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Release 4.11 drivers/scsi/vmw_pvscsi.c

Directory: drivers/scsi
/*
 * Linux driver for VMware's para-virtualized SCSI HBA.
 *
 * Copyright (C) 2008-2014, VMware, Inc. All Rights Reserved.
 *
 * 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; version 2 of the License and no 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, GOOD TITLE or
 * NON INFRINGEMENT.  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.
 *
 * Maintained by: Jim Gill <jgill@vmware.com>
 *
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/interrupt.h>
#include <linux/slab.h>
#include <linux/workqueue.h>
#include <linux/pci.h>

#include <scsi/scsi.h>
#include <scsi/scsi_host.h>
#include <scsi/scsi_cmnd.h>
#include <scsi/scsi_device.h>
#include <scsi/scsi_tcq.h>

#include "vmw_pvscsi.h"


#define PVSCSI_LINUX_DRIVER_DESC "VMware PVSCSI driver"


MODULE_DESCRIPTION(PVSCSI_LINUX_DRIVER_DESC);
MODULE_AUTHOR("VMware, Inc.");
MODULE_LICENSE("GPL");

MODULE_VERSION(PVSCSI_DRIVER_VERSION_STRING);


#define PVSCSI_DEFAULT_NUM_PAGES_PER_RING	8

#define PVSCSI_DEFAULT_NUM_PAGES_MSG_RING	1

#define PVSCSI_DEFAULT_QUEUE_DEPTH		254

#define SGL_SIZE				PAGE_SIZE


struct pvscsi_sg_list {
	
struct PVSCSISGElement sge[PVSCSI_MAX_NUM_SG_ENTRIES_PER_SEGMENT];
};


struct pvscsi_ctx {
	/*
         * The index of the context in cmd_map serves as the context ID for a
         * 1-to-1 mapping completions back to requests.
         */
	
struct scsi_cmnd	*cmd;
	
struct pvscsi_sg_list	*sgl;
	
struct list_head	list;
	
dma_addr_t		dataPA;
	
dma_addr_t		sensePA;
	
dma_addr_t		sglPA;
	
struct completion	*abort_cmp;
};


struct pvscsi_adapter {
	
char				*mmioBase;
	
u8				rev;
	
bool				use_msg;
	
bool				use_req_threshold;

	
spinlock_t			hw_lock;

	
struct workqueue_struct		*workqueue;
	
struct work_struct		work;

	
struct PVSCSIRingReqDesc	*req_ring;
	
unsigned			req_pages;
	
unsigned			req_depth;
	
dma_addr_t			reqRingPA;

	
struct PVSCSIRingCmpDesc	*cmp_ring;
	
unsigned			cmp_pages;
	
dma_addr_t			cmpRingPA;

	
struct PVSCSIRingMsgDesc	*msg_ring;
	
unsigned			msg_pages;
	
dma_addr_t			msgRingPA;

	
struct PVSCSIRingsState		*rings_state;
	
dma_addr_t			ringStatePA;

	
struct pci_dev			*dev;
	
struct Scsi_Host		*host;

	
struct list_head		cmd_pool;
	
struct pvscsi_ctx		*cmd_map;
};


/* Command line parameters */

static int pvscsi_ring_pages;

static int pvscsi_msg_ring_pages = PVSCSI_DEFAULT_NUM_PAGES_MSG_RING;

static int pvscsi_cmd_per_lun    = PVSCSI_DEFAULT_QUEUE_DEPTH;

static bool pvscsi_disable_msi;

static bool pvscsi_disable_msix;

static bool pvscsi_use_msg       = true;

static bool pvscsi_use_req_threshold = true;


#define PVSCSI_RW (S_IRUSR | S_IWUSR)

module_param_named(ring_pages, pvscsi_ring_pages, int, PVSCSI_RW);
MODULE_PARM_DESC(ring_pages, "Number of pages per req/cmp ring - (default="
		 __stringify(PVSCSI_DEFAULT_NUM_PAGES_PER_RING)
		 "[up to 16 targets],"
		 __stringify(PVSCSI_SETUP_RINGS_MAX_NUM_PAGES)
		 "[for 16+ targets])");

module_param_named(msg_ring_pages, pvscsi_msg_ring_pages, int, PVSCSI_RW);
MODULE_PARM_DESC(msg_ring_pages, "Number of pages for the msg ring - (default="
		 __stringify(PVSCSI_DEFAULT_NUM_PAGES_MSG_RING) ")");

