On Wed, Aug 26, 2009 at 03:40:59PM +0200, Arnd Bergmann wrote:
> On Tuesday 25 August 2009, Michael S. Tsirkin wrote:
> > > 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[];
>
> Hmm, variable length ioctl arguments, I'd rather not go there.
> The ioctl infrastructure already has a length argument encoded
> in the ioctl number. We can use that if we need more, e.g.
>
> /* now */
> #define VHOST_GET_FEATURES _IOR(VHOST_VIRTIO, 0x00, __u64)
> /*
> * uncomment if we run out of feature bits:
>
> struct vhost_get_features2 {
> __u64 bits[2];
> };
> #define VHOST_GET_FEATURES2 _IOR(VHOST_VIRTIO, 0x00, \
> struct vhost_get_features2)
> */
>
> > Thinking about this proposal some more, how will the guest
> > determine the size to supply the GET_FEATURES ioctl?
>
> Wait, the *guest*?
Sorry. the userspace hypervisor.
> Maybe I misunderstood something in a major way here, but
> I expected the features to be negotiated between host
> user space (qemu) and host kernel, as well as between
> guest and qemu (as they are already), but never between
> guest and kernel.
Yes.
> I would certainly expect the bits to be distinct from
> the virtio-net feature bits. E.g. stuff like TAP frame
> format opposed to TCP socket frame format (length+date)
> is something we need to negotiate here but that the
> guest does not care about.
My idea is to use virtio format for things I share with virtio (e.g.
mergeable buffers). Since we are a kind of transport, I thought that I
will use the transport bits, that is bits 28 and up for vhost things.
> > Since we are a bit tight in 32 bit space already,
> > let's just use a 64 bit integer and be done with it?
>
> Can't hurt, but don't use a struct unless you think
> we are going to need more than 64 bits.
>
> Arnd <><
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