[Qemu-devel] [PATCH 0/2] vhost-user: Back SET/GET_CONFIG with a protocol feature

[Qemu-devel] [PATCH 0/2] vhost-user: Back SET/GET_CONFIG with a protocol feature

[Maxime Coquelin]

Hi,

While reviewing DPDK series adding support to VHOST_USER_SET_CONFIG
and VHOST_USER_GET_CONFIG request, I found that it was not backed
with a dedicated protocol feature.

This series addresses this by adding a new protocol feature bit,
and by only negotiating it if the device supports it, as suggested
by Michael. Indeed, if the feature is supported by other type of
devices in the future, it would confuse the backends as it couldn't
know whether the device really support it or not.

To know whether the vhost device support config feature, the trick
is to check whether it implemented the config_ops. That's the 
reason why the first patch moves setting the config ops in
vhost-user-blk befoire calling vhost_user_init().

The series targets v2.12 release, else we may have to disable these
requests in this release.

*NOTE*: The series has only been tested as I don't have the
environment to try it. Changpeng, can you please test it?

Thanks,
Maxime

Maxime Coquelin (2):
  vhost-user-blk: set config ops before vhost-user init
  vhost-user: back SET/GET_CONFIG requests with a protocol feature

 docs/interop/vhost-user.txt | 21 ++++++++++++---------
 hw/block/vhost-user-blk.c   |  4 ++--
 hw/virtio/vhost-user.c      | 17 +++++++++++++++++
 3 files changed, 31 insertions(+), 11 deletions(-)

-- 
2.14.3

[Qemu-devel] [PATCH 1/2] vhost-user-blk: set config ops before vhost-user init

[Maxime Coquelin]

As soon as vhost-user init is done, the backend may send
VHOST_USER_SLAVE_CONFIG_CHANGE_MSG, so let's set the
notification callback before it.

Also, it will be used to know whether the device supports
the config feature to advertize it or not.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
 hw/block/vhost-user-blk.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index f840f07dfe..262baca432 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -259,6 +259,8 @@ static void vhost_user_blk_device_realize(DeviceState *dev, Error **errp)
     s->dev.vq_index = 0;
     s->dev.backend_features = 0;
 
+    vhost_dev_set_config_notifier(&s->dev, &blk_ops);
+
     ret = vhost_dev_init(&s->dev, &s->chardev, VHOST_BACKEND_TYPE_USER, 0);
     if (ret < 0) {
         error_setg(errp, "vhost-user-blk: vhost initialization failed: %s",
@@ -277,8 +279,6 @@ static void vhost_user_blk_device_realize(DeviceState *dev, Error **errp)
         s->blkcfg.num_queues = s->num_queues;
     }
 
-    vhost_dev_set_config_notifier(&s->dev, &blk_ops);
-
     return;
 
 vhost_err:
-- 
2.14.3

[Qemu-devel] [PATCH 2/2] vhost-user: back SET/GET_CONFIG requests with a protocol feature

[Maxime Coquelin]

Without a dedicated protocol feature, QEMU cannot know whether
the backend can handle VHOST_USER_SET_CONFIG and
VHOST_USER_GET_CONFIG messages.

This patch adds a protocol feature that is only advertised by
QEMU if the device implements the config ops. The backend
should only send VHOST_USER_SLAVE_CONFIG_CHANGE_MSG requests
if the protocol feature has been negotiated.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
 docs/interop/vhost-user.txt | 21 ++++++++++++---------
 hw/virtio/vhost-user.c      | 17 +++++++++++++++++
 2 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/docs/interop/vhost-user.txt b/docs/interop/vhost-user.txt
index c058c407df..534caab18a 100644
--- a/docs/interop/vhost-user.txt
+++ b/docs/interop/vhost-user.txt
@@ -379,6 +379,7 @@ Protocol features
 #define VHOST_USER_PROTOCOL_F_CROSS_ENDIAN   6
 #define VHOST_USER_PROTOCOL_F_CRYPTO_SESSION 7
 #define VHOST_USER_PROTOCOL_F_PAGEFAULT      8
+#define VHOST_USER_PROTOCOL_F_CONFIG         9
 
 Master message types
 --------------------
@@ -664,7 +665,8 @@ Master message types
       Master payload: virtio device config space
       Slave payload: virtio device config space
 
