Re: [Xen-devel] [v5][PATCH 0/5] xen: add Intel IGD passthrough support

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Mon, Jun 30, 2014 at 03:24:58PM +0800, Chen, Tiejun wrote:
> On 2014/6/30 14:48, Michael S. Tsirkin wrote:
> >On Mon, Jun 30, 2014 at 10:51:49AM +0800, Chen, Tiejun wrote:
> >>On 2014/6/26 18:03, Paolo Bonzini wrote:
> >>>Il 26/06/2014 11:18, Chen, Tiejun ha scritto:
> >>>>
> >>>>>
> >>>>>- offsets 0x0000..0x0fff map to configuration space of the host MCH
> >>>>>
> >>>>
> >>>>Are you saying the config space in the video device?
> >>>
> >>>No, I am saying in a new BAR, or at some magic offset of an existing
> >>>MMIO BAR.
> >>>
> >>
> >>As I mentioned previously, the IGD guy told me we have no any unused a
> >>offset or BAR in the config space.
> >>
> >>And guy who are responsible for the native driver seems not be accept to
> >>extend some magic offset of an existing MMIO BAR.
> >>
> >>In addition I think in a short time its not possible to migrate i440fx to
> >>q35 as a PCIe machine of xen.
> >
> >That seems like a weak motivation.  I don't see a need to get something
> >merged upstream in a short time: this seems sure to miss 2.1,
> >so you have the time to make it architecturally sound.
> >"Making existing guests work" would be a better motivation.
> 
> Yes.

So focus on this then. Existing guests will probably work
fine on a newer chipset - likely better than on i440fx.
xen management tools need to do some work to support this?
That will just give everyone more long term benefits.

If instead we create a hack that does not resemble
any existing hardware even remotely, what's the
chance that it will not break with any future
guest modification?


> >Isn't this possible with an mch chipset?
> 
> If you're saying q35, I mean AFAIK we have no any plan to migrate to this
> MCH in xen case.

q35 or a newer chipset that's closer to what guests expect.

> Additionally, I think I should follow this feature after
> q35 can work for xen scenario.

What's stopping you?

> >
> >
> >>So could we do this step by step:
> >>
> >>#1 phase: We just cover current qemu-xen implementation based on i44fx, so
> >>still provide that pseudo ISA bridge at 00:1f.0 as we already did.
> >
> >By the way there is no reason to put it at 00:1f.0 specifically I think.
> >So it seems simple: create a dummy device that gets device and
> >vendor id as properties. If you really like, add an option to get values
> 
> Yes, this is just what we did in [Xen-devel] [v5][PATCH 2/5] xen, gfx
> passthrough: create pseudo intel isa bridge. There, we fake this device just
> at 00:1f.0.
> But you guys don't like this, and shouldn't this be just this point we
> discussing now?
> 
> If you guys agree that , everything is fine.

Actually, this isn't what you did.
Don't tie it to xen, and don't tie it to 1f.
Just make it a simple stub pci device.
Whoever wants it, creates it.

The thing I worry about, is the chance this will break going forward.
So you created a system with 2 isa bridges.
This is already not something that exists on real hardware.
So it might break some guests that will get confused.
Maybe we are lucky and most guests see an unfamiliar device
and ignore it. It seems believable.

But your MCH hack overrides registers in the pci host.
Are we lucky and there's nothing in these registers
of interest to guests? This seems much more fragile.
So please poke at the spec, and compile the list
of registers you want to touch, figure out why they are
safe to override, and put this all in code comments.

And the same thing that applies to the isa bridge
applies here too. It should work without QEMU touching
hosts' hardware.



> >from sysfs: device and vendor id are world readable, so just get them
> >directly and not through xen wrappers, this way you can open the files
> >RO and not RW.
> >You seem to poke at revision as well, I don't see
> >driver looking at that - strictly necessary?
> >If yes please patch host kernel to expose that info in sysfs,
> >though we can fall back on pci config if not there.
> >
> >MCH (bridge_dev) hacks in i915 are nastier.
> >To clean them up, we really have to have an explicit driver for this
> >bridge, not a pass-through device.  Long term, the right thing to do is
> >likely to extend host driver and expose the necessary information in
> >sysfs on host kernel.
> >
> >
> 
> I'm a bit confused. Any sysfs should be filled by the associated PCIe
> device, shouldn't it? So qemu still need to emulate this PCIe device
> firstly, then set properties into sysfs.

I am talking about getting host properties into qemu.
You don't want to give qemu R/W root access to host sysfs system
of the root bridge, that's not secure.
Avoiding read only access to filesystem is a good idea too, so it
should be possible to pass all parameters in as
device properties, and let whoever starts qemu
figure out what are reasonable values.

> >
> >
> >>#2 phase: Now, we will choose a capability ID that won't be conflicting with
> >>others. To do this properly, we need to get one from PCI SIG group. To have
> >>this workable and consistently validated, this method shouldn't be virt
> >>specific. Then native driver should use the same method.
> >
> >You mean you will be able to talk sense into hardware guys?
> >I doubt that. If they could be convinced to make e.g. i915 base a
> 
> We're negotiating this, so this is just our long term solution we figure out
> currently.
> 
> >proper BAR, why didn't they?
> 
> We already have no any free BAR as we mentioned previously.

I thought you were talking about modifying hardware?

> >
> >
> >>So when xen work on
> >>q35 PCIe machine, we can walk this way.
> >
> >If you are emulating MCH anyway, pick one that is close
> >to what i915 driver expects. It would then work with existing
> 
> Looks you guys prefer we create a new MCH anyway, right? But is it necessary
> to create a new based on i440fx just for a little change?
> 
> Thanks
> Tiejun

You can inherit it. Maybe you are lucky and this happens to
work without conflicting with whatever other guests want to do.
But if you ask me, you are really just piling up hacks.
If some guest does not work on i440, you should just work on
emulating whatever it does work on.
That would have real value.

> >devices, without new capability IDs.
> >
> >
> >>Anthony,
> >>
> >>Any comments to address this in xen case?
> >>
> >>Thanks
> >>Tiejun
> >
> >

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