Virtualization - Nov 2015 - [RFC PATCH 0/2] Google extension to improve qemu-nvme performance

Hi Rob & Mihai,

I wrote vhost-nvme patches on top of Christoph's NVMe target.
vhost-nvme still uses mmio. So the guest OS can run unmodified NVMe
driver. But the tests I have done didn't show competitive performance
compared to virtio-blk/virtio-scsi. The bottleneck is in mmio. Your nvme
vendor extension patches reduces greatly the number of MMIO writes.
So I'd like to push it upstream.

I port these 2 patches to newer kernel and qemu.
I use ram disk as backend to compare performance.

qemu-nvme: 29MB/s
qemu-nvme+google-ext: 100MB/s
virtio-blk: 174MB/s
virtio-scsi: 118MB/s

I'll show you qemu-vhost-nvme+google-ext number later.

root at guest:~# cat test.job 
[global]
bs=4k
ioengine=libaio
iodepth=64
direct=1
runtime=120
time_based
rw=randread
norandommap
group_reporting
gtod_reduce=1
numjobs=2

[job1]
filename=/dev/nvme0n1
#filename=/dev/vdb
#filename=/dev/sda
rw=read

Patches also available at:
kernel:
https://git.kernel.org/cgit/linux/kernel/git/mlin/linux.git/log/?h=nvme-google-ext
qemu:
http://www.minggr.net/cgit/cgit.cgi/qemu/log/?h=nvme-google-ext 

Thanks,
Ming

From: Rob Nelson <rlnelson at google.com>

This change provides a mechanism to reduce the number of MMIO doorbell
writes for the NVMe driver. When running in a virtualized environment
like QEMU, the cost of an MMIO is quite hefy here. The main idea for
the patch is provide the device two memory location locations:
 1) to store the doorbell values so they can be lookup without the doorbell
    MMIO write
 2) to store an event index.
I believe the doorbell value is obvious, the event index not so much.
Similar to the virtio specificaiton, the virtual device can tell the
driver (guest OS) not to write MMIO unless you are writing past this
value.

FYI: doorbell values are written by the nvme driver (guest OS) and the
event index is written by the virtual device (host OS).

The patch implements a new admin command that will communicate where
these two memory locations reside. If the command fails, the nvme
driver will work as before without any optimizations.

Contributions:
  Eric Northup <digitaleric at google.com>
  Frank Swiderski <fes at google.com>
  Ted Tso <tytso at mit.edu>
  Keith Busch <keith.busch at intel.com>

Just to give an idea on the performance boost with the vendor
extension: Running fio [1], a stock NVMe driver I get about 200K read
IOPs with my vendor patch I get about 1000K read IOPs. This was
running with a null device i.e. the backing device simply returned
success on every read IO request.

[1] Running on a 4 core machine:
  fio --time_based --name=benchmark --runtime=30
  --filename=/dev/nvme0n1 --nrfiles=1 --ioengine=libaio --iodepth=32
  --direct=1 --invalidate=1 --verify=0 --verify_fatal=0 --numjobs=4
  --rw=randread --blocksize=4k --randrepeat=false

Signed-off-by: Rob Nelson <rlnelson at google.com>
[mlin: port for upstream]
Signed-off-by: Ming Lin <mlin at kernel.org>
---
 drivers/nvme/host/Kconfig |   7 +++
 drivers/nvme/host/core.c  |   1 +
 drivers/nvme/host/pci.c   | 147 ++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/nvme.h      |  21 +++++++
 4 files changed, 176 insertions(+)

diff --git a/drivers/nvme/host/Kconfig b/drivers/nvme/host/Kconfig
index 002a94a..93f9438 100644
--- a/drivers/nvme/host/Kconfig
+++ b/drivers/nvme/host/Kconfig
@@ -8,3 +8,10 @@ config BLK_DEV_NVME
 