-      Submitted by the vhost-user master to fetch the contents of the virtio
+      When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, this message is
+      submitted by the vhost-user master to fetch the contents of the virtio
       device configuration space, vhost-user slave's payload size MUST match
       master's request, vhost-user slave uses zero length of payload to
       indicate an error to vhost-user master. The vhost-user master may
@@ -677,7 +679,8 @@ Master message types
       Master payload: virtio device config space
       Slave payload: N/A
 
-      Submitted by the vhost-user master when the Guest changes the virtio
+      When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, this message is
+      submitted by the vhost-user master when the Guest changes the virtio
       device configuration space and also can be used for live migration
       on the destination host. The vhost-user slave must check the flags
       field, and slaves MUST NOT accept SET_CONFIG for read-only
@@ -766,13 +769,13 @@ Slave message types
      Slave payload: N/A
      Master payload: N/A
 
-     Vhost-user slave sends such messages to notify that the virtio device's
-     configuration space has changed, for those host devices which can support
-     such feature, host driver can send VHOST_USER_GET_CONFIG message to slave
-     to get the latest content. If VHOST_USER_PROTOCOL_F_REPLY_ACK is
-     negotiated, and slave set the VHOST_USER_NEED_REPLY flag, master must
-     respond with zero when operation is successfully completed, or non-zero
-     otherwise.
+     When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, vhost-user slave sends
+     such messages to notify that the virtio device's configuration space has
+     changed, for those host devices which can support such feature, host
+     driver can send VHOST_USER_GET_CONFIG message to slave to get the latest
+     content. If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, and slave set
+     the VHOST_USER_NEED_REPLY flag, master must respond with zero when
+     operation is successfully completed, or non-zero otherwise.
 
 VHOST_USER_PROTOCOL_F_REPLY_ACK:
 -------------------------------
diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index 44aea5c0a8..a045203b26 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -46,6 +46,7 @@ enum VhostUserProtocolFeature {
     VHOST_USER_PROTOCOL_F_CROSS_ENDIAN = 6,
     VHOST_USER_PROTOCOL_F_CRYPTO_SESSION = 7,
     VHOST_USER_PROTOCOL_F_PAGEFAULT = 8,
+    VHOST_USER_PROTOCOL_F_CONFIG = 9,
     VHOST_USER_PROTOCOL_F_MAX
 };
 
@@ -1211,6 +1212,12 @@ static int vhost_user_init(struct vhost_dev *dev, void *opaque)
 
         dev->protocol_features =
             protocol_features & VHOST_USER_PROTOCOL_FEATURE_MASK;
+
+        if (!dev->config_ops || !dev->config_ops->vhost_dev_config_notifier) {
+            /* Dont acknowledge CONFIG feature if device doesn't support it */
+            dev->protocol_features &= ~(1ULL << VHOST_USER_PROTOCOL_F_CONFIG);
+        }
+
         err = vhost_user_set_protocol_features(dev, dev->protocol_features);
         if (err < 0) {
             return err;
@@ -1405,6 +1412,11 @@ static int vhost_user_get_config(struct vhost_dev *dev, uint8_t *config,
         .hdr.size = VHOST_USER_CONFIG_HDR_SIZE + config_len,
     };
 
+    if (!virtio_has_feature(dev->protocol_features,
+                VHOST_USER_PROTOCOL_F_CONFIG)) {
+        return -1;
+    }
+
     if (config_len > VHOST_USER_MAX_CONFIG_SIZE) {
         return -1;
     }
@@ -1448,6 +1460,11 @@ static int vhost_user_set_config(struct vhost_dev *dev, const uint8_t *data,
         .hdr.size = VHOST_USER_CONFIG_HDR_SIZE + size,
     };
 
+    if (!virtio_has_feature(dev->protocol_features,
+                VHOST_USER_PROTOCOL_F_CONFIG)) {
+        return -1;
+    }
+
     if (reply_supported) {
         msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
     }
-- 
2.14.3

Re: [Qemu-devel] [PATCH 0/2] vhost-user: Back SET/GET_CONFIG with a protocol feature

[Maxime Coquelin]

