Patchwork [v6,04/12] dataplane: add virtqueue vring code

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Submitter Stefan Hajnoczi
Date Dec. 10, 2012, 1:09 p.m.
Message ID <1355144985-12897-5-git-send-email-stefanha@redhat.com>
Download mbox | patch
Permalink /patch/204882/
State New
Headers show

Comments

Stefan Hajnoczi - Dec. 10, 2012, 1:09 p.m.
The virtio-blk-data-plane cannot access memory using the usual QEMU
functions since it executes outside the global mutex and the memory APIs
are this time are not thread-safe.

This patch introduces a virtqueue module based on the kernel's vhost
vring code.  The trick is that we map guest memory ahead of time and
access it cheaply outside the global mutex.

Once the hardware emulation code can execute outside the global mutex it
will be possible to drop this code.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 hw/dataplane/Makefile.objs |   2 +-
 hw/dataplane/vring.c       | 361 +++++++++++++++++++++++++++++++++++++++++++++
 hw/dataplane/vring.h       |  63 ++++++++
 trace-events               |   3 +
 4 files changed, 428 insertions(+), 1 deletion(-)
 create mode 100644 hw/dataplane/vring.c
 create mode 100644 hw/dataplane/vring.h
Michael S. Tsirkin - Dec. 11, 2012, 2:18 p.m.
On Mon, Dec 10, 2012 at 02:09:37PM +0100, Stefan Hajnoczi wrote:
> +        iov->iov_base = hostmem_lookup(&vring->hostmem, desc.addr, desc.len,
> +                                       desc.flags & VRING_DESC_F_WRITE);
> +        if (!iov->iov_base) {
> +            error_report("Failed to map indirect descriptor"
> +                         "addr %#" PRIx64 " len %u",
> +                         (uint64_t)desc.addr, desc.len);
> +            vring->broken = true;
> +            return -EFAULT;
> +        }
> +        iov->iov_len = desc.len;
> +        iov++;

Hmm, this assumes a descriptor can not cross a memory
region boundary. Is this really guaranteed?
vhost does not make such assumptions.
Stefan Hajnoczi - Dec. 12, 2012, 3:55 p.m.
On Tue, Dec 11, 2012 at 04:18:20PM +0200, Michael S. Tsirkin wrote:
> On Mon, Dec 10, 2012 at 02:09:37PM +0100, Stefan Hajnoczi wrote:
> > +        iov->iov_base = hostmem_lookup(&vring->hostmem, desc.addr, desc.len,
> > +                                       desc.flags & VRING_DESC_F_WRITE);
> > +        if (!iov->iov_base) {
> > +            error_report("Failed to map indirect descriptor"
> > +                         "addr %#" PRIx64 " len %u",
> > +                         (uint64_t)desc.addr, desc.len);
> > +            vring->broken = true;
> > +            return -EFAULT;
> > +        }
> > +        iov->iov_len = desc.len;
> > +        iov++;
> 
> Hmm, this assumes a descriptor can not cross a memory
> region boundary. Is this really guaranteed?
> vhost does not make such assumptions.

Just responded in the other subthread.

hw/virtio.c doesn't split across memory region boundaries.  This suggest
we don't hit this in practice.  If it does we'll print the error and
disable the device until virtio reset.

I can add it to my TODO list.  hw/virtio.c also uses exit(1) for
guest-triggerable errors at runtime and I'd like it to do something like
vring->broken above.

IMO it's not worth trying to perfect these things here.  The next step
after this series should be looking at thread-safe memory API so that we
can use hw/virtio.c and improve it for all users.

Stefan
Michael S. Tsirkin - Dec. 12, 2012, 4:32 p.m.
On Wed, Dec 12, 2012 at 04:55:47PM +0100, Stefan Hajnoczi wrote:
> On Tue, Dec 11, 2012 at 04:18:20PM +0200, Michael S. Tsirkin wrote:
> > On Mon, Dec 10, 2012 at 02:09:37PM +0100, Stefan Hajnoczi wrote:
> > > +        iov->iov_base = hostmem_lookup(&vring->hostmem, desc.addr, desc.len,
> > > +                                       desc.flags & VRING_DESC_F_WRITE);
> > > +        if (!iov->iov_base) {
> > > +            error_report("Failed to map indirect descriptor"
> > > +                         "addr %#" PRIx64 " len %u",
> > > +                         (uint64_t)desc.addr, desc.len);
> > > +            vring->broken = true;
> > > +            return -EFAULT;
> > > +        }
> > > +        iov->iov_len = desc.len;
> > > +        iov++;
> > 
> > Hmm, this assumes a descriptor can not cross a memory
> > region boundary. Is this really guaranteed?
> > vhost does not make such assumptions.
> 
> Just responded in the other subthread.
> 
> hw/virtio.c doesn't split across memory region boundaries.  This suggest
> we don't hit this in practice.  If it does we'll print the error and
> disable the device until virtio reset.
> 
> I can add it to my TODO list.  hw/virtio.c also uses exit(1) for
> guest-triggerable errors at runtime and I'd like it to do something like
> vring->broken above.
> 
> IMO it's not worth trying to perfect these things here.  The next step
> after this series should be looking at thread-safe memory API so that we
> can use hw/virtio.c and improve it for all users.
> 
> Stefan