 	  To compile this driver as a module, choose M here: the
 	  module will be called nvme.
+
+config NVME_VENDOR_EXT_GOOGLE
+	tristate "NVMe Vendor Extension for Improved Virtualization"
+	depends on BLK_DEV_NVME
+	---help---
+	  Google extension to reduce the number of MMIO doorbell
+	  writes for the NVMe driver
diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 400b1ea..78ac8bb 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -160,6 +160,7 @@ int nvme_submit_sync_cmd(struct request_queue *q, struct
nvme_command *cmd,
 {
 	return __nvme_submit_sync_cmd(q, cmd, buffer, bufflen, NULL, 0);
 }
+EXPORT_SYMBOL_GPL(nvme_submit_sync_cmd);
 
 int __nvme_submit_user_cmd(struct request_queue *q, struct nvme_command *cmd,
 		void __user *ubuffer, unsigned bufflen,
diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
index 91522bb..93f1f36 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -49,6 +49,9 @@
 #define SQ_SIZE(depth)		(depth * sizeof(struct nvme_command))
 #define CQ_SIZE(depth)		(depth * sizeof(struct nvme_completion))
 
+/* Google Vendor ID is not in include/linux/pci_ids.h */
+#define PCI_VENDOR_ID_GOOGLE 0x1AE0
+
 static int use_threaded_interrupts;
 module_param(use_threaded_interrupts, int, 0);
 
@@ -106,6 +109,13 @@ struct nvme_dev {
 	unsigned long flags;
 #define NVME_CTRL_RESETTING	0
 
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+	u32 *db_mem;
+	dma_addr_t doorbell;
+	u32 *ei_mem;
+	dma_addr_t eventidx;
+#endif
+
 	struct nvme_ctrl ctrl;
 };
 
@@ -139,6 +149,12 @@ struct nvme_queue {
 	u8 cq_phase;
 	u8 cqe_seen;
 	struct async_cmd_info cmdinfo;
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+	u32 *sq_doorbell_addr;
+	u32 *sq_eventidx_addr;
+	u32 *cq_doorbell_addr;
+	u32 *cq_eventidx_addr;
+#endif
 };
 
 /*
@@ -176,6 +192,9 @@ static inline void _nvme_check_size(void)
 	BUILD_BUG_ON(sizeof(struct nvme_id_ns) != 4096);
 	BUILD_BUG_ON(sizeof(struct nvme_lba_range_type) != 64);
 	BUILD_BUG_ON(sizeof(struct nvme_smart_log) != 512);
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+	BUILD_BUG_ON(sizeof(struct nvme_doorbell_memory) != 64);
+#endif
 }
 
 /*
@@ -289,6 +308,51 @@ static void nvme_finish_aen_cmd(struct nvme_dev *dev,
struct nvme_completion *cq
 	}
 }
 
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+static int nvme_vendor_memory_size(struct nvme_dev *dev)
+{
+	return ((num_possible_cpus() + 1) * 8 * dev->db_stride);
+}
+
+static int nvme_set_doorbell_memory(struct nvme_dev *dev)
+{
+	struct nvme_command c;
+
+	memset(&c, 0, sizeof(c));
+	c.doorbell_memory.opcode = nvme_admin_doorbell_memory;
+	c.doorbell_memory.prp1 = cpu_to_le64(dev->doorbell);
+	c.doorbell_memory.prp2 = cpu_to_le64(dev->eventidx);
+
+	return nvme_submit_sync_cmd(dev->ctrl.admin_q, &c, NULL, 0);
+}
+
+static inline int nvme_ext_need_event(u16 event_idx, u16 new_idx, u16 old)
+{
+	/* Borrowed from vring_need_event */
+	return (u16)(new_idx - event_idx - 1) < (u16)(new_idx - old);
+}
+
+static void nvme_ext_write_doorbell(u16 value, u32 __iomem* q_db,
+			   u32* db_addr, volatile u32* event_idx)
+{
+	u16 old_value;
+	if (!db_addr)
+		goto ring_doorbell;
+
+	old_value = *db_addr;
+	*db_addr = value;
+
+	rmb();
+	if (!nvme_ext_need_event(*event_idx, value, old_value))
+		goto no_doorbell;
+
+ring_doorbell:
+	writel(value, q_db);
+no_doorbell:
+	return;
+}
+#endif
+
 /**
  * __nvme_submit_cmd() - Copy a command into a queue and ring the doorbell
  * @nvmeq: The queue to use
@@ -306,9 +370,19 @@ static void __nvme_submit_cmd(struct nvme_queue *nvmeq,
 	else
 		memcpy(&nvmeq->sq_cmds[tail], cmd, sizeof(*cmd));
 