On 03/28/2018 05:56 PM, Maxime Coquelin wrote:
> Hi,
> 
> While reviewing DPDK series adding support to VHOST_USER_SET_CONFIG
> and VHOST_USER_GET_CONFIG request, I found that it was not backed
> with a dedicated protocol feature.
> 
> This series addresses this by adding a new protocol feature bit,
> and by only negotiating it if the device supports it, as suggested
> by Michael. Indeed, if the feature is supported by other type of
> devices in the future, it would confuse the backends as it couldn't
> know whether the device really support it or not.
> 
> To know whether the vhost device support config feature, the trick
> is to check whether it implemented the config_ops. That's the
> reason why the first patch moves setting the config ops in
> vhost-user-blk befoire calling vhost_user_init().
> 
> The series targets v2.12 release, else we may have to disable these
> requests in this release.
> 
> *NOTE*: The series has only been tested as I don't have the

s/tested/build tested/

> environment to try it. Changpeng, can you please test it?
> 
> Thanks,
> Maxime
> 
> Maxime Coquelin (2):
>    vhost-user-blk: set config ops before vhost-user init
>    vhost-user: back SET/GET_CONFIG requests with a protocol feature
> 
>   docs/interop/vhost-user.txt | 21 ++++++++++++---------
>   hw/block/vhost-user-blk.c   |  4 ++--
>   hw/virtio/vhost-user.c      | 17 +++++++++++++++++
>   3 files changed, 31 insertions(+), 11 deletions(-)
> 

Re: [Qemu-devel] [PATCH 2/2] vhost-user: back SET/GET_CONFIG requests with a protocol feature

[Michael S. Tsirkin]

On Wed, Mar 28, 2018 at 05:56:57PM +0200, Maxime Coquelin wrote:
> Without a dedicated protocol feature, QEMU cannot know whether
> the backend can handle VHOST_USER_SET_CONFIG and
> VHOST_USER_GET_CONFIG messages.
> 
> This patch adds a protocol feature that is only advertised by
> QEMU if the device implements the config ops. The backend
> should only send VHOST_USER_SLAVE_CONFIG_CHANGE_MSG requests
> if the protocol feature has been negotiated.
> 
> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>

I presume vhost user blk should fail init if the
protocol feature isn't negotiated then.

