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Re: [Virtio-fs] [PATCH] vhost-user-fs: fix features handling

From: Anton Kuchin
Subject: Re: [Virtio-fs] [PATCH] vhost-user-fs: fix features handling
Date: Sun, 11 Apr 2021 09:21:54 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1

On 09/04/2021 18:56, Vivek Goyal wrote:
On Thu, Apr 08, 2021 at 10:55:34PM +0300, Anton Kuchin wrote:
Make virtio-fs take into account server capabilities.

Just returning requested features assumes they all of then are implemented
by server and results in setting unsupported configuration if some of them
are absent.

Signed-off-by: Anton Kuchin<antonkuchin@yandex-team.ru>
[CC stefan and qemu-devel.]

Can you give more details of what problem exactly you are facing. Or
this fix is about avoiding a future problem where device can refuse
to support a feature qemu is requesting for.

This fixes existing problem that qemu ignores features (un)supported by backend and this works fine only if backend features match features of qemu. Otherwise qemu incorrectly assumes that backend suports all of them and calls vhost_set_features() not taking into account result of previous vhost_get_features() call. This breaks protocol and can crash server or cause incorrect behavior.

This is why I hope it to be accepted in time for 6.0 release.

IIUC, this patch is preparing a list of features vhost-user-fs device
can support. Then it calls vhost_get_features() which makes sure that
all these features are support by real vhost-user device (hdev->features).
If not, then corresponding feature is reset and remaining features
are returned to caller.
When this callback is executed in virtio_bus_device_plugged() list of features that vhost-backend supports has been already obtained earlier by vhost_user_get_features() in vuf_device_realize() and stored in hdev->features. vuf_get_features() should return bitmask of features that are common for vhost backend (hdev->features) and frontend (vdev->host_features). But instead it just returns host features.
This feature negotion bit is called in so many places that I am kind of
lost that who should be doing what. Will leave it to Stefan who
understands it much better.

  hw/virtio/vhost-user-fs.c | 17 +++++++++++++----
  1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/hw/virtio/vhost-user-fs.c b/hw/virtio/vhost-user-fs.c
index ac4fc34b36..6cf983ba0e 100644
--- a/hw/virtio/vhost-user-fs.c
+++ b/hw/virtio/vhost-user-fs.c
@@ -24,6 +24,14 @@
  #include "monitor/monitor.h"
  #include "sysemu/sysemu.h"
+static const int user_feature_bits[] = {
  static void vuf_get_config(VirtIODevice *vdev, uint8_t *config)
      VHostUserFS *fs = VHOST_USER_FS(vdev);
@@ -129,11 +137,12 @@ static void vuf_set_status(VirtIODevice *vdev, uint8_t 
static uint64_t vuf_get_features(VirtIODevice *vdev,
-                                      uint64_t requested_features,
-                                      Error **errp)
+                                 uint64_t features,
Will it make sense to keep the name requested_features. This kind of
makes it clear that caller is requesting these features.

I feel there should be few lines of comments also to make it clear
what this function is actually doing.

This fix was inspired by similar functions in hw/scsi/vhost-scsi-common.c, hw/virtio/vhost-user-vsock.c, hw/block/vhost-user-blk.c and hw/net/vhost_net.c and I borrowed naming from them just to be consistent.

IMO this looks like a good place for refactoring because we have more and more vhost-user devices that duplicate that code, but it can be done after the bug is fixed.


+                                 Error **errp)
-    /* No feature bits used yet */
-    return requested_features;
+    VHostUserFS *fs = VHOST_USER_FS(vdev);
+    return vhost_get_features(&fs->vhost_dev, user_feature_bits, features);
static void vuf_handle_output(VirtIODevice *vdev, VirtQueue *vq)

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