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+	if (nvmeq->sq_doorbell_addr)
+		wmb();
+#endif
+
 	if (++tail == nvmeq->q_depth)
 		tail = 0;
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+	nvme_ext_write_doorbell(tail, nvmeq->q_db,
+		nvmeq->sq_doorbell_addr, nvmeq->sq_eventidx_addr);
+#else
 	writel(tail, nvmeq->q_db);
+#endif
 	nvmeq->sq_tail = tail;
 }
 
@@ -719,6 +793,11 @@ static int nvme_process_cq(struct nvme_queue *nvmeq)
 		u16 status = le16_to_cpu(cqe.status);
 		struct request *req;
 
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+		if (to_pci_dev(nvmeq->dev->dev)->vendor == PCI_VENDOR_ID_GOOGLE)
+			rmb();
+#endif
+
 		if ((status & 1) != phase)
 			break;
 		nvmeq->sq_head = le16_to_cpu(cqe.sq_head);
@@ -764,7 +843,12 @@ static int nvme_process_cq(struct nvme_queue *nvmeq)
 	if (head == nvmeq->cq_head && phase == nvmeq->cq_phase)
 		return 0;
 
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+	nvme_ext_write_doorbell(head, nvmeq->q_db + nvmeq->dev->db_stride,
+		nvmeq->cq_doorbell_addr, nvmeq->cq_eventidx_addr);
+#else
 	writel(head, nvmeq->q_db + nvmeq->dev->db_stride);
+#endif
 	nvmeq->cq_head = head;
 	nvmeq->cq_phase = phase;
 
@@ -1111,6 +1195,17 @@ static struct nvme_queue *nvme_alloc_queue(struct
nvme_dev *dev, int qid,
 	nvmeq->cq_vector = -1;
 	dev->queues[qid] = nvmeq;
 
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+	if (dev->db_mem && dev->ei_mem && qid != 0) {
+		nvmeq->sq_doorbell_addr = &dev->db_mem[qid * 2 *
dev->db_stride];
+		nvmeq->cq_doorbell_addr +			&dev->db_mem[(qid * 2 + 1) *
dev->db_stride];
+		nvmeq->sq_eventidx_addr = &dev->ei_mem[qid * 2 *
dev->db_stride];
+		nvmeq->cq_eventidx_addr +			&dev->ei_mem[(qid * 2 + 1) *
dev->db_stride];
+	}
+#endif
+
 	/* make sure queue descriptor is set before queue count, for kthread */
 	mb();
 	dev->queue_count++;
@@ -1145,6 +1240,16 @@ static void nvme_init_queue(struct nvme_queue *nvmeq, u16
qid)
 	nvmeq->cq_head = 0;
 	nvmeq->cq_phase = 1;
 	nvmeq->q_db = &dev->dbs[qid * 2 * dev->db_stride];
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+	if (to_pci_dev(dev->dev)->vendor == PCI_VENDOR_ID_GOOGLE && qid
!= 0) {
+		nvmeq->sq_doorbell_addr = &dev->db_mem[qid * 2 *
dev->db_stride];
+		nvmeq->cq_doorbell_addr +			&dev->db_mem[(qid * 2 + 1) *
dev->db_stride];
+		nvmeq->sq_eventidx_addr = &dev->ei_mem[qid * 2 *
dev->db_stride];
+		nvmeq->cq_eventidx_addr +			&dev->ei_mem[(qid * 2 + 1) *
dev->db_stride];
+	}
+#endif
 	memset((void *)nvmeq->cqes, 0, CQ_SIZE(nvmeq->q_depth));
 	dev->online_queues++;
 	spin_unlock_irq(&nvmeq->q_lock);
@@ -1565,6 +1670,19 @@ static int nvme_dev_add(struct nvme_dev *dev)
 		if (blk_mq_alloc_tag_set(&dev->tagset))
 			return 0;
 		dev->ctrl.tagset = &dev->tagset;
+
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+		if (to_pci_dev(dev->dev)->vendor == PCI_VENDOR_ID_GOOGLE) {
+			int res = nvme_set_doorbell_memory(dev);
+			if (res) {
+				// Free memory and continue on.
+				dma_free_coherent(dev->dev, 8192, dev->db_mem, dev->doorbell);
+				dma_free_coherent(dev->dev, 8192, dev->ei_mem, dev->doorbell);
+				dev->db_mem = 0;
+				dev->ei_mem = 0;
+			}
+		}
+#endif
 	}
 	queue_work(nvme_workq, &dev->scan_work);
 	return 0;
@@ -1618,7 +1736,28 @@ static int nvme_dev_map(struct nvme_dev *dev)
 	if (readl(dev->bar + NVME_REG_VS) >= NVME_VS(1, 2))
 		dev->cmb = nvme_map_cmb(dev);
 
