vq, fmt...)                    \
+        do { dev_err(&vq->vq.vdev->dev, fmt); BUG(); } while(0)
+#define START_USE(vq) \
+        do { if ((vq)->in_use) panic("in_use = %i\n", (vq)->in_use); (vq)->in_use = __LINE__; mb(); } while(0)
+#define END_USE(vq) \
+        do { BUG_ON(!(vq)->in_use); (vq)->in_use = 0; mb(); } while(0)
+#else
+#define BAD_RING(vq, fmt...)                    \
+        do { dev_err(&vq->vq.vdev->dev, fmt); (vq)->broken = true; } while(0)
+#define START_USE(vq)
+#define END_USE(vq)
+#endif
+
+struct vring_virtqueue
+{
+        struct virtqueue vq;
+
+        /* Actual memory layout for this queue */
+        struct vring vring;
+
+        /* Other side has made a mess, don't try any more. */
+        bool broken;
+
+        /* Number of free buffers */
+        unsigned int num_free;
+        /* Head of free buffer list. */
+        unsigned int free_head;
+        /* Number we've added since last sync. */
+        unsigned int num_added;
+
+        /* Last used index we've seen. */
+        unsigned int last_used_idx;
+
+        /* How to notify other side. FIXME: commonalize hcalls! */
+        void (*notify)(struct virtqueue *vq);
+
+#ifdef DEBUG
+        /* They're supposed to lock for us. */
+        unsigned int in_use;
+#endif
+
+        /* Tokens for callbacks. */
+        void *data[];
+};
+
+#define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
+
+static int vring_add_buf(struct virtqueue *_vq,
+                         struct scatterlist sg[],
+                         unsigned int out,
+                         unsigned int in,
+                         void *data)
+{
+        struct vring_virtqueue *vq = to_vvq(_vq);
+        unsigned int i, avail, head, uninitialized_var(prev);
+
+        BUG_ON(data == NULL);
+        BUG_ON(out + in > vq->vring.num);
+        BUG_ON(out + in == 0);
+
+        START_USE(vq);
+
+        if (vq->num_free < out + in) {
+                pr_debug("Can't add buf len %i - avail = %i\n",
+                         out + in, vq->num_free);
+                END_USE(vq);
+                return -ENOSPC;
+        }
+
+        /* We're about to use some buffers from the free list. */
+        vq->num_free -= out + in;
+
+        head = vq->free_head;
+        for (i = vq->free_head; out; i = vq->vring.desc[i].next, out--) {
+                vq->vring.desc[i].flags = VRING_DESC_F_NEXT;
+                vq->vring.desc[i].addr = (page_to_pfn(sg_page(sg))<<PAGE_SHIFT)
+                        + sg->offset;
+                vq->vring.desc[i].len = sg->length;
+                prev = i;
+                sg++;
+        }
+        for (; in; i = vq->vring.desc[i].next, in--) {
+                vq->vring.desc[i].flags = VRING_DESC_F_NEXT|VRING_DESC_F_WRITE;
+                vq->vring.desc[i].addr = (page_to_pfn(sg_page(sg))<<PAGE_SHIFT)
+                        + sg->offset;
+                vq->vring.desc[i].len = sg->length;
+                prev = i;
+                sg++;
+        }
+        /* Last one doesn't continue. */
+        vq->vring.desc[prev].flags &= ~VRING_DESC_F_NEXT;
+
+        /* Update free pointer */
+        vq->free_head = i;
+
+        /* Set token. */
+        vq->data[head] = data;
+
+        /* Put entry in available array (but don't update avail->idx until they
+         * do sync).  FIXME: avoid modulus here? */
+        avail = (vq->vring.avail->idx + vq->num_added++) % vq->vring.num;
+        vq->vring.avail->ring[avail] = head;
+
+        pr_debug("Added buffer head %i to %p\n", head, vq);
+        END_USE(vq);
+        return 0;
+}
+
+static void vring_kick(struct virtqueue *_vq)
+{
+        struct vring_virtqueue *vq = to_vvq(_vq);
+        START_USE(vq);
+        /* Descriptors and available array need to be set before we expose the
+         * new available array entries. */
+        wmb();
+
+        vq->vring.avail->idx += vq->num_added;
+        vq->num_added = 0;
+
+        /* Need to update avail index before checking if we should notify */
+        mb();
+
+        if (!(vq->vring.used->flags & VRING_USED_F_NO_NOTIFY))
+                /* Prod other side to tell it about changes. */
+                vq->notify(&vq->vq);
+
+        END_USE(vq);
+}
+
+static void detach_buf(struct vring_virtqueue *vq, unsigned int head)
+{
+        unsigned int i;
+
+        /* Clear data ptr. */
+        vq->data[head] = NULL;
+
+        /* Put back on free list: find end */
+        i = head;
+        while (vq->vring.desc[i].flags & VRING_DESC_F_NEXT) {
+                i = vq->vring.desc[i].next;
+                vq->num_free++;
+        }
+
+        vq->vring.desc[i].next = vq->free_head;