OK

Patch

diff --git a/hw/dataplane/Makefile.objs b/hw/dataplane/Makefile.objs
index 8c8dea1..34e6d57 100644
--- a/hw/dataplane/Makefile.objs
+++ b/hw/dataplane/Makefile.objs
@@ -1,3 +1,3 @@ 
 ifeq ($(CONFIG_VIRTIO), y)
-common-obj-$(CONFIG_VIRTIO_BLK_DATA_PLANE) += hostmem.o
+common-obj-$(CONFIG_VIRTIO_BLK_DATA_PLANE) += hostmem.o vring.o
 endif
diff --git a/hw/dataplane/vring.c b/hw/dataplane/vring.c
new file mode 100644
index 0000000..8321c70
--- /dev/null
+++ b/hw/dataplane/vring.c
@@ -0,0 +1,361 @@ 
+/* Copyright 2012 Red Hat, Inc.
+ * Copyright IBM, Corp. 2012
+ *
+ * Based on Linux 2.6.39 vhost code:
+ * Copyright (C) 2009 Red Hat, Inc.
+ * Copyright (C) 2006 Rusty Russell IBM Corporation
+ *
+ * Author: Michael S. Tsirkin <mst@redhat.com>
+ *         Stefan Hajnoczi <stefanha@redhat.com>
+ *
+ * Inspiration, some code, and most witty comments come from
+ * Documentation/virtual/lguest/lguest.c, by Rusty Russell
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.
+ */
+
+#include "trace.h"
+#include "hw/dataplane/vring.h"
+
+/* Map the guest's vring to host memory */
+bool vring_setup(Vring *vring, VirtIODevice *vdev, int n)
+{
+    hwaddr vring_addr = virtio_queue_get_ring_addr(vdev, n);
+    hwaddr vring_size = virtio_queue_get_ring_size(vdev, n);
+    void *vring_ptr;
+
+    vring->broken = false;
+
+    hostmem_init(&vring->hostmem);
+    vring_ptr = hostmem_lookup(&vring->hostmem, vring_addr, vring_size, true);
+    if (!vring_ptr) {
+        error_report("Failed to map vring "
+                     "addr %#" HWADDR_PRIx " size %" HWADDR_PRIu,
+                     vring_addr, vring_size);
+        vring->broken = true;
+        return false;
+    }
+
+    vring_init(&vring->vr, virtio_queue_get_num(vdev, n), vring_ptr, 4096);
+
+    vring->last_avail_idx = 0;
+    vring->last_used_idx = 0;
+    vring->signalled_used = 0;
+    vring->signalled_used_valid = false;
+
+    trace_vring_setup(virtio_queue_get_ring_addr(vdev, n),
+                      vring->vr.desc, vring->vr.avail, vring->vr.used);
+    return true;
+}
+
+void vring_teardown(Vring *vring)
+{
+    hostmem_finalize(&vring->hostmem);
+}
+
+/* Disable guest->host notifies */
+void vring_disable_notification(VirtIODevice *vdev, Vring *vring)
+{
+    if (!(vdev->guest_features & (1 << VIRTIO_RING_F_EVENT_IDX))) {
+        vring->vr.used->flags |= VRING_USED_F_NO_NOTIFY;
+    }
+}
+
+/* Enable guest->host notifies
+ *
+ * Return true if the vring is empty, false if there are more requests.
+ */
+bool vring_enable_notification(VirtIODevice *vdev, Vring *vring)
+{
+    if (vdev->guest_features & (1 << VIRTIO_RING_F_EVENT_IDX)) {
+        vring_avail_event(&vring->vr) = vring->vr.avail->idx;
+    } else {
+        vring->vr.used->flags &= ~VRING_USED_F_NO_NOTIFY;
+    }
+    smp_mb(); /* ensure update is seen before reading avail_idx */
+    return !vring_more_avail(vring);
+}
+
+/* This is stolen from linux/drivers/vhost/vhost.c:vhost_notify() */
+bool vring_should_notify(VirtIODevice *vdev, Vring *vring)
+{
+    uint16_t old, new;
+    bool v;
+    /* Flush out used index updates. This is paired
+     * with the barrier that the Guest executes when enabling
+     * interrupts. */
+    smp_mb();
+
+    if ((vdev->guest_features & VIRTIO_F_NOTIFY_ON_EMPTY) &&
+        unlikely(vring->vr.avail->idx == vring->last_avail_idx)) {
+        return true;
+    }
+
+    if (!(vdev->guest_features & VIRTIO_RING_F_EVENT_IDX)) {
+        return !(vring->vr.avail->flags & VRING_AVAIL_F_NO_INTERRUPT);
+    }
+    old = vring->signalled_used;
+    v = vring->signalled_used_valid;
+    new = vring->signalled_used = vring->last_used_idx;
+    vring->signalled_used_valid = true;
+
+    if (unlikely(!v)) {
+        return true;
+    }
+
+    return vring_need_event(vring_used_event(&vring->vr), new, old);
+}
+
+/* This is stolen from linux/drivers/vhost/vhost.c. */
+static int get_indirect(Vring *vring,