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+	if (pdev->vendor == PCI_VENDOR_ID_GOOGLE) {
+		int mem_size = nvme_vendor_memory_size(dev);
+		dev->db_mem = dma_alloc_coherent(&pdev->dev, mem_size,
&dev->doorbell, GFP_KERNEL);
+		if (!dev->db_mem) {
+			result = -ENOMEM;
+			goto unmap;
+		}
+		dev->ei_mem = dma_alloc_coherent(&pdev->dev, mem_size,
&dev->eventidx, GFP_KERNEL);
+		if (!dev->ei_mem) {
+			result = -ENOMEM;
+			goto dma_free;
+		}
+	}
+
+	return 0;
+
+ dma_free:
+	dma_free_coherent(&pdev->dev, nvme_vendor_memory_size(dev),
dev->db_mem, dev->doorbell);
+	dev->db_mem = 0;
 	return 0;
+#endif
 
  unmap:
 	iounmap(dev->bar);
@@ -1633,6 +1772,14 @@ static int nvme_dev_map(struct nvme_dev *dev)
 static void nvme_dev_unmap(struct nvme_dev *dev)
 {
 	struct pci_dev *pdev = to_pci_dev(dev->dev);
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+	int mem_size = nvme_vendor_memory_size(dev);
+
+	if (dev->db_mem)
+		dma_free_coherent(&pdev->dev, mem_size, dev->db_mem,
dev->doorbell);
+	if (dev->ei_mem)
+		dma_free_coherent(&pdev->dev, mem_size, dev->ei_mem,
dev->eventidx);
+#endif
 
 	if (pdev->msi_enabled)
 		pci_disable_msi(pdev);
diff --git a/include/linux/nvme.h b/include/linux/nvme.h
index a55986f..d3a8289 100644
--- a/include/linux/nvme.h
+++ b/include/linux/nvme.h
@@ -387,6 +387,9 @@ enum nvme_admin_opcode {
 	nvme_admin_format_nvm		= 0x80,
 	nvme_admin_security_send	= 0x81,
 	nvme_admin_security_recv	= 0x82,
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+	nvme_admin_doorbell_memory	= 0xC0,
+#endif
 };
 
 enum {
@@ -516,6 +519,18 @@ struct nvme_format_cmd {
 	__u32			rsvd11[5];
 };
 
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+struct nvme_doorbell_memory {
+	__u8			opcode;
+	__u8			flags;
+	__u16			command_id;
+	__u32			rsvd1[5];
+	__le64			prp1;
+	__le64			prp2;
+	__u32			rsvd12[6];
+};
+#endif
+
 struct nvme_command {
 	union {
 		struct nvme_common_command common;
@@ -529,6 +544,9 @@ struct nvme_command {
 		struct nvme_format_cmd format;
 		struct nvme_dsm_cmd dsm;
 		struct nvme_abort_cmd abort;
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+		struct nvme_doorbell_memory doorbell_memory;
+#endif
 	};
 };
 
