Messages in this thread | | | Date | Tue, 21 Sep 2010 15:14:22 +0200 | From | "Michael S. Tsirkin" <> | Subject | Re: [RFC PATCH v9 12/16] Add mp(mediate passthru) device. |
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On Tue, Sep 21, 2010 at 09:39:31AM +0800, Xin, Xiaohui wrote: > >From: Michael S. Tsirkin [mailto:mst@redhat.com] > >Sent: Monday, September 20, 2010 7:37 PM > >To: Xin, Xiaohui > >Cc: netdev@vger.kernel.org; kvm@vger.kernel.org; linux-kernel@vger.kernel.org; > >mingo@elte.hu; davem@davemloft.net; herbert@gondor.hengli.com.au; > >jdike@linux.intel.com > >Subject: Re: [RFC PATCH v9 12/16] Add mp(mediate passthru) device. > > > >On Mon, Sep 20, 2010 at 04:08:48PM +0800, xiaohui.xin@intel.com wrote: > >> From: Xin Xiaohui <xiaohui.xin@intel.com> > >> > >> --- > >> 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
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