module_param_named(cmd_per_lun, pvscsi_cmd_per_lun, int, PVSCSI_RW);
MODULE_PARM_DESC(cmd_per_lun, "Maximum commands per lun - (default="
		 __stringify(PVSCSI_DEFAULT_QUEUE_DEPTH) ")");

module_param_named(disable_msi, pvscsi_disable_msi, bool, PVSCSI_RW);
MODULE_PARM_DESC(disable_msi, "Disable MSI use in driver - (default=0)");

module_param_named(disable_msix, pvscsi_disable_msix, bool, PVSCSI_RW);
MODULE_PARM_DESC(disable_msix, "Disable MSI-X use in driver - (default=0)");

module_param_named(use_msg, pvscsi_use_msg, bool, PVSCSI_RW);
MODULE_PARM_DESC(use_msg, "Use msg ring when available - (default=1)");

module_param_named(use_req_threshold, pvscsi_use_req_threshold,
		   bool, PVSCSI_RW);
MODULE_PARM_DESC(use_req_threshold, "Use driver-based request coalescing if configured - (default=1)");


static const struct pci_device_id pvscsi_pci_tbl[] = {
	{ PCI_VDEVICE(VMWARE, PCI_DEVICE_ID_VMWARE_PVSCSI) },
	{ 0 }
};

MODULE_DEVICE_TABLE(pci, pvscsi_pci_tbl);



static struct device *

pvscsi_dev(const struct pvscsi_adapter *adapter)
{
	return &(adapter->dev->dev);
}


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static struct pvscsi_ctx *

pvscsi_find_context(const struct pvscsi_adapter *adapter, struct scsi_cmnd *cmd)
{
	struct pvscsi_ctx *ctx, *end;

	end = &adapter->cmd_map[adapter->req_depth];
	for (ctx = adapter->cmd_map; ctx < end; ctx++)
		if (ctx->cmd == cmd)
			return ctx;

	return NULL;
}


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static struct pvscsi_ctx *

pvscsi_acquire_context(struct pvscsi_adapter *adapter, struct scsi_cmnd *cmd)
{
	struct pvscsi_ctx *ctx;

	if (list_empty(&adapter->cmd_pool))
		return NULL;

	ctx = list_first_entry(&adapter->cmd_pool, struct pvscsi_ctx, list);
	ctx->cmd = cmd;
	list_del(&ctx->list);

	return ctx;
}


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static void pvscsi_release_context(struct pvscsi_adapter *adapter,
				   struct pvscsi_ctx *ctx)
{
	ctx->cmd = NULL;
	ctx->abort_cmp = NULL;
	list_add(&ctx->list, &adapter->cmd_pool);
}


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/*
 * Map a pvscsi_ctx struct to a context ID field value; we map to a simple
 * non-zero integer. ctx always points to an entry in cmd_map array, hence
 * the return value is always >=1.
 */



static u64 pvscsi_map_context(const struct pvscsi_adapter *adapter,
			      const struct pvscsi_ctx *ctx)
{
	return ctx - adapter->cmd_map + 1;
}


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static struct pvscsi_ctx *

pvscsi_get_context(const struct pvscsi_adapter *adapter, u64 context)
{
	return &adapter->cmd_map[context - 1];
}


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static void pvscsi_reg_write(const struct pvscsi_adapter *adapter,
			     u32 offset, u32 val)
{
	writel(val, adapter->mmioBase + offset);
}


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static u32 pvscsi_reg_read(const struct pvscsi_adapter *adapter, u32 offset)
{
	return readl(adapter->mmioBase + offset);
}


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static u32 pvscsi_read_intr_status(const struct pvscsi_adapter *adapter)
{
	return pvscsi_reg_read(adapter, PVSCSI_REG_OFFSET_INTR_STATUS);
}


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static void pvscsi_write_intr_status(const struct pvscsi_adapter *adapter,
				     u32 val)
{
	pvscsi_reg_write(adapter, PVSCSI_REG_OFFSET_INTR_STATUS, val);
}


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static void pvscsi_unmask_intr(const struct pvscsi_adapter *adapter)
{
	u32 intr_bits;

	intr_bits = PVSCSI_INTR_CMPL_MASK;
	if (adapter->use_msg)
		intr_bits |= PVSCSI_INTR_MSG_MASK;

	pvscsi_reg_write(adapter, PVSCSI_REG_OFFSET_INTR_MASK, intr_bits);
}


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static void pvscsi_mask_intr(const struct pvscsi_adapter *adapter)
{
	pvscsi_reg_write(adapter, PVSCSI_REG_OFFSET_INTR_MASK, 0);
}