> ---
>  docs/interop/vhost-user.txt | 21 ++++++++++++---------
>  hw/virtio/vhost-user.c      | 17 +++++++++++++++++
>  2 files changed, 29 insertions(+), 9 deletions(-)
> 
> diff --git a/docs/interop/vhost-user.txt b/docs/interop/vhost-user.txt
> index c058c407df..534caab18a 100644
> --- a/docs/interop/vhost-user.txt
> +++ b/docs/interop/vhost-user.txt
> @@ -379,6 +379,7 @@ Protocol features
>  #define VHOST_USER_PROTOCOL_F_CROSS_ENDIAN   6
>  #define VHOST_USER_PROTOCOL_F_CRYPTO_SESSION 7
>  #define VHOST_USER_PROTOCOL_F_PAGEFAULT      8
> +#define VHOST_USER_PROTOCOL_F_CONFIG         9
>  
>  Master message types
>  --------------------
> @@ -664,7 +665,8 @@ Master message types
>        Master payload: virtio device config space
>        Slave payload: virtio device config space
>  
> -      Submitted by the vhost-user master to fetch the contents of the virtio
> +      When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, this message is
> +      submitted by the vhost-user master to fetch the contents of the virtio
>        device configuration space, vhost-user slave's payload size MUST match
>        master's request, vhost-user slave uses zero length of payload to
>        indicate an error to vhost-user master. The vhost-user master may
> @@ -677,7 +679,8 @@ Master message types
>        Master payload: virtio device config space
>        Slave payload: N/A
>  
> -      Submitted by the vhost-user master when the Guest changes the virtio
> +      When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, this message is
> +      submitted by the vhost-user master when the Guest changes the virtio
>        device configuration space and also can be used for live migration
>        on the destination host. The vhost-user slave must check the flags
>        field, and slaves MUST NOT accept SET_CONFIG for read-only
> @@ -766,13 +769,13 @@ Slave message types
>       Slave payload: N/A
>       Master payload: N/A
>  
> -     Vhost-user slave sends such messages to notify that the virtio device's
> -     configuration space has changed, for those host devices which can support
> -     such feature, host driver can send VHOST_USER_GET_CONFIG message to slave
> -     to get the latest content. If VHOST_USER_PROTOCOL_F_REPLY_ACK is
> -     negotiated, and slave set the VHOST_USER_NEED_REPLY flag, master must
> -     respond with zero when operation is successfully completed, or non-zero
> -     otherwise.
> +     When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, vhost-user slave sends
> +     such messages to notify that the virtio device's configuration space has
> +     changed, for those host devices which can support such feature, host
> +     driver can send VHOST_USER_GET_CONFIG message to slave to get the latest
> +     content. If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, and slave set
> +     the VHOST_USER_NEED_REPLY flag, master must respond with zero when
> +     operation is successfully completed, or non-zero otherwise.
>  
>  VHOST_USER_PROTOCOL_F_REPLY_ACK:
>  -------------------------------
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index 44aea5c0a8..a045203b26 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -46,6 +46,7 @@ enum VhostUserProtocolFeature {
>      VHOST_USER_PROTOCOL_F_CROSS_ENDIAN = 6,
>      VHOST_USER_PROTOCOL_F_CRYPTO_SESSION = 7,
>      VHOST_USER_PROTOCOL_F_PAGEFAULT = 8,
> +    VHOST_USER_PROTOCOL_F_CONFIG = 9,
>      VHOST_USER_PROTOCOL_F_MAX
>  };
>  
> @@ -1211,6 +1212,12 @@ static int vhost_user_init(struct vhost_dev *dev, void *opaque)
>  
>          dev->protocol_features =
>              protocol_features & VHOST_USER_PROTOCOL_FEATURE_MASK;
> +
> +        if (!dev->config_ops || !dev->config_ops->vhost_dev_config_notifier) {
> +            /* Dont acknowledge CONFIG feature if device doesn't support it */
> +            dev->protocol_features &= ~(1ULL << VHOST_USER_PROTOCOL_F_CONFIG);
> +        }
> +
>          err = vhost_user_set_protocol_features(dev, dev->protocol_features);
>          if (err < 0) {
>              return err;
> @@ -1405,6 +1412,11 @@ static int vhost_user_get_config(struct vhost_dev *dev, uint8_t *config,
>          .hdr.size = VHOST_USER_CONFIG_HDR_SIZE + config_len,
>      };
>  
> +    if (!virtio_has_feature(dev->protocol_features,
> +                VHOST_USER_PROTOCOL_F_CONFIG)) {
> +        return -1;
> +    }
> +
>      if (config_len > VHOST_USER_MAX_CONFIG_SIZE) {
>          return -1;
>      }
> @@ -1448,6 +1460,11 @@ static int vhost_user_set_config(struct vhost_dev *dev, const uint8_t *data,
>          .hdr.size = VHOST_USER_CONFIG_HDR_SIZE + size,
>      };
>  
> +    if (!virtio_has_feature(dev->protocol_features,
> +                VHOST_USER_PROTOCOL_F_CONFIG)) {
> +        return -1;
> +    }
> +
>      if (reply_supported) {
>          msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
>      }
> -- 
> 2.14.3

Re: [Qemu-devel] [PATCH 2/2] vhost-user: back SET/GET_CONFIG requests with a protocol feature

[Maxime Coquelin]

On 03/28/2018 06:55 PM, Michael S. Tsirkin wrote:
> On Wed, Mar 28, 2018 at 05:56:57PM +0200, Maxime Coquelin wrote:
>> Without a dedicated protocol feature, QEMU cannot know whether
>> the backend can handle VHOST_USER_SET_CONFIG and
>> VHOST_USER_GET_CONFIG messages.
>>
>> This patch adds a protocol feature that is only advertised by
>> QEMU if the device implements the config ops. The backend
>> should only send VHOST_USER_SLAVE_CONFIG_CHANGE_MSG requests
>> if the protocol feature has been negotiated.
>>
>> Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> 
> I presume vhost user blk should fail init if the
> protocol feature isn't negotiated then.

I did that and finally removed it.
In the future, if for example we add config support for net device,
we will want init to succeed even with old backend version that
does not support it, right?

For the vhost user blk case, its init will fail right after,
because it tries to get config, but will get an error instead.

As we only have vhost-user-blk supporting it for now, and since it
is a mandatory feature, I fine to post a v2 that makes
vhost_user_init() to fail.

