Re: [Xen-devel] [Hackathon minutes] PV network improvements

[Tim Deegan <tim@xxxxxxx>]

At 10:39 +0100 on 21 May (1369132774), Wei Liu wrote:
> On Tue, May 21, 2013 at 10:26:00AM +0100, Tim Deegan wrote:
> > At 19:31 +0100 on 20 May (1369078279), Wei Liu wrote:
> > > On Mon, May 20, 2013 at 03:08:05PM +0100, Stefano Stabellini wrote:
> > > > J) Map the whole physical memory of the machine in dom0
> > > > If mapping/unmapping or copying slows us down, could we just keep the
> > > > whole physical memory of the machine mapped in dom0 (with corresponding
> > > > IOMMU entries)?
> > > > At that point the frontend could just pass mfn numbers to the backend,
> > > > and the backend would already have them mapped.
> > > > >From a security perspective it doesn't change anything when running
> > > > the backend in dom0, because dom0 is already capable of mapping random
> > > > pages of any guests. QEMU instances do that all the time.
> > > > But it would take away one of the benefits of deploying driver domains:
> > > > we wouldn't be able to run the backends at a lower privilege level.
> > > > However it might still be worth considering as an option? The backend is
> > > > still trusted and protected from the frontend, but the frontend wouldn't
> > > > be protected from the backend.
> > > > 
> > > 
> > > I think Dom0 mapping all machine memory is a good starting point.
> > 
> > I _strongly_ disagree.  The opportunity for disaggregation and reduction
> > of privilege in backends is probably Xen's biggest techical advantage
> > and we should not be taking any backward steps there.
> > 
> 
> I agree with you that disaggregation and reduction of privilege is Xen's
> biggest technical advantage.
> 
> Just to make clear, this idea was summerized from a discussion among
> George, Stefano and I on the way back from hackathon. We want to see if
> things like mapping / unmapping incur heavy performance penalty.

Ah, I see. :)  As an experiment to measure the overheads it's obviously
a Good Thing.  I thought you were considering it as a _solution_ to the
perf problem!

Cheers,

Tim.

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