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static void pvscsi_write_cmd_desc(const struct pvscsi_adapter *adapter,
				  u32 cmd, const void *desc, size_t len)
{
	const u32 *ptr = desc;
	size_t i;

	len /= sizeof(*ptr);
	pvscsi_reg_write(adapter, PVSCSI_REG_OFFSET_COMMAND, cmd);
	for (i = 0; i < len; i++)
		pvscsi_reg_write(adapter,
				 PVSCSI_REG_OFFSET_COMMAND_DATA, ptr[i]);
}


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static void pvscsi_abort_cmd(const struct pvscsi_adapter *adapter,
			     const struct pvscsi_ctx *ctx)
{
	struct PVSCSICmdDescAbortCmd cmd = { 0 };

	cmd.target = ctx->cmd->device->id;
	cmd.context = pvscsi_map_context(adapter, ctx);

	pvscsi_write_cmd_desc(adapter, PVSCSI_CMD_ABORT_CMD, &cmd, sizeof(cmd));
}


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static void pvscsi_kick_rw_io(const struct pvscsi_adapter *adapter)
{
	pvscsi_reg_write(adapter, PVSCSI_REG_OFFSET_KICK_RW_IO, 0);
}


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static void pvscsi_process_request_ring(const struct pvscsi_adapter *adapter)
{
	pvscsi_reg_write(adapter, PVSCSI_REG_OFFSET_KICK_NON_RW_IO, 0);
}


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static int scsi_is_rw(unsigned char op)
{
	return op == READ_6  || op == WRITE_6 ||
	       op == READ_10 || op == WRITE_10 ||
	       op == READ_12 || op == WRITE_12 ||
	       op == READ_16 || op == WRITE_16;
}


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static void pvscsi_kick_io(const struct pvscsi_adapter *adapter,
			   unsigned char op)
{
	if (scsi_is_rw(op)) {
		struct PVSCSIRingsState *s = adapter->rings_state;

		if (!adapter->use_req_threshold ||
		    s->reqProdIdx - s->reqConsIdx >= s->reqCallThreshold)
			pvscsi_kick_rw_io(adapter);
	} else {
		pvscsi_process_request_ring(adapter);
	}
}


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static void ll_adapter_reset(const struct pvscsi_adapter *adapter)
{
	dev_dbg(pvscsi_dev(adapter), "Adapter Reset on %p\n", adapter);

	pvscsi_write_cmd_desc(adapter, PVSCSI_CMD_ADAPTER_RESET, NULL, 0);
}


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static void ll_bus_reset(const struct pvscsi_adapter *adapter)
{
	dev_dbg(pvscsi_dev(adapter), "Resetting bus on %p\n", adapter);

	pvscsi_write_cmd_desc(adapter, PVSCSI_CMD_RESET_BUS, NULL, 0);
}


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static void ll_device_reset(const struct pvscsi_adapter *adapter, u32 target)
{
	struct PVSCSICmdDescResetDevice cmd = { 0 };

	dev_dbg(pvscsi_dev(adapter), "Resetting device: target=%u\n", target);

	cmd.target = target;

	pvscsi_write_cmd_desc(adapter, PVSCSI_CMD_RESET_DEVICE,
			      &cmd, sizeof(cmd));
}


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static void pvscsi_create_sg(struct pvscsi_ctx *ctx,
			     struct scatterlist *sg, unsigned count)
{
	unsigned i;
	struct PVSCSISGElement *sge;

	BUG_ON(count > PVSCSI_MAX_NUM_SG_ENTRIES_PER_SEGMENT);

	sge = &ctx->sgl->sge[0];
	for (i = 0; i < count; i++, sg++) {
		sge[i].addr   = sg_dma_address(sg);
		sge[i].length = sg_dma_len(sg);
		sge[i].flags  = 0;
	}
}


