[qemu-xen staging] file-posix: Allow byte-aligned O_DIRECT with NFS

[patchbot@xxxxxxx]

commit 5edc85571e7b7269dce408735eba7507f18ac666
Author:     Kevin Wolf <kwolf@xxxxxxxxxx>
AuthorDate: Thu Jul 16 16:26:01 2020 +0200
Commit:     Kevin Wolf <kwolf@xxxxxxxxxx>
CommitDate: Fri Jul 17 14:20:57 2020 +0200

    file-posix: Allow byte-aligned O_DIRECT with NFS
    
    Since commit a6b257a08e3 ('file-posix: Handle undetectable alignment'),
    we assume that if we open a file with O_DIRECT and alignment probing
    returns 1, we just couldn't find out the real alignment requirement
    because some filesystems make the requirement only for allocated blocks.
    In this case, a safe default of 4k is used.
    
    This is too strict for NFS, which does actually allow byte-aligned
    requests even with O_DIRECT. Because we can't distinguish both cases
    with generic code, let's just look at the file system magic and disable
    s->needs_alignment for NFS. This way, O_DIRECT can still be used on NFS
    for images that are not aligned to 4k.
    
    Signed-off-by: Kevin Wolf <kwolf@xxxxxxxxxx>
    Reviewed-by: Eric Blake <eblake@xxxxxxxxxx>
    Message-Id: <20200716142601.111237-3-kwolf@xxxxxxxxxx>
    Reviewed-by: Max Reitz <mreitz@xxxxxxxxxx>
    Signed-off-by: Kevin Wolf <kwolf@xxxxxxxxxx>
---
 block/file-posix.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 8067e238cb..ae8190edab 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -62,10 +62,12 @@
 #include <sys/ioctl.h>
 #include <sys/param.h>
 #include <sys/syscall.h>
+#include <sys/vfs.h>
 #include <linux/cdrom.h>
 #include <linux/fd.h>
 #include <linux/fs.h>
 #include <linux/hdreg.h>
+#include <linux/magic.h>
 #include <scsi/sg.h>
 #ifdef __s390__
 #include <asm/dasd.h>
@@ -300,6 +302,28 @@ static int probe_physical_blocksize(int fd, unsigned int 
*blk_size)
 #endif
 }
 
+/*
+ * Returns true if no alignment restrictions are necessary even for files
+ * opened with O_DIRECT.
+ *
+ * raw_probe_alignment() probes the required alignment and assume that 1 means
+ * the probing failed, so it falls back to a safe default of 4k. This can be
+ * avoided if we know that byte alignment is okay for the file.
+ */
+static bool dio_byte_aligned(int fd)
+{
+#ifdef __linux__
+    struct statfs buf;
+    int ret;
+
+    ret = fstatfs(fd, &buf);
+    if (ret == 0 && buf.f_type == NFS_SUPER_MAGIC) {
+        return true;
+    }
+#endif
+    return false;
+}
+
 /* Check if read is allowed with given memory buffer and length.
  *
  * This function is used to check O_DIRECT memory buffer and request alignment.
@@ -629,7 +653,7 @@ static int raw_open_common(BlockDriverState *bs, QDict 
*options,
 
     s->has_discard = true;
     s->has_write_zeroes = true;
-    if ((bs->open_flags & BDRV_O_NOCACHE) != 0) {
+    if ((bs->open_flags & BDRV_O_NOCACHE) != 0 && !dio_byte_aligned(s->fd)) {
         s->needs_alignment = true;
     }
 
--
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