+        vq->free_head = head;
+        /* Plus final descriptor */
+        vq->num_free++;
+}
+
+/* FIXME: We need to tell other side about removal, to synchronize. */
+static void vring_shutdown(struct virtqueue *_vq)
+{
+        struct vring_virtqueue *vq = to_vvq(_vq);
+        unsigned int i;
+
+        for (i = 0; i < vq->vring.num; i++)
+                detach_buf(vq, i);
+}
+
+static inline bool more_used(const struct vring_virtqueue *vq)
+{
+        return vq->last_used_idx != vq->vring.used->idx;
+}
+
+static void *vring_get_buf(struct virtqueue *_vq, unsigned int *len)
+{
+        struct vring_virtqueue *vq = to_vvq(_vq);
+        void *ret;
+        unsigned int i;
+
+        START_USE(vq);
+
+        if (!more_used(vq)) {
+                pr_debug("No more buffers in queue\n");
+                END_USE(vq);
+                return NULL;
+        }
+
+        i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id;
+        *len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len;
+
+        if (unlikely(i >= vq->vring.num)) {
+                BAD_RING(vq, "id %u out of range\n", i);
+                return NULL;
+        }
+        if (unlikely(!vq->data[i])) {
+                BAD_RING(vq, "id %u is not a head!\n", i);
+                return NULL;
+        }
+
+        /* detach_buf clears data, so grab it now. */
+        ret = vq->data[i];
+        detach_buf(vq, i);
+        vq->last_used_idx++;
+        END_USE(vq);
+        return ret;
+}
+
+static bool vring_restart(struct virtqueue *_vq)
+{
+        struct vring_virtqueue *vq = to_vvq(_vq);
+
+        START_USE(vq);
+        BUG_ON(!(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT));
+
+        /* We optimistically turn back on interrupts, then check if there was
+         * more to do. */
+        vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
+        mb();
+        if (unlikely(more_used(vq))) {
+                vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
+                END_USE(vq);
+                return false;
+        }
+
+        END_USE(vq);
+        return true;
+}
+
+irqreturn_t vring_interrupt(int irq, void *_vq)
+{
+        struct vring_virtqueue *vq = to_vvq(_vq);
+
+        if (!more_used(vq)) {
+                pr_debug("virtqueue interrupt with no work for %p\n", vq);
+                return IRQ_NONE;
+        }
+
+        if (unlikely(vq->broken))
+                return IRQ_HANDLED;
+
+        pr_debug("virtqueue callback for %p (%p)\n", vq, vq->vq.callback);
+        if (vq->vq.callback && !vq->vq.callback(&vq->vq))
+                vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
+
+        return IRQ_HANDLED;
+}
+
+static struct virtqueue_ops vring_vq_ops = {
+        .add_buf = vring_add_buf,
+        .get_buf = vring_get_buf,
+        .kick = vring_kick,
+        .restart = vring_restart,
+        .shutdown = vring_shutdown,
+};
+
+struct virtqueue *vring_new_virtqueue(unsigned int num,
+                                      struct virtio_device *vdev,
+                                      void *pages,
+                                      void (*notify)(struct virtqueue *),
+                                      bool (*callback)(struct virtqueue *))
+{
+        struct vring_virtqueue *vq;
+        unsigned int i;
+
+        vq = kmalloc(sizeof(*vq) + sizeof(void *)*num, GFP_KERNEL);
+        if (!vq)
+                return NULL;
+
+        vring_init(&vq->vring, num, pages);
+        vq->vq.callback = callback;
+        vq->vq.vdev = vdev;
+        vq->vq.vq_ops = &vring_vq_ops;
+        vq->notify = notify;
+        vq->broken = false;
+        vq->last_used_idx = 0;
+        vq->num_added = 0;
+#ifdef DEBUG
+        vq->in_use = false;
+#endif
+
+        /* No callback?  Tell other side not to bother us. */
+        if (!callback)
+                vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
+
+        /* Put everything in free lists. */
+        vq->num_free = num;
+        vq->free_head = 0;
+        for (i = 0; i < num-1; i++)
+                vq->vring.desc[i].next = i+1;
+
+        return &vq->vq;
+}
+
+void vring_del_virtqueue(struct virtqueue *vq)
+{
+        kfree(to_vvq(vq));
+}
+
diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
new file mode 100644
index 000000000000..ac69e7bb5a14
--- /dev/null
+++ b/include/linux/virtio_ring.h
@@ -0,0 +1,119 @@
+#ifndef _LINUX_VIRTIO_RING_H
+#define _LINUX_VIRTIO_RING_H
+/* An interface for efficient virtio implementation, currently for use by KVM
+ * and lguest, but hopefully others soon.  Do NOT change this since it will
+ * break existing servers and clients.
