[PATCH v2 101/109] virtiofsd: use fuse_buf_writev to replace fuse_buf_write for better performance

[Dr. David Alan Gilbert (git)]

From: piaojun <piao...@huawei.com>

fuse_buf_writev() only handles the normal write in which src is buffer
and dest is fd. Specially if src buffer represents guest physical
address that can't be mapped by the daemon process, IO must be bounced
back to the VMM to do it by fuse_buf_copy().

Signed-off-by: Jun Piao <piao...@huawei.com>
Suggested-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
Suggested-by: Stefan Hajnoczi <stefa...@redhat.com>
Reviewed-by: Daniel P. Berrangé <berra...@redhat.com>
---
 tools/virtiofsd/buffer.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/tools/virtiofsd/buffer.c b/tools/virtiofsd/buffer.c
index 70980550d2..b75f871680 100644
--- a/tools/virtiofsd/buffer.c
+++ b/tools/virtiofsd/buffer.c
@@ -33,7 +33,6 @@ size_t fuse_buf_size(const struct fuse_bufvec *bufv)
     return size;
 }
 
-__attribute__((unused))
 static ssize_t fuse_buf_writev(struct fuse_buf *out_buf,
                                struct fuse_bufvec *in_buf)
 {
@@ -262,12 +261,29 @@ static int fuse_bufvec_advance(struct fuse_bufvec *bufv, 
size_t len)
 ssize_t fuse_buf_copy(struct fuse_bufvec *dstv, struct fuse_bufvec *srcv,
                       enum fuse_buf_copy_flags flags)
 {
-    size_t copied = 0;
+    size_t copied = 0, i;
 
     if (dstv == srcv) {
         return fuse_buf_size(dstv);
     }
 
+    /*
+     * use writev to improve bandwidth when all the
+     * src buffers already mapped by the daemon
+     * process
+     */
+    for (i = 0; i < srcv->count; i++) {
+        if (srcv->buf[i].flags & FUSE_BUF_IS_FD) {
+            break;
+        }
+    }
+    if ((i == srcv->count) && (dstv->count == 1) &&
+        (dstv->idx == 0) &&
+        (dstv->buf[0].flags & FUSE_BUF_IS_FD)) {
+        dstv->buf[0].pos += dstv->off;
+        return fuse_buf_writev(&dstv->buf[0], srcv);
+    }
+
     for (;;) {
         const struct fuse_buf *src = fuse_bufvec_current(srcv);
         const struct fuse_buf *dst = fuse_bufvec_current(dstv);
-- 
2.24.1