@@ -575,6 +593,9 @@ enum {
 	NVME_SC_BAD_ATTRIBUTES		= 0x180,
 	NVME_SC_INVALID_PI		= 0x181,
 	NVME_SC_READ_ONLY		= 0x182,
+#ifdef CONFIG_NVME_VENDOR_EXT_GOOGLE
+	NVME_SC_DOORBELL_MEMORY_INVALID	= 0x1C0,
+#endif
 	NVME_SC_WRITE_FAULT		= 0x280,
 	NVME_SC_READ_ERROR		= 0x281,
 	NVME_SC_GUARD_CHECK		= 0x282,
-- 
1.9.1

From: Mihai Rusu <dizzy at google.com>

This implements the device side for an NVMe vendor extension that
reduces the number of MMIO writes which can result in a very large
performance benefit in virtualized environments.

See the following link for a description of the mechanism and the
kernel NVMe driver changes to support this vendor extension:
http://lists.infradead.org/pipermail/linux-nvme/2014-July/001076.html

On my workstation (3.2Ghz Xeon E5-1650), running QEMU:
$ bin/opt/native/x86_64-softmmu/qemu-system-x86_64 \
    -enable-kvm -m 2048 -smp 4 \
    -drive if=virtio,file=debian.raw,cache=none \
    -drive file=nvme.raw,if=none,id=nvme-dev \
    -device nvme,drive=nvme-dev,serial=nvme-serial

Using "fio":
vm # fio -time_based --name=benchmark --ioengine=libaio --iodepth=32 \
    --numjobs=1 --runtime=30 --blocksize=4k --filename=/dev/nvme0n1 \
    --nrfiles=1 --invalidate=1 --verify=0 --direct=1 --rw=randread

I get about 20k IOPs with the original code and about 85k IOPs with
the vendor extension changes applied (and running a vendor extension
supporting 3.14 based guest kernel).

Signed-off-by: Mihai Rusu <dizzy at google.com>
[fixed for a merging into different tree; added VID/DID params]
Signed-off-by: Keith Busch <keith.busch at intel.com>
[mlin: port for upstream]
Signed-off-by: Ming Lin <mlin at kernel.org>
---
 hw/block/nvme.c | 92 ++++++++++++++++++++++++++++++++++++++++++++++++++++++---
 hw/block/nvme.h | 18 +++++++++++
 2 files changed, 106 insertions(+), 4 deletions(-)

diff --git a/hw/block/nvme.c b/hw/block/nvme.c
index 169e4fa..3e1c38d 100644
--- a/hw/block/nvme.c
+++ b/hw/block/nvme.c
@@ -20,6 +20,7 @@
  *      -device
nvme,drive=<drive_id>,serial=<serial>,id=<id[optional]>
  */
 
+#include <exec/memory.h>
 #include <hw/block/block.h>
 #include <hw/hw.h>
 #include <hw/pci/msix.h>
@@ -158,6 +159,14 @@ static uint16_t nvme_dma_read_prp(NvmeCtrl *n, uint8_t
*ptr, uint32_t len,
     return NVME_SUCCESS;
 }
 
+static void nvme_update_cq_head(NvmeCQueue *cq)
+{
+    if (cq->db_addr) {
+        pci_dma_read(&cq->ctrl->parent_obj, cq->db_addr,
+                     &cq->head, sizeof(cq->head));
+    }
+}
+
 static void nvme_post_cqes(void *opaque)
 {
     NvmeCQueue *cq = opaque;
@@ -168,6 +177,8 @@ static void nvme_post_cqes(void *opaque)
         NvmeSQueue *sq;
         hwaddr addr;
 