> 
>> ---
>>   docs/interop/vhost-user.txt | 21 ++++++++++++---------
>>   hw/virtio/vhost-user.c      | 17 +++++++++++++++++
>>   2 files changed, 29 insertions(+), 9 deletions(-)
>>
>> diff --git a/docs/interop/vhost-user.txt b/docs/interop/vhost-user.txt
>> index c058c407df..534caab18a 100644
>> --- a/docs/interop/vhost-user.txt
>> +++ b/docs/interop/vhost-user.txt
>> @@ -379,6 +379,7 @@ Protocol features
>>   #define VHOST_USER_PROTOCOL_F_CROSS_ENDIAN   6
>>   #define VHOST_USER_PROTOCOL_F_CRYPTO_SESSION 7
>>   #define VHOST_USER_PROTOCOL_F_PAGEFAULT      8
>> +#define VHOST_USER_PROTOCOL_F_CONFIG         9
>>   
>>   Master message types
>>   --------------------
>> @@ -664,7 +665,8 @@ Master message types
>>         Master payload: virtio device config space
>>         Slave payload: virtio device config space
>>   
>> -      Submitted by the vhost-user master to fetch the contents of the virtio
>> +      When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, this message is
>> +      submitted by the vhost-user master to fetch the contents of the virtio
>>         device configuration space, vhost-user slave's payload size MUST match
>>         master's request, vhost-user slave uses zero length of payload to
>>         indicate an error to vhost-user master. The vhost-user master may
>> @@ -677,7 +679,8 @@ Master message types
>>         Master payload: virtio device config space
>>         Slave payload: N/A
>>   
>> -      Submitted by the vhost-user master when the Guest changes the virtio
>> +      When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, this message is
>> +      submitted by the vhost-user master when the Guest changes the virtio
>>         device configuration space and also can be used for live migration
>>         on the destination host. The vhost-user slave must check the flags
>>         field, and slaves MUST NOT accept SET_CONFIG for read-only
>> @@ -766,13 +769,13 @@ Slave message types
>>        Slave payload: N/A
>>        Master payload: N/A
>>   
>> -     Vhost-user slave sends such messages to notify that the virtio device's
>> -     configuration space has changed, for those host devices which can support
>> -     such feature, host driver can send VHOST_USER_GET_CONFIG message to slave
>> -     to get the latest content. If VHOST_USER_PROTOCOL_F_REPLY_ACK is
>> -     negotiated, and slave set the VHOST_USER_NEED_REPLY flag, master must
>> -     respond with zero when operation is successfully completed, or non-zero
>> -     otherwise.
>> +     When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, vhost-user slave sends
>> +     such messages to notify that the virtio device's configuration space has
>> +     changed, for those host devices which can support such feature, host
>> +     driver can send VHOST_USER_GET_CONFIG message to slave to get the latest
>> +     content. If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, and slave set
>> +     the VHOST_USER_NEED_REPLY flag, master must respond with zero when
>> +     operation is successfully completed, or non-zero otherwise.
>>   
>>   VHOST_USER_PROTOCOL_F_REPLY_ACK:
>>   -------------------------------
>> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
>> index 44aea5c0a8..a045203b26 100644
>> --- a/hw/virtio/vhost-user.c
>> +++ b/hw/virtio/vhost-user.c
>> @@ -46,6 +46,7 @@ enum VhostUserProtocolFeature {
>>       VHOST_USER_PROTOCOL_F_CROSS_ENDIAN = 6,
>>       VHOST_USER_PROTOCOL_F_CRYPTO_SESSION = 7,
>>       VHOST_USER_PROTOCOL_F_PAGEFAULT = 8,
>> +    VHOST_USER_PROTOCOL_F_CONFIG = 9,
>>       VHOST_USER_PROTOCOL_F_MAX
>>   };
>>   
>> @@ -1211,6 +1212,12 @@ static int vhost_user_init(struct vhost_dev *dev, void *opaque)
>>   
>>           dev->protocol_features =
>>               protocol_features & VHOST_USER_PROTOCOL_FEATURE_MASK;
>> +
>> +        if (!dev->config_ops || !dev->config_ops->vhost_dev_config_notifier) {
>> +            /* Dont acknowledge CONFIG feature if device doesn't support it */
>> +            dev->protocol_features &= ~(1ULL << VHOST_USER_PROTOCOL_F_CONFIG);
>> +        }
>> +
>>           err = vhost_user_set_protocol_features(dev, dev->protocol_features);
>>           if (err < 0) {
>>               return err;
>> @@ -1405,6 +1412,11 @@ static int vhost_user_get_config(struct vhost_dev *dev, uint8_t *config,
>>           .hdr.size = VHOST_USER_CONFIG_HDR_SIZE + config_len,
>>       };
>>   
>> +    if (!virtio_has_feature(dev->protocol_features,
>> +                VHOST_USER_PROTOCOL_F_CONFIG)) {
>> +        return -1;
>> +    }
>> +
>>       if (config_len > VHOST_USER_MAX_CONFIG_SIZE) {
>>           return -1;
>>       }
>> @@ -1448,6 +1460,11 @@ static int vhost_user_set_config(struct vhost_dev *dev, const uint8_t *data,
>>           .hdr.size = VHOST_USER_CONFIG_HDR_SIZE + size,
>>       };
>>   
>> +    if (!virtio_has_feature(dev->protocol_features,
>> +                VHOST_USER_PROTOCOL_F_CONFIG)) {
>> +        return -1;
>> +    }
>> +
>>       if (reply_supported) {
>>           msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
>>       }
>> -- 
>> 2.14.3