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/*
 * Map all data buffers for a command into PCI space and
 * setup the scatter/gather list if needed.
 */



static int pvscsi_map_buffers(struct pvscsi_adapter *adapter,
			      struct pvscsi_ctx *ctx, struct scsi_cmnd *cmd,
			      struct PVSCSIRingReqDesc *e)
{
	unsigned count;
	unsigned bufflen = scsi_bufflen(cmd);
	struct scatterlist *sg;

	e->dataLen = bufflen;
	e->dataAddr = 0;
	if (bufflen == 0)
		return 0;

	sg = scsi_sglist(cmd);
	count = scsi_sg_count(cmd);
	if (count != 0) {
		int segs = scsi_dma_map(cmd);

		if (segs == -ENOMEM) {
			scmd_printk(KERN_ERR, cmd,
				    "vmw_pvscsi: Failed to map cmd sglist for DMA.\n");
			return -ENOMEM;
		} else if (segs > 1) {
			pvscsi_create_sg(ctx, sg, segs);

			e->flags |= PVSCSI_FLAG_CMD_WITH_SG_LIST;
			ctx->sglPA = pci_map_single(adapter->dev, ctx->sgl,
						    SGL_SIZE, PCI_DMA_TODEVICE);
			if (pci_dma_mapping_error(adapter->dev, ctx->sglPA)) {
				scmd_printk(KERN_ERR, cmd,
					    "vmw_pvscsi: Failed to map ctx sglist for DMA.\n");
				scsi_dma_unmap(cmd);
				ctx->sglPA = 0;
				return -ENOMEM;
			}
			e->dataAddr = ctx->sglPA;
		} else
			e->dataAddr = sg_dma_address(sg);
	} else {
		/*
                 * In case there is no S/G list, scsi_sglist points
                 * directly to the buffer.
                 */
		ctx->dataPA = pci_map_single(adapter->dev, sg, bufflen,
					     cmd->sc_data_direction);
		if (pci_dma_mapping_error(adapter->dev, ctx->dataPA)) {
			scmd_printk(KERN_ERR, cmd,
				    "vmw_pvscsi: Failed to map direct data buffer for DMA.\n");
			return -ENOMEM;
		}
		e->dataAddr = ctx->dataPA;
	}

	return 0;
}


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static void pvscsi_unmap_buffers(const struct pvscsi_adapter *adapter,
				 struct pvscsi_ctx *ctx)
{
	struct scsi_cmnd *cmd;
	unsigned bufflen;

	cmd = ctx->cmd;
	bufflen = scsi_bufflen(cmd);

	if (bufflen != 0) {
		unsigned count = scsi_sg_count(cmd);

		if (count != 0) {
			scsi_dma_unmap(cmd);
			if (ctx->sglPA) {
				pci_unmap_single(adapter->dev, ctx->sglPA,
						 SGL_SIZE, PCI_DMA_TODEVICE);
				ctx->sglPA = 0;
			}
		} else
			pci_unmap_single(adapter->dev, ctx->dataPA, bufflen,
					 cmd->sc_data_direction);
	}
	if (cmd->sense_buffer)
		pci_unmap_single(adapter->dev, ctx->sensePA,
				 SCSI_SENSE_BUFFERSIZE, PCI_DMA_FROMDEVICE);
}


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static int pvscsi_allocate_rings(struct pvscsi_adapter *adapter)
{
	adapter->rings_state = pci_alloc_consistent(adapter->dev, PAGE_SIZE,
						    &adapter->ringStatePA);
	if (!adapter->rings_state)
		return -ENOMEM;

	adapter->req_pages = min(PVSCSI_MAX_NUM_PAGES_REQ_RING,
				 pvscsi_ring_pages);
	adapter->req_depth = adapter->req_pages
					* PVSCSI_MAX_NUM_REQ_ENTRIES_PER_PAGE;
	adapter->req_ring = pci_alloc_consistent(adapter->dev,
						 adapter->req_pages * PAGE_SIZE,
						 &adapter->reqRingPA);
	if (!adapter->req_ring)
		return -ENOMEM;

	adapter->cmp_pages = min(PVSCSI_MAX_NUM_PAGES_CMP_RING,
				 pvscsi_ring_pages);
	adapter->cmp_ring = pci_alloc_consistent(adapter->dev,
						 adapter->cmp_pages * PAGE_SIZE,
						 &adapter->cmpRingPA);
	if (!adapter->cmp_ring)
		return -ENOMEM;