+        nvme_update_cq_head(cq);
+
         if (nvme_cq_full(cq)) {
             break;
         }
@@ -350,6 +361,8 @@ static void nvme_init_sq(NvmeSQueue *sq, NvmeCtrl *n,
uint64_t dma_addr,
         QTAILQ_INSERT_TAIL(&(sq->req_list), &sq->io_req[i],
entry);
     }
     sq->timer = timer_new_ns(QEMU_CLOCK_VIRTUAL, nvme_process_sq, sq);
+    sq->db_addr = 0;
+    sq->eventidx_addr = 0;
 
     assert(n->cq[cqid]);
     cq = n->cq[cqid];
@@ -430,6 +443,8 @@ static void nvme_init_cq(NvmeCQueue *cq, NvmeCtrl *n,
uint64_t dma_addr,
     cq->head = cq->tail = 0;
     QTAILQ_INIT(&cq->req_list);
     QTAILQ_INIT(&cq->sq_list);
+    cq->db_addr = 0;
+    cq->eventidx_addr = 0;
     msix_vector_use(&n->parent_obj, cq->vector);
     n->cq[cqid] = cq;
     cq->timer = timer_new_ns(QEMU_CLOCK_VIRTUAL, nvme_post_cqes, cq);
@@ -528,6 +543,40 @@ static uint16_t nvme_set_feature(NvmeCtrl *n, NvmeCmd *cmd,
NvmeRequest *req)
     return NVME_SUCCESS;
 }
 
+static uint16_t nvme_set_db_memory(NvmeCtrl *n, const NvmeCmd *cmd)
+{
+    uint64_t db_addr = le64_to_cpu(cmd->prp1);
+    uint64_t eventidx_addr = le64_to_cpu(cmd->prp2);
+    int i;
+
+    /* Addresses should not be NULL and should be page aligned. */
+    if (db_addr == 0 || db_addr & (n->page_size - 1) ||
+        eventidx_addr == 0 || eventidx_addr & (n->page_size - 1)) {
+        return NVME_INVALID_MEMORY_ADDRESS | NVME_DNR;
+    }
+
+    /* This assumes all I/O queues are created before this command is handled.
+     * We skip the admin queues. */
+    for (i = 1; i < n->num_queues; i++) {
+        NvmeSQueue *sq = n->sq[i];
+        NvmeCQueue *cq = n->cq[i];
+
+        if (sq != NULL) {
+            /* Submission queue tail pointer location, 2 * QID * stride. */
+            sq->db_addr = db_addr + 2 * i * 4;
+            sq->eventidx_addr = eventidx_addr + 2 * i * 4;
+        }
+
+        if (cq != NULL) {
+            /* Completion queue head pointer location, (2 * QID + 1) * stride.
+             */
+            cq->db_addr = db_addr + (2 * i + 1) * 4;
+            cq->eventidx_addr = eventidx_addr + (2 * i + 1) * 4;
+        }
+    }
+    return NVME_SUCCESS;
+}
+
 static uint16_t nvme_admin_cmd(NvmeCtrl *n, NvmeCmd *cmd, NvmeRequest *req)
 {
     switch (cmd->opcode) {
@@ -545,11 +594,29 @@ static uint16_t nvme_admin_cmd(NvmeCtrl *n, NvmeCmd *cmd,
NvmeRequest *req)
         return nvme_set_feature(n, cmd, req);
     case NVME_ADM_CMD_GET_FEATURES:
         return nvme_get_feature(n, cmd, req);
+    case NVME_ADM_CMD_SET_DB_MEMORY:
+        return nvme_set_db_memory(n, cmd);
     default:
         return NVME_INVALID_OPCODE | NVME_DNR;
     }
 }
 
+static void nvme_update_sq_eventidx(const NvmeSQueue *sq)
+{
+    if (sq->eventidx_addr) {
+        pci_dma_write(&sq->ctrl->parent_obj, sq->eventidx_addr,
+                      &sq->tail, sizeof(sq->tail));
+    }
+}
+
+static void nvme_update_sq_tail(NvmeSQueue *sq)
+{
+    if (sq->db_addr) {
+        pci_dma_read(&sq->ctrl->parent_obj, sq->db_addr,
+                     &sq->tail, sizeof(sq->tail));
+    }
+}
+
 static void nvme_process_sq(void *opaque)
 {
     NvmeSQueue *sq = opaque;
@@ -561,6 +628,8 @@ static void nvme_process_sq(void *opaque)
     NvmeCmd cmd;
     NvmeRequest *req;
 