Re: [Qemu-devel] [PATCH 2/2] vhost-user: back SET/GET_CONFIG requests with a protocol feature

[Michael S. Tsirkin]

On Wed, Mar 28, 2018 at 07:08:32PM +0200, Maxime Coquelin wrote:
> 
> 
> On 03/28/2018 06:55 PM, Michael S. Tsirkin wrote:
> > On Wed, Mar 28, 2018 at 05:56:57PM +0200, Maxime Coquelin wrote:
> > > Without a dedicated protocol feature, QEMU cannot know whether
> > > the backend can handle VHOST_USER_SET_CONFIG and
> > > VHOST_USER_GET_CONFIG messages.
> > > 
> > > This patch adds a protocol feature that is only advertised by
> > > QEMU if the device implements the config ops. The backend
> > > should only send VHOST_USER_SLAVE_CONFIG_CHANGE_MSG requests
> > > if the protocol feature has been negotiated.
> > > 
> > > Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> > 
> > I presume vhost user blk should fail init if the
> > protocol feature isn't negotiated then.
> 
> I did that and finally removed it.
> In the future, if for example we add config support for net device,
> we will want init to succeed even with old backend version that
> does not support it, right?
> 
> For the vhost user blk case, its init will fail right after,
> because it tries to get config, but will get an error instead.
> 
> As we only have vhost-user-blk supporting it for now, and since it
> is a mandatory feature, I fine to post a v2 that makes
> vhost_user_init() to fail.

Seems safer. We can remove restrictions but not add new ones.