+                        struct iovec iov[], struct iovec *iov_end,
+                        unsigned int *out_num, unsigned int *in_num,
+                        struct vring_desc *indirect)
+{
+    struct vring_desc desc;
+    unsigned int i = 0, count, found = 0;
+
+    /* Sanity check */
+    if (unlikely(indirect->len % sizeof(desc))) {
+        error_report("Invalid length in indirect descriptor: "
+                     "len %#x not multiple of %#zx",
+                     indirect->len, sizeof(desc));
+        vring->broken = true;
+        return -EFAULT;
+    }
+
+    count = indirect->len / sizeof(desc);
+    /* Buffers are chained via a 16 bit next field, so
+     * we can have at most 2^16 of these. */
+    if (unlikely(count > USHRT_MAX + 1)) {
+        error_report("Indirect buffer length too big: %d", indirect->len);
+        vring->broken = true;
+        return -EFAULT;
+    }
+
+    do {
+        struct vring_desc *desc_ptr;
+
+        /* Translate indirect descriptor */
+        desc_ptr = hostmem_lookup(&vring->hostmem,
+                                  indirect->addr + found * sizeof(desc),
+                                  sizeof(desc), false);
+        if (!desc_ptr) {
+            error_report("Failed to map indirect descriptor "
+                         "addr %#" PRIx64 " len %zu",
+                         (uint64_t)indirect->addr + found * sizeof(desc),
+                         sizeof(desc));
+            vring->broken = true;
+            return -EFAULT;
+        }
+        desc = *desc_ptr;
+
+        /* Ensure descriptor has been loaded before accessing fields */
+        barrier(); /* read_barrier_depends(); */
+
+        if (unlikely(++found > count)) {
+            error_report("Loop detected: last one at %u "
+                         "indirect size %u", i, count);
+            vring->broken = true;
+            return -EFAULT;
+        }
+
+        if (unlikely(desc.flags & VRING_DESC_F_INDIRECT)) {
+            error_report("Nested indirect descriptor");
+            vring->broken = true;
+            return -EFAULT;
+        }
+
+        /* Stop for now if there are not enough iovecs available. */
+        if (iov >= iov_end) {
+            return -ENOBUFS;
+        }
+
+        iov->iov_base = hostmem_lookup(&vring->hostmem, desc.addr, desc.len,
+                                       desc.flags & VRING_DESC_F_WRITE);
+        if (!iov->iov_base) {
+            error_report("Failed to map indirect descriptor"
+                         "addr %#" PRIx64 " len %u",
+                         (uint64_t)desc.addr, desc.len);
+            vring->broken = true;
+            return -EFAULT;
+        }
+        iov->iov_len = desc.len;
+        iov++;
+
+        /* If this is an input descriptor, increment that count. */
+        if (desc.flags & VRING_DESC_F_WRITE) {
+            *in_num += 1;
+        } else {
+            /* If it's an output descriptor, they're all supposed
+             * to come before any input descriptors. */
+            if (unlikely(*in_num)) {
+                error_report("Indirect descriptor "
+                             "has out after in: idx %u", i);
+                vring->broken = true;
+                return -EFAULT;
+            }
+            *out_num += 1;
+        }
+        i = desc.next;
+    } while (desc.flags & VRING_DESC_F_NEXT);
+    return 0;
+}
+
+/* This looks in the virtqueue and for the first available buffer, and converts
+ * it to an iovec for convenient access.  Since descriptors consist of some
+ * number of output then some number of input descriptors, it's actually two
+ * iovecs, but we pack them into one and note how many of each there were.
+ *
+ * This function returns the descriptor number found, or vq->num (which is
+ * never a valid descriptor number) if none was found.  A negative code is
+ * returned on error.
+ *
+ * Stolen from linux/drivers/vhost/vhost.c.