+    nvme_update_sq_tail(sq);
+
     while (!(nvme_sq_empty(sq) || QTAILQ_EMPTY(&sq->req_list))) {
         addr = sq->dma_addr + sq->head * n->sqe_size;
         pci_dma_read(&n->parent_obj, addr, (void *)&cmd,
sizeof(cmd));
@@ -578,6 +647,9 @@ static void nvme_process_sq(void *opaque)
             req->status = status;
             nvme_enqueue_req_completion(cq, req);
         }
+
+        nvme_update_sq_eventidx(sq);
+        nvme_update_sq_tail(sq);
     }
 }
 
@@ -726,7 +798,11 @@ static void nvme_process_db(NvmeCtrl *n, hwaddr addr, int
val)
         }
 
         start_sqs = nvme_cq_full(cq) ? 1 : 0;
-        cq->head = new_head;
+        /* When the mapped pointer memory area is setup, we don't rely on
+         * the MMIO written values to update the head pointer. */
+        if (!cq->db_addr) {
+            cq->head = new_head;
+        }
         if (start_sqs) {
             NvmeSQueue *sq;
             QTAILQ_FOREACH(sq, &cq->sq_list, entry) {
@@ -752,7 +828,11 @@ static void nvme_process_db(NvmeCtrl *n, hwaddr addr, int
val)
             return;
         }
 
-        sq->tail = new_tail;
+        /* When the mapped pointer memory area is setup, we don't rely on
+         * the MMIO written values to update the tail pointer. */
+        if (!sq->db_addr) {
+            sq->tail = new_tail;
+        }
         timer_mod(sq->timer, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + 500);
     }
 }
@@ -805,6 +885,8 @@ static int nvme_init(PCIDevice *pci_dev)
     pci_conf = pci_dev->config;
     pci_conf[PCI_INTERRUPT_PIN] = 1;
     pci_config_set_prog_interface(pci_dev->config, 0x2);
+    pci_config_set_vendor_id(pci_dev->config, n->vid);
+    pci_config_set_device_id(pci_dev->config, n->did);
     pci_config_set_class(pci_dev->config, PCI_CLASS_STORAGE_EXPRESS);
     pcie_endpoint_cap_init(&n->parent_obj, 0x80);
 
@@ -885,9 +967,13 @@ static void nvme_exit(PCIDevice *pci_dev)
     msix_uninit_exclusive_bar(pci_dev);
 }
 
+#define PCI_VENDOR_ID_GOOGLE 0x1AE0
+
 static Property nvme_props[] = {
     DEFINE_BLOCK_PROPERTIES(NvmeCtrl, conf),
     DEFINE_PROP_STRING("serial", NvmeCtrl, serial),
+    DEFINE_PROP_UINT16("vid", NvmeCtrl, vid, PCI_VENDOR_ID_GOOGLE),
+    DEFINE_PROP_UINT16("did", NvmeCtrl, did, 0x5845),
     DEFINE_PROP_END_OF_LIST(),
 };
 
@@ -905,8 +991,6 @@ static void nvme_class_init(ObjectClass *oc, void *data)
     pc->exit = nvme_exit;
     pc->class_id = PCI_CLASS_STORAGE_EXPRESS;
     pc->vendor_id = PCI_VENDOR_ID_INTEL;
-    pc->device_id = 0x5845;
-    pc->revision = 1;
     pc->is_express = 1;
 
     set_bit(DEVICE_CATEGORY_STORAGE, dc->categories);
diff --git a/hw/block/nvme.h b/hw/block/nvme.h
index bf3a3cc..82aeab4 100644
--- a/hw/block/nvme.h
+++ b/hw/block/nvme.h
@@ -170,6 +170,7 @@ enum NvmeAdminCommands {
     NVME_ADM_CMD_FORMAT_NVM     = 0x80,
     NVME_ADM_CMD_SECURITY_SEND  = 0x81,
     NVME_ADM_CMD_SECURITY_RECV  = 0x82,
+    NVME_ADM_CMD_SET_DB_MEMORY  = 0xC0,  /* Vendor specific. */
 };
 