	BUG_ON(!IS_ALIGNED(adapter->ringStatePA, PAGE_SIZE));
	BUG_ON(!IS_ALIGNED(adapter->reqRingPA, PAGE_SIZE));
	BUG_ON(!IS_ALIGNED(adapter->cmpRingPA, PAGE_SIZE));

	if (!adapter->use_msg)
		return 0;

	adapter->msg_pages = min(PVSCSI_MAX_NUM_PAGES_MSG_RING,
				 pvscsi_msg_ring_pages);
	adapter->msg_ring = pci_alloc_consistent(adapter->dev,
						 adapter->msg_pages * PAGE_SIZE,
						 &adapter->msgRingPA);
	if (!adapter->msg_ring)
		return -ENOMEM;
	BUG_ON(!IS_ALIGNED(adapter->msgRingPA, PAGE_SIZE));

	return 0;
}


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static void pvscsi_setup_all_rings(const struct pvscsi_adapter *adapter)
{
	struct PVSCSICmdDescSetupRings cmd = { 0 };
	dma_addr_t base;
	unsigned i;

	cmd.ringsStatePPN   = adapter->ringStatePA >> PAGE_SHIFT;
	cmd.reqRingNumPages = adapter->req_pages;
	cmd.cmpRingNumPages = adapter->cmp_pages;

	base = adapter->reqRingPA;
	for (i = 0; i < adapter->req_pages; i++) {
		cmd.reqRingPPNs[i] = base >> PAGE_SHIFT;
		base += PAGE_SIZE;
	}

	base = adapter->cmpRingPA;
	for (i = 0; i < adapter->cmp_pages; i++) {
		cmd.cmpRingPPNs[i] = base >> PAGE_SHIFT;
		base += PAGE_SIZE;
	}

	memset(adapter->rings_state, 0, PAGE_SIZE);
	memset(adapter->req_ring, 0, adapter->req_pages * PAGE_SIZE);
	memset(adapter->cmp_ring, 0, adapter->cmp_pages * PAGE_SIZE);

	pvscsi_write_cmd_desc(adapter, PVSCSI_CMD_SETUP_RINGS,
			      &cmd, sizeof(cmd));

	if (adapter->use_msg) {
		struct PVSCSICmdDescSetupMsgRing cmd_msg = { 0 };

		cmd_msg.numPages = adapter->msg_pages;

		base = adapter->msgRingPA;
		for (i = 0; i < adapter->msg_pages; i++) {
			cmd_msg.ringPPNs[i] = base >> PAGE_SHIFT;
			base += PAGE_SIZE;
		}
		memset(adapter->msg_ring, 0, adapter->msg_pages * PAGE_SIZE);

		pvscsi_write_cmd_desc(adapter, PVSCSI_CMD_SETUP_MSG_RING,
				      &cmd_msg, sizeof(cmd_msg));
	}
}


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static int pvscsi_change_queue_depth(struct scsi_device *sdev, int qdepth)
{
	if (!sdev->tagged_supported)
		qdepth = 1;
	return scsi_change_queue_depth(sdev, qdepth);
}


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/*
 * Pull a completion descriptor off and pass the completion back
 * to the SCSI mid layer.
 */



static void pvscsi_complete_request(struct pvscsi_adapter *adapter,
				    const struct PVSCSIRingCmpDesc *e)
{
	struct pvscsi_ctx *ctx;
	struct scsi_cmnd *cmd;
	struct completion *abort_cmp;
	u32 btstat = e->hostStatus;
	u32 sdstat = e->scsiStatus;

	ctx = pvscsi_get_context(adapter, e->context);
	cmd = ctx->cmd;
	abort_cmp = ctx->abort_cmp;
	pvscsi_unmap_buffers(adapter, ctx);
	pvscsi_release_context(adapter, ctx);
	if (abort_cmp) {
		/*
                 * The command was requested to be aborted. Just signal that
                 * the request completed and swallow the actual cmd completion
                 * here. The abort handler will post a completion for this
                 * command indicating that it got successfully aborted.
                 */
		complete(abort_cmp);
		return;
	}

	cmd->result = 0;
	if (sdstat != SAM_STAT_GOOD &&
	    (btstat == BTSTAT_SUCCESS ||
	     btstat == BTSTAT_LINKED_COMMAND_COMPLETED ||
	     btstat == BTSTAT_LINKED_COMMAND_COMPLETED_WITH_FLAG)) {
		cmd->result = (DID_OK << 16) | sdstat;
		if (sdstat == SAM_STAT_CHECK_CONDITION && cmd->sense_buffer)
			cmd->result |= (DRIVER_SENSE << 24);
	} else
		switch (btstat) {
		case BTSTAT_SUCCESS:
		case BTSTAT_LINKED_COMMAND_COMPLETED:
		case BTSTAT_LINKED_COMMAND_COMPLETED_WITH_FLAG:
			/* If everything went fine, let's move on..  */
			cmd->result = (DID_OK << 16);
			break;

