Subject: Re: [PATCHv4 2/2] vhost_net: a kernel-level virtio server

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 25 Aug 2009 20:50:16 +0300

On Tue, Aug 25, 2009 at 09:40:40PM +0930, Rusty Russell wrote:
> > +	u32 __user *featurep = argp;
> > +	int __user *fdp = argp;
> > +	u32 features;
> > +	int fd, r;
> > +	switch (ioctl) {
> > +	case VHOST_NET_SET_SOCKET:
> > +		r = get_user(fd, fdp);
> > +		if (r < 0)
> > +			return r;
> > +		return vhost_net_set_socket(n, fd);
> > +	case VHOST_GET_FEATURES:
> > +		/* No features for now */
> > +		features = 0;
> > +		return put_user(features, featurep);
> 
> We may well get more than 32 feature bits, at least for virtio_net, which will
> force us to do some trickery in virtio_pci.

Unlike PCI, if we ever run out of bits we can just
add FEATURES_EXTENDED ioctl, no need for trickery.

>  I'd like to avoid that here,
> though it's kind of ugly.  We'd need VHOST_GET_FEATURES (and ACK) to take a
> struct like:
> 
> 	u32 feature_size;
> 	u32 features[];

Thinking about this proposal some more, how will the guest
determine the size to supply the GET_FEATURES ioctl?

Since we are a bit tight in 32 bit space already,
let's just use a 64 bit integer and be done with it?
Right?

-- 
MST