On Mon, 9 Nov 2009 05:40:32 pm Michael S. Tsirkin wrote: > On Mon, Nov 9, 2009 at 8:17 AM, Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote: > > There's something about the 'acked' which rubs me the wrong way. > > "enabled_features" is perhaps a better term than "acked_features"; "acked" > > seems more a user point-of-view, "enabled" seems more driver POV? > > Hmm. Are you happy with the ioctl name? If yes I think being consistent > with that is important. I think in my original comments I noted that I preferred GET / SET, rather than GET/ACK. > > Actually, this looks wrong to me: > > > > + case VHOST_SET_VRING_BASE: > > ... > > + vq->avail_idx = vq->last_avail_idx = s.num; > > > > The last_avail_idx is part of the state of the driver. It needs to be saved > > and restored over susp/resume. > > > Exactly. That's what VHOST_GET/SET_VRING_BASE does. avail_idx is just a > cached value for notify on empty, so what this does is clear the value. Ah, you actually refresh it every time anyway. Hmm, could you do my poor brain a favor and either just get_user it in vhost_trigger_irq(), or call it 'cached_avail_idx' or something? > > The only reason it's not in the ring itself > > is because I figured the other side doesn't need to see it (which is true, but > > missed debugging opportunities as well as man-in-the-middle issues like this > > one). I had a patch which put this field at the end of the ring, I might > > resurrect it to avoid this problem. This is backwards compatible with all > > implementations. See patch at end. > > Yes, I remember that patch. There seems to be little point though, at > this stage. Well, it avoids this ioctl, by exposing all the state. We may well need it later, to expand the ring in other ways. > > I would drop avail_idx altogether: get_user is basically free, and simplifies > > a lot. As most state is in the ring, all you need is an ioctl to save/restore > > the last_avail_idx. > > avail_idx is there for notify on empty: I had this thought that it's > better to leave the avail cache line alone when we are triggering > interrupt to avoid bouncing it around if guest is updating it meanwhile > on another CPU, and I think my testing showed that it helped > performance, but could be a mistake. You don't believe this can help? I believe it could help, but this is YA case where it would have been nice to have a dumb basic patch and this as a patch on top. But I am going to ask you to re-run that measurement, see if it stacks up (because it's an interesting lesson if it does..) Thanks! Rusty. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
