[PATCH 4/5] vhost: add per VQ memory region caching

[Igor Mammedov <imammedo@redhat.com>]

that brings down translate_desc() cost to around 210ns
if accessed descriptors are from the same memory region.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
that's what netperf/iperf workloads were during testing.
---
 drivers/vhost/vhost.c | 16 +++++++++++++---
 drivers/vhost/vhost.h |  1 +
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 6a18c92..68c1c88 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -200,6 +200,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
 	vq->call = NULL;
 	vq->log_ctx = NULL;
 	vq->memory = NULL;
+	vq->cached_reg = 0;
 }
 
 static int vhost_worker(void *data)
@@ -649,6 +650,7 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
 	for (i = 0; i < d->nvqs; ++i) {
 		mutex_lock(&d->vqs[i]->mutex);
 		d->vqs[i]->memory = newmem;
+		d->vqs[i]->cached_reg = 0;
 		mutex_unlock(&d->vqs[i]->mutex);
 	}
 	kvfree(oldmem);
@@ -936,11 +938,17 @@ done:
 EXPORT_SYMBOL_GPL(vhost_dev_ioctl);
 
 static const struct vhost_memory_region *find_region(struct vhost_memory *mem,
-						     __u64 addr, __u32 len)
+						     __u64 addr, __u32 len,
+						     int *cached_reg)
 {
 	const struct vhost_memory_region *reg;
 	int start = 0, end = mem->nregions;
 
+	reg = mem->regions + *cached_reg;
+	if (likely(addr >= reg->guest_phys_addr &&
+		reg->guest_phys_addr + reg->memory_size > addr))
+		return reg;
+
 	while (start < end) {
 		int slot = start + (end - start) / 2;
 		reg = mem->regions + slot;
@@ -952,8 +960,10 @@ static const struct vhost_memory_region *find_region(struct vhost_memory *mem,
 
 	reg = mem->regions + start;
 	if (addr >= reg->guest_phys_addr &&
-		reg->guest_phys_addr + reg->memory_size > addr)
+		reg->guest_phys_addr + reg->memory_size > addr) {
+		*cached_reg = start;
 		return reg;
+	}
 	return NULL;
 }
 
@@ -1107,7 +1117,7 @@ static int translate_desc(struct vhost_virtqueue *vq, u64 addr, u32 len,
 			ret = -ENOBUFS;
 			break;
 		}
-		reg = find_region(mem, addr, len);
+		reg = find_region(mem, addr, len, &vq->cached_reg);
 		if (unlikely(!reg)) {
 			ret = -EFAULT;
 			break;
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index 8c1c792..68bd00f 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -106,6 +106,7 @@ struct vhost_virtqueue {
 	/* Log write descriptors */
 	void __user *log_base;
 	struct vhost_log *log;
+	int cached_reg;
 };
 
 struct vhost_dev {
-- 
1.8.3.1