On Tue, Sep 21, 2010 at 09:39:31AM +0800, Xin, Xiaohui wrote: > >From: Michael S. Tsirkin [mailto:mst@xxxxxxxxxx] > >Sent: Monday, September 20, 2010 7:37 PM > >To: Xin, Xiaohui > >Cc: netdev@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > >mingo@xxxxxxx; davem@xxxxxxxxxxxxx; herbert@xxxxxxxxxxxxxxxxxxxx; > >jdike@xxxxxxxxxxxxxxx > >Subject: Re: [RFC PATCH v9 12/16] Add mp(mediate passthru) device. > > > >On Mon, Sep 20, 2010 at 04:08:48PM +0800, xiaohui.xin@xxxxxxxxx wrote: > >> From: Xin Xiaohui <xiaohui.xin@xxxxxxxxx> > >> > >> --- > >> Michael, > >> I have move the ioctl to configure the locked memory to vhost > > > >It's ok to move this to vhost but vhost does not > >know how much memory is needed by the backend. > > I think the backend here you mean is mp device. > Actually, the memory needed is related to vq->num to run zero-copy > smoothly. > That means mp device did not know it but vhost did. Well, this might be so if you insist on locking all posted buffers immediately. However, let's assume I have a very large ring and prepost a ton of RX buffers: there's no need to lock all of them directly: if we have buffers A and B, we can lock A, pass it to hardware, and when A is consumed unlock A, lock B and pass it to hardware. It's not really critical. But note we can always have userspace tell MP device all it wants to know, after all. > And the rlimt stuff is per process, we use current pointer to set > and check the rlimit, the operations should be in the same process. Well no, the ring is handled from the kernel thread: we switch the mm to point to the owner task so copy from/to user and friends work, but you can't access the rlimit etc. > Now the check operations are in vhost process, as mp_recvmsg() or > mp_sendmsg() are called by vhost. Hmm, what do you mean by the check operations? send/recv are data path operations, they shouldn't do any checks, should they? > So set operations should be in > vhost process too, it's natural. > > >So I think we'll need another ioctl in the backend > >to tell userspace how much memory is needed? > > > Except vhost tells it to mp device, mp did not know > how much memory is needed to run zero-copy smoothly. > Is userspace interested about the memory mp is needed? Couldn't parse this last question. I think userspace generally does want control over how much memory we'll lock. We should not just lock as much as we can. -- MST -- 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
