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

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

On Mon, Jun 30, 2014 at 05:38:21PM +0800, Chen, Tiejun wrote:
> On 2014/6/30 17:05, Michael S. Tsirkin wrote:
> >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?
> 
> I mean if you want create an new machine based on q35, actually this is
> equal to start making xen to migrate to q35 now. Right? I can't image how
> much effort should be done.

I don't see why you don't try. Sounds like a more robust solution to me.

> So this is a reason why I'm saying I'd like to follow this feature after q35
> can work with xen completely.

Then we'll end up with more configurations to support, and to what end?

> >
> >>>
> >>>
> >>>>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.
> 
> I don't set class type to claim this is an ISA bridge, just a normal PCI
> device.
> +static int create_pseudo_pch_isa_bridge(PCIBus *bus, XenHostPCIDevice
> *hdev)
> +{
> +    struct PCIDevice *dev;
> +
> +    char rid;
> +
> +    /* We havt to use a simple PCI device to fake this ISA bridge
> +     * to avoid making some confusion to BIOS and ACPI.
> +     */
> +    dev = pci_create(bus, PCI_DEVFN(0x1f, 0),
> "pseudo-intel-pch-isa-bridge");
> +
> +    qdev_init_nofail(&dev->qdev);
> +
> +    pci_config_set_vendor_id(dev->config, hdev->vendor_id);
> +    pci_config_set_device_id(dev->config, hdev->device_id);
> +
> +    xen_host_pci_get_block(hdev, PCI_REVISION_ID, (uint8_t *)&rid, 1);
> +
> +    pci_config_set_revision(dev->config, rid);
> +
> +    XEN_PT_LOG(dev, "The pseudo Intel PCH ISA bridge created.\n");
> +    return 0;
> +}

Then I don't see how it's supposed to work.
Doesn't i915 look for an isa bridge?

        /*
         * The reason to probe ISA bridge instead of Dev31:Fun0 is to
         * make graphics device passthrough work easy for VMM, that only
         * need to expose ISA bridge to let driver know the real hardware
         * underneath. This is a requirement from virtualization team.
         *
         * In some virtualized environments (e.g. XEN), there is irrelevant
         * ISA bridge in the system. To work reliably, we should scan trhough
         * all the ISA bridge devices and check for the first match, instead
         * of only checking the first one.
         */
        while ((pch = pci_get_class(PCI_CLASS_BRIDGE_ISA << 8, pch))) {
                if (pch->vendor == PCI_VENDOR_ID_INTEL) {
                        unsigned short id = pch->device & 
INTEL_PCH_DEVICE_ID_MASK;
                        dev_priv->pch_id = id;

                        if (id == INTEL_PCH_IBX_DEVICE_ID_TYPE) {
                                dev_priv->pch_type = PCH_IBX;

etc



> >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.
> 
> We just try to write *one* register we already confirm this is safe enough.

This should go in code in form of comments:
document what this register does on 440fx
and why it's safe to override.
We don't see what you
confirmed off-line.

> Other register are read-only.

Doesn't matter, need to document these as well.

> >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
> 
> We just expose its own vendor/device ids here. We don't access to change
> anything in 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.
> 
> igd_pci_read()
>       |
>       + xen_host_pci_get_block()
>               |
>               + xen_host_pci_config_read(()
>                       |
>                       +  pread()
> 
> So shouldn't we already expose these information via sysfs?

That's poking at sysfs of a real device, and after opening it RW.

> >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?
> 
> Yes.
> 
> Tiejun

And they can't figure out how to stick extra info in an existing BAR?
Oh well, that's hardware for you.

> >
> >>>
> >>>
> >>>>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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