Re: [Qemu-devel] [PATCH v4 1/4] vhost-user: add new vhost user messages to support virtio config space

[Stefan Hajnoczi]

On Thu, Oct 19, 2017 at 06:36:00PM +0300, Michael S. Tsirkin wrote:
> On Thu, Oct 19, 2017 at 04:09:35PM +0200, Stefan Hajnoczi wrote:
> > On Thu, Oct 19, 2017 at 01:24:07PM +0800, Changpeng Liu wrote:
> > > @@ -922,6 +931,91 @@ static void vhost_user_set_iotlb_callback(struct 
> > > vhost_dev *dev, int enabled)
> > >      /* No-op as the receive channel is not dedicated to IOTLB messages. 
> > > */
> > >  }
> > >  
> > > +static int vhost_user_get_config(struct vhost_dev *dev, uint8_t *config,
> > > +                                 size_t config_len)
> > > +{
> > > +    VhostUserMsg msg = {
> > > +        .request = VHOST_USER_GET_CONFIG,
> > > +        .flags = VHOST_USER_VERSION,
> > > +        .size = config_len,
> > > +    };
> > > +
> > > +    if (config_len == 0 || config_len > VHOST_USER_PAYLOAD_SIZE) {
> > 
> > config_len should be limited to 256 bytes:
> > 
> >   if (config_len == 0 || config_len > sizeof(msg.payload.config) {
> 
> I would just limit it to a reasonable value, acceptable to
> both master and slave, not fail if it's bigger.
> 
> 
> > > +        error_report("bad config length");
> > > +        return -1;
> > > +    }
> > > +
> > > +    if (vhost_user_write(dev, &msg, NULL, 0) < 0) {
> > > +        return -1;
> > > +    }
> > > +
> > > +    if (vhost_user_read(dev, &msg) < 0) {
> > > +        return -1;
> > > +    }
> > > +
> > > +    if (msg.request != VHOST_USER_GET_CONFIG) {
> > > +        error_report("Received unexpected msg type. Expected %d received 
> > > %d",
> > > +                     VHOST_USER_GET_CONFIG, msg.request);
> > > +        return -1;
> > > +    }
> > > +
> > > +    if (msg.size != config_len) {
> > > +        error_report("Received bad msg size.");
> > > +        return -1;
> > > +    }
> > > +
> > > +    memcpy(config, &msg.payload.config, config_len);
> > 
> > There is some complexity here: different virtio devices use different
> > amounts of config space.  Devices may append new fields to the config
> > space to support new features.
> > 
> > Therefore I think the simplest protocol is to always fetch the full
> > 256-byte configuration space.  This way the vhost-user slave process can
> > implement feature bits that the master process does not know about.
> > 
> > In other words, I don't think the master process knows how much of the
> > config space is used so it should always request 256 bytes.
> 
> Each device knows the max config space size.
> 
>     vdev->config_len = config_size;

I see you're referring to the field that is set in:

  void virtio_init(VirtIODevice *vdev, const char *name,
                   uint16_t device_id, size_t config_size)

How does this work for vhost-user where different slave programs may
offer different config sizes?

The QEMU master process does not know the correct size ahead of time.
The size depends on the vhost-user slave process.  This is why I suggest
using the full 256 bytes that the VIRTIO spec defines.