Commit 0a8a69dd authored by Rusty Russell's avatar Rusty Russell
Browse files

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: default avatarRusty Russell <rusty@rustcorp.com.au>
Cc: Dor Laor <dor.laor@qumranet.com>
parent b01d9f28

arch/x86/lguest/Kconfig

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Original line number Diff line number Diff line
@@ -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

drivers/virtio/Kconfig

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# Virtio always gets selected by whoever wants it.

config VIRTIO

	bool



# Similarly the virtio ring implementation.

config VIRTIO_RING

	bool

	depends on VIRTIO

drivers/virtio/Makefile

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Original line number Diff line number Diff line 
obj-$(CONFIG_VIRTIO) += virtio.o

obj-$(CONFIG_VIRTIO_RING) += virtio_ring.o

drivers/virtio/virtio_ring.c

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Original line number Diff line number Diff line 
/* 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));

}

include/linux/virtio_ring.h

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Original line number Diff line number Diff line 
#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 */

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