+ */
+int vring_pop(VirtIODevice *vdev, Vring *vring,
+              struct iovec iov[], struct iovec *iov_end,
+              unsigned int *out_num, unsigned int *in_num)
+{
+    struct vring_desc desc;
+    unsigned int i, head, found = 0, num = vring->vr.num;
+    uint16_t avail_idx, last_avail_idx;
+
+    /* If there was a fatal error then refuse operation */
+    if (vring->broken) {
+        return -EFAULT;
+    }
+
+    /* Check it isn't doing very strange things with descriptor numbers. */
+    last_avail_idx = vring->last_avail_idx;
+    avail_idx = vring->vr.avail->idx;
+    barrier(); /* load indices now and not again later */
+
+    if (unlikely((uint16_t)(avail_idx - last_avail_idx) > num)) {
+        error_report("Guest moved used index from %u to %u",
+                     last_avail_idx, avail_idx);
+        vring->broken = true;
+        return -EFAULT;
+    }
+
+    /* If there's nothing new since last we looked. */
+    if (avail_idx == last_avail_idx) {
+        return -EAGAIN;
+    }
+
+    /* Only get avail ring entries after they have been exposed by guest. */
+    smp_rmb();
+
+    /* Grab the next descriptor number they're advertising, and increment
+     * the index we've seen. */
+    head = vring->vr.avail->ring[last_avail_idx % num];
+
+    /* If their number is silly, that's an error. */
+    if (unlikely(head >= num)) {
+        error_report("Guest says index %u > %u is available", head, num);
+        vring->broken = true;
+        return -EFAULT;
+    }
+
+    if (vdev->guest_features & (1 << VIRTIO_RING_F_EVENT_IDX)) {
+        vring_avail_event(&vring->vr) = vring->vr.avail->idx;
+    }
+
+    /* When we start there are none of either input nor output. */
+    *out_num = *in_num = 0;
+
+    i = head;
+    do {
+        if (unlikely(i >= num)) {
+            error_report("Desc index is %u > %u, head = %u", i, num, head);
+            vring->broken = true;
+            return -EFAULT;
+        }
+        if (unlikely(++found > num)) {
+            error_report("Loop detected: last one at %u vq size %u head %u",
+                         i, num, head);
+            vring->broken = true;
+            return -EFAULT;
+        }
+        desc = vring->vr.desc[i];
+
+        /* Ensure descriptor is loaded before accessing fields */
+        barrier();
+
+        if (desc.flags & VRING_DESC_F_INDIRECT) {
+            int ret = get_indirect(vring, iov, iov_end, out_num, in_num, &desc);
+            if (ret < 0) {
+                return ret;
+            }
+            continue;
+        }
+
+        /* If there are not enough iovecs left, stop for now.  The caller
+         * should check if there are more descs available once they have dealt
+         * with the current set.
+         */
+        if (iov >= iov_end) {
+            return -ENOBUFS;
+        }
+
+        iov->iov_base = hostmem_lookup(&vring->hostmem, desc.addr, desc.len,
+                                       desc.flags & VRING_DESC_F_WRITE);
+        if (!iov->iov_base) {
+            error_report("Failed to map vring desc addr %#" PRIx64 " len %u",
+                         (uint64_t)desc.addr, desc.len);
+            vring->broken = true;
+            return -EFAULT;
+        }
+        iov->iov_len  = desc.len;
+        iov++;
+
+        if (desc.flags & VRING_DESC_F_WRITE) {
+            /* If this is an input descriptor,
+             * increment that count. */
+            *in_num += 1;
+        } else {
+            /* If it's an output descriptor, they're all supposed
+             * to come before any input descriptors. */
+            if (unlikely(*in_num)) {
+                error_report("Descriptor has out after in: idx %d", i);
+                vring->broken = true;
+                return -EFAULT;
+            }
+            *out_num += 1;
+        }
+        i = desc.next;
+    } while (desc.flags & VRING_DESC_F_NEXT);
+