 enum NvmeIoCommands {
@@ -381,6 +382,7 @@ enum NvmeStatusCodes {
     NVME_CONFLICTING_ATTRS      = 0x0180,
     NVME_INVALID_PROT_INFO      = 0x0181,
     NVME_WRITE_TO_RO            = 0x0182,
+    NVME_INVALID_MEMORY_ADDRESS = 0x01C0,  /* Vendor extension. */
     NVME_WRITE_FAULT            = 0x0280,
     NVME_UNRECOVERED_READ       = 0x0281,
     NVME_E2E_GUARD_ERROR        = 0x0282,
@@ -658,6 +660,13 @@ typedef struct NvmeSQueue {
     QTAILQ_HEAD(sq_req_list, NvmeRequest) req_list;
     QTAILQ_HEAD(out_req_list, NvmeRequest) out_req_list;
     QTAILQ_ENTRY(NvmeSQueue) entry;
+    /* Mapped memory location where the tail pointer is stored by the guest
+     * without triggering MMIO exits. */
+    uint64_t    db_addr;
+    /* virtio-like eventidx pointer, guest updates to the tail pointer that
+     * do not go over this value will not result in MMIO writes (but will
+     * still write the tail pointer to the "db_addr" location above).
*/
+    uint64_t    eventidx_addr;
 } NvmeSQueue;
 
 typedef struct NvmeCQueue {
@@ -673,6 +682,13 @@ typedef struct NvmeCQueue {
     QEMUTimer   *timer;
     QTAILQ_HEAD(sq_list, NvmeSQueue) sq_list;
     QTAILQ_HEAD(cq_req_list, NvmeRequest) req_list;
+    /* Mapped memory location where the head pointer is stored by the guest
+     * without triggering MMIO exits. */
+    uint64_t    db_addr;
+    /* virtio-like eventidx pointer, guest updates to the head pointer that
+     * do not go over this value will not result in MMIO writes (but will
+     * still write the head pointer to the "db_addr" location above).
*/
+    uint64_t    eventidx_addr;
 } NvmeCQueue;
 
 typedef struct NvmeNamespace {
@@ -699,6 +715,8 @@ typedef struct NvmeCtrl {
     uint32_t    num_queues;
     uint32_t    max_q_ents;
     uint64_t    ns_size;
+    uint16_t    vid;
+    uint16_t    did;
 
     char            *serial;
     NvmeNamespace   *namespaces;
-- 
1.9.1

On 18/11/2015 06:47, Ming Lin wrote:
> @@ -726,7 +798,11 @@ static void nvme_process_db(NvmeCtrl *n, hwaddr addr,
int val)
>          }
>  
>          start_sqs = nvme_cq_full(cq) ? 1 : 0;
> -        cq->head = new_head;
> +        /* When the mapped pointer memory area is setup, we don't rely
on
> +         * the MMIO written values to update the head pointer. */
> +        if (!cq->db_addr) {
> +            cq->head = new_head;
> +        }
You are still checking

        if (new_head >= cq->size) {
            return;
        }

above.  I think this is incorrect when the extension is present, and
furthermore it's the only case where val is being used.

If you're not using val, you could use ioeventfd for the MMIO.  An
ioeventfd cuts the MMIO cost by at least 55% and up to 70%. Here are
quick and dirty measurements from kvm-unit-tests's vmexit.flat
benchmark, on two very different machines:

			Haswell-EP		Ivy Bridge i7
  MMIO memory write	5100 -> 2250 (55%)	7000 -> 3000 (58%)
  I/O port write	3800 -> 1150 (70%)	4100 -> 1800 (57%)

You would need to allocate two eventfds for each qid, one for the sq and
one for the cq.  Also, processing the queues is now bounced to the QEMU
iothread, so you can probably get rid of sq->timer and cq->timer.

Paolo