+ *
+ * This header is BSD licensed so anyone can use the definitions to implement
+ * compatible drivers/servers.
+ *
+ * Copyright Rusty Russell IBM Corporation 2007. */
+#include <linux/types.h>
+
+/* This marks a buffer as continuing via the next field. */
+#define VRING_DESC_F_NEXT       1
+/* This marks a buffer as write-only (otherwise read-only). */
+#define VRING_DESC_F_WRITE      2
+
+/* This means don't notify other side when buffer added. */
+#define VRING_USED_F_NO_NOTIFY  1
+/* This means don't interrupt guest when buffer consumed. */
+#define VRING_AVAIL_F_NO_INTERRUPT      1
+
+/* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */
+struct vring_desc
+{
+        /* Address (guest-physical). */
+        __u64 addr;
+        /* Length. */
+        __u32 len;
+        /* The flags as indicated above. */
+        __u16 flags;
+        /* We chain unused descriptors via this, too */
+        __u16 next;
+};
+
+struct vring_avail
+{
+        __u16 flags;
+        __u16 idx;
+        __u16 ring[];
+};
+
+/* u32 is used here for ids for padding reasons. */
+struct vring_used_elem
+{
+        /* Index of start of used descriptor chain. */
+        __u32 id;
+        /* Total length of the descriptor chain which was used (written to) */
+        __u32 len;
+};
+
+struct vring_used
+{
+        __u16 flags;
+        __u16 idx;
+        struct vring_used_elem ring[];
+};
+
+struct vring {
+        unsigned int num;
+
+        struct vring_desc *desc;
+
+        struct vring_avail *avail;
+
+        struct vring_used *used;
+};
+
+/* The standard layout for the ring is a continuous chunk of memory which looks
+ * like this.  The used fields will be aligned to a "num+1" boundary.
+ *
+ * struct vring
+ * {
+ *      // The actual descriptors (16 bytes each)
+ *      struct vring_desc desc[num];
+ *
+ *      // A ring of available descriptor heads with free-running index.
+ *      __u16 avail_flags;
+ *      __u16 avail_idx;
+ *      __u16 available[num];
+ *
+ *      // Padding so a correctly-chosen num value will cache-align used_idx.
+ *      char pad[sizeof(struct vring_desc) - sizeof(avail_flags)];
+ *
+ *      // A ring of used descriptor heads with free-running index.
+ *      __u16 used_flags;
+ *      __u16 used_idx;
+ *      struct vring_used_elem used[num];
+ * };
+ */
+static inline void vring_init(struct vring *vr, unsigned int num, void *p)
+{
+        vr->num = num;
+        vr->desc = p;
+        vr->avail = p + num*sizeof(struct vring);
+        vr->used = p + (num+1)*(sizeof(struct vring) + sizeof(__u16));
+}
+
+static inline unsigned vring_size(unsigned int num)
+{
+        return (num + 1) * (sizeof(struct vring_desc) + sizeof(__u16))
+                + sizeof(__u32) + num * sizeof(struct vring_used_elem);
+}
+
+#ifdef __KERNEL__
+#include <linux/irqreturn.h>
+struct virtio_device;
+struct virtqueue;
+
+struct virtqueue *vring_new_virtqueue(unsigned int num,
+                                      struct virtio_device *vdev,
+                                      void *pages,
+                                      void (*notify)(struct virtqueue *vq),
+                                      bool (*callback)(struct virtqueue *vq));
+void vring_del_virtqueue(struct virtqueue *vq);
+
+irqreturn_t vring_interrupt(int irq, void *_vq);
+#endif /* __KERNEL__ */
+#endif /* _LINUX_VIRTIO_RING_H */