+    /* On success, increment avail index. */
+    vring->last_avail_idx++;
+    return head;
+}
+
+/* After we've used one of their buffers, we tell them about it.
+ *
+ * Stolen from linux/drivers/vhost/vhost.c.
+ */
+void vring_push(Vring *vring, unsigned int head, int len)
+{
+    struct vring_used_elem *used;
+    uint16_t new;
+
+    /* Don't touch vring if a fatal error occurred */
+    if (vring->broken) {
+        return;
+    }
+
+    /* The virtqueue contains a ring of used buffers.  Get a pointer to the
+     * next entry in that used ring. */
+    used = &vring->vr.used->ring[vring->last_used_idx % vring->vr.num];
+    used->id = head;
+    used->len = len;
+
+    /* Make sure buffer is written before we update index. */
+    smp_wmb();
+
+    new = vring->vr.used->idx = ++vring->last_used_idx;
+    if (unlikely((int16_t)(new - vring->signalled_used) < (uint16_t)1)) {
+        vring->signalled_used_valid = false;
+    }
+}
diff --git a/hw/dataplane/vring.h b/hw/dataplane/vring.h
new file mode 100644
index 0000000..1a1164a
--- /dev/null
+++ b/hw/dataplane/vring.h
@@ -0,0 +1,63 @@ 
+/* Copyright 2012 Red Hat, Inc. and/or its affiliates
+ * Copyright IBM, Corp. 2012
+ *
+ * Based on Linux 2.6.39 vhost code:
+ * Copyright (C) 2009 Red Hat, Inc.
+ * Copyright (C) 2006 Rusty Russell IBM Corporation
+ *
+ * Author: Michael S. Tsirkin <mst@redhat.com>
+ *         Stefan Hajnoczi <stefanha@redhat.com>
+ *
+ * Inspiration, some code, and most witty comments come from
+ * Documentation/virtual/lguest/lguest.c, by Rusty Russell
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.
+ */
+
+#ifndef VRING_H
+#define VRING_H
+
+#include <linux/virtio_ring.h>
+#include "qemu-common.h"
+#include "qemu-barrier.h"
+#include "hw/dataplane/hostmem.h"
+#include "hw/virtio.h"
+
+typedef struct {
+    Hostmem hostmem;                /* guest memory mapper */
+    struct vring vr;                /* virtqueue vring mapped to host memory */
+    uint16_t last_avail_idx;        /* last processed avail ring index */
+    uint16_t last_used_idx;         /* last processed used ring index */
+    uint16_t signalled_used;        /* EVENT_IDX state */
+    bool signalled_used_valid;
+    bool broken;                    /* was there a fatal error? */
+} Vring;
+
+static inline unsigned int vring_get_num(Vring *vring)
+{
+    return vring->vr.num;
+}
+
+/* Are there more descriptors available? */
+static inline bool vring_more_avail(Vring *vring)
+{
+    return vring->vr.avail->idx != vring->last_avail_idx;
+}
+
+/* Fail future vring_pop() and vring_push() calls until reset */
+static inline void vring_set_broken(Vring *vring)
+{
+    vring->broken = true;
+}
+
+bool vring_setup(Vring *vring, VirtIODevice *vdev, int n);
+void vring_teardown(Vring *vring);
+void vring_disable_notification(VirtIODevice *vdev, Vring *vring);
+bool vring_enable_notification(VirtIODevice *vdev, Vring *vring);
+bool vring_should_notify(VirtIODevice *vdev, Vring *vring);
+int vring_pop(VirtIODevice *vdev, Vring *vring,
+              struct iovec iov[], struct iovec *iov_end,
+              unsigned int *out_num, unsigned int *in_num);
+void vring_push(Vring *vring, unsigned int head, int len);
+
+#endif /* VRING_H */
diff --git a/trace-events b/trace-events
index 6c6cbf1..a9a791b 100644
--- a/trace-events
+++ b/trace-events
@@ -98,6 +98,9 @@  virtio_blk_rw_complete(void *req, int ret) "req %p ret %d"
 virtio_blk_handle_write(void *req, uint64_t sector, size_t nsectors) "req %p sector %"PRIu64" nsectors %zu"
 virtio_blk_handle_read(void *req, uint64_t sector, size_t nsectors) "req %p sector %"PRIu64" nsectors %zu"
 
+# hw/dataplane/vring.c
+vring_setup(uint64_t physical, void *desc, void *avail, void *used) "vring physical %#"PRIx64" desc %p avail %p used %p"
+
 # thread-pool.c
 thread_pool_submit(void *req, void *opaque) "req %p opaque %p"
 thread_pool_complete(void *req, void *opaque, int ret) "req %p opaque %p ret %d"