Re: [Xen-devel] intel IGD driver intel_detect_pch() failure

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 11 Dec 2012, G.R. wrote:
> On Tue, Dec 11, 2012 at 8:01 PM, Stefano Stabellini 
> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> >
> > On Tue, 11 Dec 2012, G.R. wrote:
> > >
> > > On Tue, Dec 11, 2012 at 2:15 AM, Kay, Allen M <allen.m.kay@xxxxxxxxx> 
> > > wrote:
> > > Â Â Â My understanding is that i915 driver needs to looks at the real 
> > > PCH's device ID to apply HW workarounds.
> > >
> > > Â Â Â One way to fix this is to make the device ID of the first PCH's 
> > > (00:01.0) device ID reflect the device ID of
> > > Â Â Â 00:1f.0 on the host. ÂThis way, when i915 running as a guest will 
> > > find the valid PCH device ID to make workaround
> > > Â Â Â decisions with.
> > >
> > >
> > > Â Â Â I don't know why it would make a difference if i915 is built into 
> > > the kernel or as a module though.
> > >
> > > Â Â Â Allen
> > >
> > > Thanks Allen for your input.
> > > But module v.s. built-in is not the only difference. Another difference 
> > > is the PVHVM build vs. pure HVM build.
> > > Both share the same PCI layout but different result. Any theory how to 
> > > explain the difference? What makes the PVHVM version
> > > work?
> >
> > Please don't use HTML in emails.
> >
> 
> I'm using gmail and not sure how to control html formatting.
> I explicitly use 'remove formatting' this time, does it look better now?
> Sorry for the inconvenience.

It is still html. You just need to click on "<< Plain Text".


> > PVHVM Linux guests setup interrupts differently: they request an event
> > channel for each legacy interrupt or MSI/MSI-X, then the hypervisor uses
> > these event channels to inject notifications into the guest, rather
> > than emulated interrupts or emulated MSIs.
> >
> Will this affect the result of pci_get_class() as called by the intel driver?
> If not, this can still not explain the different behavior.
> Maybe I need to do one more experiment when I got time.

No, it doesn't.


> > Reading again the description of the bug, wouldn't it be better to just
> > the fix the problem in Linux?
> > In fact this looks like a bug in intel_detect_pch(): QEMU is emulating a
> > PCI-PCI bridge and the driver is checking for an PCI-ISA bridge (to help
> > with virtualization?). Moreover it only checks the first PCI-ISA bridge.
> >
> > As far as I know Xen has never exposed a PCI-ISA bridge with vendor ==
> > Intel to the guest.
> >
> But why do xen expose the PCI-ISA bridge in host as a PCI-PCI bridge, doesn't 
> it sounds strange?

Yes, it does sound strange, possibly wrong.

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