[Qemu-devel] [PATCH 2/2] virtio-scsi: Optimize virtio_scsi_init_req

[Fam Zheng <famz@redhat.com>]

The VirtQueueElement is a very big structure (> 4k), since it will be
initialzed by virtqueue_pop, we can save the expensive zeroing here.

This saves a few nanoseconds per request in my test:

[fio-test]      rw         bs         iodepth    jobs       bw         iops       latency
--------------------------------------------------------------------------------------------
Before          read       4k         1          1          110        28269      34
After           read       4k         1          1          131        33745      28

virtio-blk      read       4k         1          1          217        55673      16

Signed-off-by: Fam Zheng <famz@redhat.com>
---
 hw/scsi/virtio-scsi.c           | 17 ++++++++++-------
 include/hw/virtio/virtio-scsi.h |  1 +
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 86aba88..0792529 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -24,12 +24,12 @@
 typedef struct VirtIOSCSIReq {
     VirtIOSCSI *dev;
     VirtQueue *vq;
-    VirtQueueElement elem;
     QEMUSGList qsgl;
+    QEMUIOVector resp_iov;
+    VirtQueueElement elem;
     SCSIRequest *sreq;
     size_t resp_size;
     enum SCSIXferMode mode;
-    QEMUIOVector resp_iov;
     union {
         VirtIOSCSICmdResp     cmd;
         VirtIOSCSICtrlTMFResp tmf;
@@ -44,6 +44,7 @@ typedef struct VirtIOSCSIReq {
         VirtIOSCSICtrlTMFReq  tmf;
         VirtIOSCSICtrlANReq   an;
     } req;
+    uint8_t cdb[VIRTIO_SCSI_CDB_SIZE_MAX];
 } VirtIOSCSIReq;
 
 QEMU_BUILD_BUG_ON(offsetof(VirtIOSCSIReq, req.cdb) !=
@@ -68,15 +69,16 @@ static inline SCSIDevice *virtio_scsi_device_find(VirtIOSCSI *s, uint8_t *lun)
 static VirtIOSCSIReq *virtio_scsi_init_req(VirtIOSCSI *s, VirtQueue *vq)
 {
     VirtIOSCSIReq *req;
-    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(s);
-
-    req = g_malloc0(sizeof(*req) + vs->cdb_size);
+    const size_t zero_skip = offsetof(VirtIOSCSIReq, elem)
+                             + sizeof(VirtQueueElement);
 
+    req = g_slice_new(VirtIOSCSIReq);
     req->vq = vq;
     req->dev = s;
     req->sreq = NULL;
     qemu_sglist_init(&req->qsgl, DEVICE(s), 8, &address_space_memory);
     qemu_iovec_init(&req->resp_iov, 1);
+    memset((uint8_t *)req + zero_skip, 0, sizeof(*req) - zero_skip);
     return req;
 }
 
@@ -84,7 +86,7 @@ static void virtio_scsi_free_req(VirtIOSCSIReq *req)
 {
     qemu_iovec_destroy(&req->resp_iov);
     qemu_sglist_destroy(&req->qsgl);
-    g_free(req);
+    g_slice_free(VirtIOSCSIReq, req);
 }
 
 static void virtio_scsi_complete_req(VirtIOSCSIReq *req)
@@ -532,7 +534,8 @@ static void virtio_scsi_set_config(VirtIODevice *vdev,
     VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(vdev);
 
     if ((uint32_t) virtio_ldl_p(vdev, &scsiconf->sense_size) >= 65536 ||
-        (uint32_t) virtio_ldl_p(vdev, &scsiconf->cdb_size) >= 256) {
+        (uint32_t) virtio_ldl_p(vdev, &scsiconf->cdb_size)
+                   >= VIRTIO_SCSI_CDB_SIZE_MAX) {
         error_report("bad data written to virtio-scsi configuration space");
         exit(1);
     }
diff --git a/include/hw/virtio/virtio-scsi.h b/include/hw/virtio/virtio-scsi.h
index 188a2d9..6e876f4 100644
--- a/include/hw/virtio/virtio-scsi.h
+++ b/include/hw/virtio/virtio-scsi.h
@@ -37,6 +37,7 @@
 
 #define VIRTIO_SCSI_VQ_SIZE     128
 #define VIRTIO_SCSI_CDB_SIZE    32
+#define VIRTIO_SCSI_CDB_SIZE_MAX 256
 #define VIRTIO_SCSI_SENSE_SIZE  96
 #define VIRTIO_SCSI_MAX_CHANNEL 0
 #define VIRTIO_SCSI_MAX_TARGET  255
-- 
1.9.3