		case BTSTAT_DATARUN:
		case BTSTAT_DATA_UNDERRUN:
			/* Report residual data in underruns */
			scsi_set_resid(cmd, scsi_bufflen(cmd) - e->dataLen);
			cmd->result = (DID_ERROR << 16);
			break;

		case BTSTAT_SELTIMEO:
			/* Our emulation returns this for non-connected devs */
			cmd->result = (DID_BAD_TARGET << 16);
			break;

		case BTSTAT_LUNMISMATCH:
		case BTSTAT_TAGREJECT:
		case BTSTAT_BADMSG:
			cmd->result = (DRIVER_INVALID << 24);
			/* fall through */

		case BTSTAT_HAHARDWARE:
		case BTSTAT_INVPHASE:
		case BTSTAT_HATIMEOUT:
		case BTSTAT_NORESPONSE:
		case BTSTAT_DISCONNECT:
		case BTSTAT_HASOFTWARE:
		case BTSTAT_BUSFREE:
		case BTSTAT_SENSFAILED:
			cmd->result |= (DID_ERROR << 16);
			break;

		case BTSTAT_SENTRST:
		case BTSTAT_RECVRST:
		case BTSTAT_BUSRESET:
			cmd->result = (DID_RESET << 16);
			break;

		case BTSTAT_ABORTQUEUE:
			cmd->result = (DID_ABORT << 16);
			break;

		case BTSTAT_SCSIPARITY:
			cmd->result = (DID_PARITY << 16);
			break;

		default:
			cmd->result = (DID_ERROR << 16);
			scmd_printk(KERN_DEBUG, cmd,
				    "Unknown completion status: 0x%x\n",
				    btstat);
	}

	dev_dbg(&cmd->device->sdev_gendev,
		"cmd=%p %x ctx=%p result=0x%x status=0x%x,%x\n",
		cmd, cmd->cmnd[0], ctx, cmd->result, btstat, sdstat);

	cmd->scsi_done(cmd);
}


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Arvind Kumar246.09%150.00%


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/*
 * barrier usage : Since the PVSCSI device is emulated, there could be cases
 * where we may want to serialize some accesses between the driver and the
 * emulation layer. We use compiler barriers instead of the more expensive
 * memory barriers because PVSCSI is only supported on X86 which has strong
 * memory access ordering.
 */



static void pvscsi_process_completion_ring(struct pvscsi_adapter *adapter)
{
	struct PVSCSIRingsState *s = adapter->rings_state;
	struct PVSCSIRingCmpDesc *ring = adapter->cmp_ring;
	u32 cmp_entries = s->cmpNumEntriesLog2;

	while (s->cmpConsIdx != s->cmpProdIdx) {
		struct PVSCSIRingCmpDesc *e = ring + (s->cmpConsIdx &
						      MASK(cmp_entries));
		/*
                 * This barrier() ensures that *e is not dereferenced while
                 * the device emulation still writes data into the slot.
                 * Since the device emulation advances s->cmpProdIdx only after
                 * updating the slot we want to check it first.
                 */
		barrier();
		pvscsi_complete_request(adapter, e);
		/*
                 * This barrier() ensures that compiler doesn't reorder write
                 * to s->cmpConsIdx before the read of (*e) inside
                 * pvscsi_complete_request. Otherwise, device emulation may
                 * overwrite *e before we had a chance to read it.
                 */
		barrier();
		s->cmpConsIdx++;
	}
}


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/*
 * Translate a Linux SCSI request into a request ring entry.
 */



static int pvscsi_queue_ring(struct pvscsi_adapter *adapter,
			     struct pvscsi_ctx *ctx, struct scsi_cmnd *cmd)
{
	struct PVSCSIRingsState *s;
	struct PVSCSIRingReqDesc *e;
	struct scsi_device *sdev;
	u32 req_entries;

	s = adapter->rings_state;
	sdev = cmd->device;
	req_entries = s->reqNumEntriesLog2;

	/*
         * If this condition holds, we might have room on the request ring, but
         * we might not have room on the completion ring for the response.
         * However, we have already ruled out this possibility - we would not
         * have successfully allocated a context if it were true, since we only
         * have one context per request entry.  Check for it anyway, since it
         * would be a serious bug.
         */
	if (s->reqProdIdx - s->cmpConsIdx >= 1 <<