virtiofsd: Add -o allow_direct_io|no_allow_direct_io options

Due to the commit 65da4539803373ec4eec97ffc49ee90083e56efd, the O_DIRECT
open flag of guest applications will be discarded by virtiofsd. While
this behavior makes it consistent with the virtio-9p scheme when guest
applications use direct I/O, we no longer have any chance to bypass the
host page cache.

Therefore, we add a flag 'allow_direct_io' to lo_data. If '-o
 no_allow_direct_io' option is added, or none of '-o allow_direct_io' or
 '-o no_allow_direct_io' is added, the 'allow_direct_io' will be set to
 0, and virtiofsd discards O_DIRECT as before. If '-o allow_direct_io'
is added to the starting command-line, 'allow_direct_io' will be set to
1, so that the O_DIRECT flags will be retained and host page cache can
be bypassed.

Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20200824105957.61265-1-zhangjiachen.jaycee@bytedance.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/tools/virtiofsd/helper.c b/tools/virtiofsd/helper.c
index 7bc5d7dc5a..85770d63f1 100644
--- a/tools/virtiofsd/helper.c
+++ b/tools/virtiofsd/helper.c
@@ -178,6 +178,10 @@ void fuse_cmdline_help(void)
            "                               (0 leaves rlimit unchanged)\n"
            "                               default: min(1000000, fs.file-max - 16384)\n"
            "                                        if the current rlimit is lower\n"
+           "    -o allow_direct_io|no_allow_direct_io\n"
+           "                               retain/discard O_DIRECT flags passed down\n"
+           "                               to virtiofsd from guest applications.\n"
+           "                               default: no_allow_direct_io\n"
            );
 }