> > 
> > > ---
> > >   docs/interop/vhost-user.txt | 21 ++++++++++++---------
> > >   hw/virtio/vhost-user.c      | 17 +++++++++++++++++
> > >   2 files changed, 29 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/docs/interop/vhost-user.txt b/docs/interop/vhost-user.txt
> > > index c058c407df..534caab18a 100644
> > > --- a/docs/interop/vhost-user.txt
> > > +++ b/docs/interop/vhost-user.txt
> > > @@ -379,6 +379,7 @@ Protocol features
> > >   #define VHOST_USER_PROTOCOL_F_CROSS_ENDIAN   6
> > >   #define VHOST_USER_PROTOCOL_F_CRYPTO_SESSION 7
> > >   #define VHOST_USER_PROTOCOL_F_PAGEFAULT      8
> > > +#define VHOST_USER_PROTOCOL_F_CONFIG         9
> > >   Master message types
> > >   --------------------
> > > @@ -664,7 +665,8 @@ Master message types
> > >         Master payload: virtio device config space
> > >         Slave payload: virtio device config space
> > > -      Submitted by the vhost-user master to fetch the contents of the virtio
> > > +      When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, this message is
> > > +      submitted by the vhost-user master to fetch the contents of the virtio
> > >         device configuration space, vhost-user slave's payload size MUST match
> > >         master's request, vhost-user slave uses zero length of payload to
> > >         indicate an error to vhost-user master. The vhost-user master may
> > > @@ -677,7 +679,8 @@ Master message types
> > >         Master payload: virtio device config space
> > >         Slave payload: N/A
> > > -      Submitted by the vhost-user master when the Guest changes the virtio
> > > +      When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, this message is
> > > +      submitted by the vhost-user master when the Guest changes the virtio
> > >         device configuration space and also can be used for live migration
> > >         on the destination host. The vhost-user slave must check the flags
> > >         field, and slaves MUST NOT accept SET_CONFIG for read-only
> > > @@ -766,13 +769,13 @@ Slave message types
> > >        Slave payload: N/A
> > >        Master payload: N/A
> > > -     Vhost-user slave sends such messages to notify that the virtio device's
> > > -     configuration space has changed, for those host devices which can support
> > > -     such feature, host driver can send VHOST_USER_GET_CONFIG message to slave
> > > -     to get the latest content. If VHOST_USER_PROTOCOL_F_REPLY_ACK is
> > > -     negotiated, and slave set the VHOST_USER_NEED_REPLY flag, master must
> > > -     respond with zero when operation is successfully completed, or non-zero
> > > -     otherwise.
> > > +     When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, vhost-user slave sends
> > > +     such messages to notify that the virtio device's configuration space has
> > > +     changed, for those host devices which can support such feature, host
> > > +     driver can send VHOST_USER_GET_CONFIG message to slave to get the latest
> > > +     content. If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, and slave set
> > > +     the VHOST_USER_NEED_REPLY flag, master must respond with zero when
> > > +     operation is successfully completed, or non-zero otherwise.
> > >   VHOST_USER_PROTOCOL_F_REPLY_ACK:
> > >   -------------------------------
> > > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> > > index 44aea5c0a8..a045203b26 100644
> > > --- a/hw/virtio/vhost-user.c
> > > +++ b/hw/virtio/vhost-user.c
> > > @@ -46,6 +46,7 @@ enum VhostUserProtocolFeature {
> > >       VHOST_USER_PROTOCOL_F_CROSS_ENDIAN = 6,
> > >       VHOST_USER_PROTOCOL_F_CRYPTO_SESSION = 7,
> > >       VHOST_USER_PROTOCOL_F_PAGEFAULT = 8,
> > > +    VHOST_USER_PROTOCOL_F_CONFIG = 9,
> > >       VHOST_USER_PROTOCOL_F_MAX
> > >   };
> > > @@ -1211,6 +1212,12 @@ static int vhost_user_init(struct vhost_dev *dev, void *opaque)
> > >           dev->protocol_features =
> > >               protocol_features & VHOST_USER_PROTOCOL_FEATURE_MASK;
> > > +
> > > +        if (!dev->config_ops || !dev->config_ops->vhost_dev_config_notifier) {
> > > +            /* Dont acknowledge CONFIG feature if device doesn't support it */
> > > +            dev->protocol_features &= ~(1ULL << VHOST_USER_PROTOCOL_F_CONFIG);
> > > +        }
> > > +
> > >           err = vhost_user_set_protocol_features(dev, dev->protocol_features);
> > >           if (err < 0) {
> > >               return err;
> > > @@ -1405,6 +1412,11 @@ static int vhost_user_get_config(struct vhost_dev *dev, uint8_t *config,
> > >           .hdr.size = VHOST_USER_CONFIG_HDR_SIZE + config_len,
> > >       };
> > > +    if (!virtio_has_feature(dev->protocol_features,
> > > +                VHOST_USER_PROTOCOL_F_CONFIG)) {
> > > +        return -1;
> > > +    }
> > > +
> > >       if (config_len > VHOST_USER_MAX_CONFIG_SIZE) {
> > >           return -1;
> > >       }
> > > @@ -1448,6 +1460,11 @@ static int vhost_user_set_config(struct vhost_dev *dev, const uint8_t *data,
> > >           .hdr.size = VHOST_USER_CONFIG_HDR_SIZE + size,
> > >       };
> > > +    if (!virtio_has_feature(dev->protocol_features,
> > > +                VHOST_USER_PROTOCOL_F_CONFIG)) {
> > > +        return -1;
> > > +    }
> > > +
> > >       if (reply_supported) {
> > >           msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
> > >       }
> > > -- 
> > > 2.14.3