[PATCH 2/2] hw/9pfs: fix alignment issue when host filesystem block size is larger than client msize

[Jevon Qiao <scaleqiao@gmail.com>]

The following patch is to fix alignment issue when host filesystem block 
size
is larger than client msize.

Thanks,
Jevon

From: Jevon Qiao <scaleqiao@gmail.com>
Date: Sun, 14 Feb 2016 15:11:08 +0800
Subject: [PATCH] hw/9pfs: fix alignment issue when host filesystem block 
size
  is larger than client msize.

Per the previous implementation, iounit will be assigned to be 0 after the
first if statement as (s->msize - P9_IOHDRSZ)/stbuf.f_bsize will be zero 
when
host filesystem block size is larger than msize. Finally, iounit will be 
equal
to s->msize - P9_IOHDRSZ, which is usually not aligned.

Signed-off-by: Jevon Qiao <scaleqiao@gmail.com>
---
  hw/9pfs/virtio-9p.c | 19 ++++++++++++++++---
  1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/hw/9pfs/virtio-9p.c b/hw/9pfs/virtio-9p.c
index f972731..005d3a8 100644
--- a/hw/9pfs/virtio-9p.c
+++ b/hw/9pfs/virtio-9p.c
@@ -1326,7 +1326,7 @@ out_nofid:
  static int32_t get_iounit(V9fsPDU *pdu, V9fsPath *path)
  {
      struct statfs stbuf;
-    int32_t iounit = 0;
+    int32_t iounit = 0, unit = 0;
      V9fsState *s = pdu->s;

      /*
@@ -1334,8 +1334,21 @@ static int32_t get_iounit(V9fsPDU *pdu, V9fsPath 
*path)
       * and as well as less than (client msize - P9_IOHDRSZ))
       */
      if (!v9fs_co_statfs(pdu, path, &stbuf)) {
-        iounit = stbuf.f_bsize;
-        iounit *= (s->msize - P9_IOHDRSZ)/stbuf.f_bsize;
+    /*
+     * If host filesystem block size is larger than client msize,
+     * we will use PAGESIZE as the unit. The reason why we choose
+     * PAGESIZE is because the data will be splitted in terms of
+     * PAGESIZE in the virtio layer. In this case, the final
+     * iounit is equal to the value of ((msize/unit) - 1) * unit.
+     */
+    if (stbuf.f_bsize > s->msize) {
+        iounit = 4096;
+        unit = 4096;
+    } else {
+            iounit = stbuf.f_bsize;
+        unit = stbuf.f_bsize;
+    }
+        iounit *= (s->msize - P9_IOHDRSZ)/unit;
      }
      if (!iounit) {
          iounit = s->msize - P9_IOHDRSZ;
--