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Re: [Qemu-devel] device assignment for embedded Power
From: |
Benjamin Herrenschmidt |
Subject: |
Re: [Qemu-devel] device assignment for embedded Power |
Date: |
Sat, 02 Jul 2011 09:05:46 +1000 |
On Fri, 2011-07-01 at 21:59 +0100, Paul Brook wrote:
> > On Fri, 1 Jul 2011 18:03:01 +0100
> >
> > Paul Brook <address@hidden> wrote:
> > > Basically you should start by implementing full emulation of a device
> > > with similar characteristics to the one you want to passthrough.
> >
> > That's not going to happen.
>
> Why is your device so unique? How does it interact with the guest system and
> what features does it require that doen't exist in any device that can be
> emulated?
Do you guys only support PCI pass-through by doing full emulation of the
all possible supported PCI devices first ? :-)
> I'm also extremely sceptical of anything that only works in a kvm
> environment.
> Makes me think it's an unmaintainable hack, and almost certainly going to
> cause you immense amounts of pain later.
See above question...
Cheers,
Ben.
> > > I doubt you're going to get generic passthrough of arbitrary devices
> > > working in a useful way.
> >
> > It's usefully working for us internally -- we're just trying to find a way
> > to improve it for upstream, with a better configuration mechanism.
>
> I don't believe that either. More likely you've got passthrough of device
> hanging off your specific CPU bus, using only (or even a subset of) the
> facilities provided by that bus.
>
> > > Basically you have to emulate everything that is different between the
> > > host and guest.
> >
> > Directly assigning a device means you don't get to have differences between
> > the actual hardware device and what the guest sees. The kind of thin
> > wrapper you're suggesting might have some use cases, but it's a different
> > problem from what we're trying to solve.
>
> That's the problem. You've skipped several steps and gone startigh for
> optimization before you've even got basic functionality working.
>
> You've also missed the point I was making. In order to do device passthrough
> you need to define a boundary allong which the emulated machine state can be
> fully replicated on the host machine. Anything inside this boundary is (by
> definition) that same on both the host and guest systems (we're effectively
> using host hardware to emulate a device for us). Outside that boundary the
> host and guest systems will diverge.
>
> For a device that merely responds to CPU initiated MMIO transfers this is
> pretty simple, it's the point at which MMIO transfers are generated. So the
> guest gets a proxy device that intercepts accesses to that memory region, and
> the host proxies some way for qemu to poke values at the host device.
>
> > > Once you've done all the above, host device passthrough should be
> > > relatively straightforward. Just replace the emulation bits in the
> > > above device with code that pokes at a real device via the relevant
> > > kernel API.
> >
> > That's not what we mean by direct device assignment.
>
> Maybe, but IMO but it's a necessary prerequisite. You're trying to run before
> you can walk.
>
> > We're talking about directly mapping the registers into the guest. The
> > whole point is performance.
>
> That's an additional step after you get passthrough working the normal way.
> We already have mechanisms (or at least patches) for mapping file-like
> objects
> into guest physical memory. That's largely independent of device
> passthrough.
> It's a relatively minor tweak to how the passthrough device sets up its MMIO
> regions.
>
> Mapping host device MMIO regions into guest space is entirely uninteresting
> unless we already have some way of creating guest-host passthrough devices.
> Creating guest-device passthrough devices isn't going to happen until the can
> create arbitrary devices (within the set emulated by qemu) that interact with
> the rest of the emulated machine in a similar way.
>
> Paul
- Re: [Qemu-devel] device assignment for embedded Power, (continued)
- Re: [Qemu-devel] device assignment for embedded Power, Anthony Liguori, 2011/07/01
- Re: [Qemu-devel] device assignment for embedded Power, Paul Brook, 2011/07/01
- Re: [Qemu-devel] device assignment for embedded Power, Scott Wood, 2011/07/01
- Re: [Qemu-devel] device assignment for embedded Power, Paul Brook, 2011/07/01
- Re: [Qemu-devel] device assignment for embedded Power, Scott Wood, 2011/07/01
- Re: [Qemu-devel] device assignment for embedded Power, Paul Brook, 2011/07/01
- Re: [Qemu-devel] device assignment for embedded Power, Scott Wood, 2011/07/01
- Re: [Qemu-devel] device assignment for embedded Power, Paul Brook, 2011/07/01
- Re: [Qemu-devel] device assignment for embedded Power,
Benjamin Herrenschmidt <=
- Re: [Qemu-devel] device assignment for embedded Power, Paul Brook, 2011/07/01
- Re: [Qemu-devel] device assignment for embedded Power, Alexander Graf, 2011/07/01
- Re: [Qemu-devel] device assignment for embedded Power, Paul Brook, 2011/07/02
- Re: [Qemu-devel] device assignment for embedded Power, Anthony Liguori, 2011/07/01
- Re: [Qemu-devel] device assignment for embedded Power, Anthony Liguori, 2011/07/01
- Re: [Qemu-devel] device assignment for embedded Power, Scott Wood, 2011/07/05
Re: [Qemu-devel] device assignment for embedded Power, Scott Wood, 2011/07/01
Re: [Qemu-devel] device assignment for embedded Power, Yoder Stuart-B08248, 2011/07/05